Recent Change—North Sea
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This chapter discusses past and ongoing change in the following physical variables within the North Sea: temperature, salinity and stratification; currents and circulation; mean sea level; and extreme sea levels. Also considered are carbon dioxide; pH and nutrients; oxygen; suspended particulate matter and turbidity; coastal erosion, sedimentation and morphology; and sea ice. The distinctive character of the Wadden Sea is addressed, with a particular focus on nutrients and sediments. This chapter covers the past 200 years and focuses on the historical development of evidence (measurements, process understanding and models), the form, duration and accuracy of the evidence available, and what the evidence shows in terms of the state and trends in the respective variables. Much work has focused on detecting long-term change in the North Sea region, either from measurements or with models. Attempts to attribute such changes to, for example, anthropogenic forcing are still missing for the North Sea. Studies are urgently needed to assess consistency between observed changes and current expectations, in order to increase the level of confidence in projections of expected future conditions.
KeywordsSuspend Particulate Matter Dissolve Inorganic Carbon Atlantic Meridional Overturning Circulation North Atlantic Oscillation German Bight
Physical variables, most obviously sea temperature, relate closely to climate change and strongly affect other properties and life in the sea. This chapter discusses past and ongoing change in the following physical variables within the North Sea: temperature, salinity and stratification (Sect. 3.2), currents and circulation (Sect. 3.3), mean sea level (Sect. 3.4) and extreme sea levels, i.e. contributions from wind-generated waves and storm surges (Sect. 3.5). Also considered are carbon dioxide (CO2), pH, and nutrients (Sect. 3.6), oxygen (Sect. 3.7), suspended particulate matter and turbidity (Sect. 3.8), coastal erosion, sedimentation and morphology (Sect. 3.9) and sea ice (Sect. 3.10). The distinctive character of the Wadden Sea is addressed in Sect. 3.11, with a particular focus on sediments and nutrients. The chapter covers the past 200 years. Chapter 1 described the North Sea context and physical process understanding, so the focus of the present chapter is on the historical development of evidence (measurements, process understanding and models), the form, duration and accuracy of the evidence available (further detailed in Electronic (E-)Supplement S3) and what the evidence shows in terms of the state and trends in the respective variables.
3.2 Temperature, Salinity and Stratification
3.2.1 Historical Perspective
Observations of sea-surface temperature (SST) have been made in the North Sea since 1823, but were sparse initially. The typical number of observations per month (from ships, and moored and drifting buoys) increased from a few hundred in the 19th century to more than 10,000 in recent decades, despite the Voluntary Observing Ship (VOS) fleet declining from a peak of about 7700 ships worldwide in 1984/85 to about 4000 in 2009 (www.vos.noaa.gov/vos_scheme.shtml). Early SST observations used buckets (Kent et al. 2010); adjustments of up to ~0.3 °C in the annual mean, and 0.6 °C in winter, may be needed for these early data owing to sample heat loss or gain (Folland and Parker 1995; Smith and Reynolds 2002; Kennedy et al. 2011a, b). The adjustments depend on large-scale forcing and assumptions about measurement methods—local variations add uncertainty. Cooling water intake temperatures have been measured on ships since the 1920s but data quality is variable, sometimes poor (Kent et al. 1993). Temperature sensors on ships’ hulls became more numerous in recent decades (Kent et al. 2010). About 70 % of in situ observations in 2006 came from moored and drifting buoys (Kennedy et al. 2011b). Other modern shipboard methods include radiation thermometers, expendable bathythermographs (XBTs) and towed thermistors (Woodruff et al. 2011). Satellite estimates of SST are regularly available using Advanced Very High Resolution Radiometers (AVHRR; from 1981) and passive microwave radiometers (with little cloud attenuation; from 1997).
Below the sea surface, temperature was measured by reversing (mercury) thermometers until the 1960s. Since then, electronic instruments lowered from ships (conductivity-temperature-depth profilers; CTDs) enable near-continuous measurements. Since about 2005, multi-decadal model runs have become increasingly available and now provide useful information on temperature distribution to complement the observational evidence (see E-Supplement Sect. S3.1).
Early salinity estimates used titration-based chemical analysis of recovered water samples (from buckets and water intakes) and from lowered sample bottles. Titration estimates usually depended on assuming a constant relation between chlorinity and total dissolved salts (a subject of discussion since 1900), with typical error O(0.01 ‰). Since the 1960s–1970s lowered CTD conductivity cells enable near-continuous measurements, calibrated by comparing the conductivity of water samples against standardised sea water; typical error O(0.001 ‰). Consistent definition of salinity has continued to be a research topic (Pawlowicz et al. 2012).
Thermistors and conductivity cells as on CTDs now record temperature and salinity of (near-surface) intake water on ships. Since the late 1990s, CTDs on profiling ‘Argo’ floats have greatly increased available temperature and salinity data for the upper 2000 m of the open ocean (www.argo.ucsd.edu). Although not available for the North Sea, these data greatly improve estimates of open-ocean temperature and salinity and thereby North Sea model estimates by better specifying open-ocean boundary conditions.
The history of stratification estimates, based on profiles of temperature and salinity (or at least near-surface and near-bottom values), corresponds with that of subsurface temperature and salinity.
Detail on time-series evidence for coastal and offshore temperature and salinity variations is given in E-Supplement S3.1 and S3.2.
3.2.2 Temperature Variability and Trends
18.104.22.168 Northeast Atlantic
West and north of Britain, the HadISST data set shows an SST trend of 0.2–0.3 °C decade−1 over the period 1983–2012, which is higher than the global average (Rayner et al. 2003; see Dye et al. 2013a among several references). Thus positive temperature anomalies exceeding one standard deviation (based on the period 1981–2010) were widespread in adjacent Atlantic Water and the northern North Sea during 2003–2012 (Beszczynska-Möller and Dye 2013). In fact, several authors suggest an inverse relation between Subpolar Gyre strength and the extent of warm saline water (e.g. Hátún et al. 2005; Johnson and Gruber 2007; Haekkinen et al. 2011).
22.214.171.124 North Sea
126.96.36.199 Regional Variations
The rise in North Sea SST since the 1980s increased from north (trend <0.2 °C decade−1) to south (trend 0.8 °C decade−1; Fig. 3.6; McQuatters-Gollop et al. 2007). Based on HadISST1 for the period 1987–2011, the EEA (2012) showed warming of 0.3 °C decade−1 in the Channel, 0.4 °C decade−1 off the Dutch coast, and less than 0.2 °C decade−1 at 60°N off Norway.
The German Bight shows the largest warming trend in recent decades (Fig. 3.6) with a rapid SST rise in the late 1980s (Wiltshire et al. 2008; Meyer et al. 2011). Variability is also large, between years O(1 °C) and longer term (Wiltshire et al. 2008; Meyer et al. 2011; Holt et al. 2012). At Helgoland Roads Station (54° 11′N, 7° 54′E) decadal SST trends since 1873 show the warming after the early 1980s was the strongest.
The Dutch coastal zone shows a trend of rising SST since 1982 (van Aken 2010), despite a very cold winter in 1996 (January–March; about 4 °C below the 1969–2008 average; van Hal et al. 2010). Factors contributing to this rise are thermal inertia (seasonally), winds and cloudiness or bright sunshine (van Aken 2010). The 1956–2003 Marsdiep winter temperature (Tsimplis et al. 2006) and Wadden Sea winter and spring temperature (van Aken 2008) were significantly correlated with the winter North Atlantic Oscillation (NAO) index (see Annex 1). However, decadal to centennial temperature variations (a cooling of about 1.5 °C over the period 1860–1890 and a similar warming in the last 25 years) were not related to long-term changes in the NAO.
The western English Channel (50.03°N, 4.37°W) warmed in the 1920s and 1930s (Southward 1960); after a dip it warmed again in the 1950s, cooled in the 1960s and warmed over the full water column from the mid-1980s to the early 2000s (0.6 °C decade−1, Smyth et al. 2010; see E-Supplement Fig. S3.2). The greatest (1990s) temperature rise coincided with a decrease in median wind speed (from 3.5 to 2.75 m s−1) and an increase in surface solar irradiation (of about 20 %), both correlated with changes in the NAO (Smyth et al. 2010).
Off northern Denmark and Norway, coastal waters in winter (JFM) were 0.8–1.3 °C warmer in the period 2000–2009 than the period 1961–1990 (Albretsen et al. 2012); the corresponding rise at 200 m depth was 0.55–0.8 °C. Winter–spring observed SST in the Kattegat and Danish Straits rose by about 1 °C between 1897–1901 and the 1980s, and again by about 1 °C to the 1990s–2006 period (Henriksen 2009). Summer–autumn trends were not as clear.
3.2.3 Salinity Variability and Trends
188.8.131.52 Northeast Atlantic
North Atlantic surface salinity shows pronounced interannual and multi-decadal variability. In the Subpolar Gyre salinity variations are correlated with SST such that high salinities usually coincide with anomalously warm water and vice versa (such as in Rockall Trough; Beszczynska-Möller and Dye 2013). On decadal time scales, upper-layer salinity is also positively correlated with the winter NAO, especially in the eastern part of the gyre (Holliday et al. 2011). Shelf-sea and oceanic surface waters to the north and west of the UK had a salinity maximum in the early 1960s and a relatively fresh period in the 1970s, associated with the so-called Great Salinity Anomaly (Dickson et al. 1988). In Rockall Trough the minimum occurred about 1975 (Dickson et al. 1988) and was followed by increasing salinities, interrupted by a mid-1990s minimum (Holliday et al. 2010; Hughes et al. 2012; Sherwin et al. 2012).
Correspondingly, the Fair Isle—Munken section (~2°W 59.5°N to 6°W 61°N across the Faroe-Shetland Channel) at 50–100 m depth showed an upward salinity trend of 0.075 decade−1 during the period 1994–2011 (Fig. 3.1; Berx et al. 2013). Likewise, the salinity of Atlantic water inflow to the Nordic Seas through Svinøy section (to the north-west off Norway through ~4°E 63°N) has increased by about 0.15 since the 1970s (Holliday et al. 2008; Beszczynska-Möller and Dye 2013), for example by 0.08 from 1992 to 2009 (Mork and Skagseth 2010).
184.108.40.206 North Sea
On the western side of the Norwegian Trench and in the central northern North Sea (Utsira section, 59.3°N), influenced by Atlantic water, salinity has increased by about 0.05 since the late 1970s (when values were relatively stable after the Great Salinity Anomaly; Beszczynska-Möller and Dye 2013). On the other hand, salinity in the Fair Isle Current shows interannual variability and no clear long-term trend (Fig. 3.2), being influenced by the fresher waters of the Scottish Coastal Current from west of Scotland.
The western English Channel (50.03°N, 4.37°W), away from the coast, is influenced by North Atlantic water, showing a similar increase in salinity in recent years (Holliday et al. 2010). Local weather effects (mixed vertically by tidal currents) add to interannual salinity variability which is much greater than in the open ocean. For example, station L4 off Plymouth experiences pulses of surface freshening after intense summer rain increases riverine input (Smyth et al. 2010). However, there is no clear trend over a century of measurements (see also E-Supplement Fig. S3.3, E-Supplement Sect. S3.2).
In the Kattegat and Skagerrak, salinities are affected by low-salinity Baltic Sea outflow. Skagerrak coastal waters in winter (January–March) were up to 0.5 more saline in the period 2000–2009 than the period 1961–1990, but further west and north around Norway their salinity decreased slightly (Albretsen et al. 2012). Shorter-term variability is larger. Salinity variability in the Kattegat and Skagerrak exceeds that in Atlantic water, owing to varying Baltic outflow (see Sect. 3.3) and net precipitation minus evaporation in catchments.
Salinity variability on all time scales to multi-decadal exceeds and obscures any potential long-term trend. For example, in winter 2005, a series of storms drove much high-salinity Atlantic water across the north-west boundary into the North Sea as far south as Dogger Bank and bottom-water salinity exceeded 35 in 63 % of the North Sea area (Loewe 2009). Adjacent Atlantic waters in the period 2002–2010 (Hughes et al. 2011) show positive salinity anomalies of more than two (one) standard deviation in Rockall Trough (Faroe-Shetland Channel) while the North Sea has no comparably clear signal.
3.2.4 Stratification Variability and Trends
Annual time series of ECOHAM4 simulated thermocline characteristics averaged over the North Sea were reported by Lorkowski et al. (2012). The maximum depth of the thermocline2 is much more variable interannually than its mean depth. Thermocline intensity shows no trend and only moderate variability. The annual number of days with a mean thermocline greater than 0.2 °C m−1 ranged from 31 to 101. The warmest summer in the period simulated (2003) hardly shows in any thermocline characteristics (Lorkowski et al. 2012). In the north-western North Sea, the strength of thermal stratification varies interannually (with no clear trend but periodicity of about 7–8 years; Sharples et al. 2010). The multi-decadal hindcast by Meyer et al. (2011) for the North Sea confirmed that variability in stratification is mainly interannual. In seasonally stratified regions, Holt et al. (2012) modelling showed 1985–2004 warming trends to be greater at the surface than at depth (reflecting an increase in stratification), especially in the central North Sea, at frontal areas of Dogger Bank, in an area north-east of Scotland and in inflow to the Skagerrak. They also found this pattern in annual trends of ICES (International Council for the Exploration of the Sea) data, albeit limited by a lack of seasonal resolution.
In estuarine outflow regions, strong short-term and interannual variability in precipitation (hence fluvial inputs) and tidal mixing mask any longer-term trends in stratification (timing or strength).
3.3 Currents and Circulation
3.3.1 Historical Perspective
The earliest evidence for circulation comes from hydrographic sections, for time scales longer than a day, and from drifters, observed by chance or deliberately deployed. Prior to satellite tracking (of floats or drogued buoys), typically only drifters’ start and end points would be known; temporal and spatial resolution were lacking. Moored current meters record time series at one location; their use was rare until the 1960s. Within the area (5°W–13°E, 48°N–62°N) the international current meter inventory at the British Oceanographic Data Centre3 records just 27 year-long records and 3025 month-long records to 2008; by decade from the 1950s, the numbers of month-long records are 1, 32, 1306, 1201, 381, 124. Occasionally, submarine cables have monitored approximate transport across a section (notably for flow through Dover Strait; e.g. Robinson 1976; Prandle 1978a) and HF radar has given spatial coverage for surface currents within a limited range (Prandle and Player 1993).
Detail on evidence for currents, circulation and their variations is given in E-Supplement Sect. S3.3.
3.3.2 Circulation: Variability and Trends
The Atlantic Meridional Overturning Circulation (AMOC), and its warm north-eastern limb in the Subpolar Gyre, influence the flow and properties of Atlantic Water bordering and partly flowing onto the north-west European shelf and into the North Sea. The AMOC has much seasonal and some interannual variability: mean 18.5 Sv (SD ~ 3 Sv) for April 2004 to March 2009 (Sv is Sverdrup, 106 m3 s−1) (McCarthy et al. 2012). The AMOC probably also varies on decadal time scales (e.g. Latif et al. 2006). Longer-term trends are not yet determined (Cunningham et al. 2010) even though Smeed et al. (2014) found the mean for April 2008 to March 2012 to be significantly less than for the previous four years. The Subpolar Gyre extent correlates with the NAO (Lozier and Stewart 2008). It strengthened overall from the 1960s to the mid-1990s, then decreased (Hátún et al. 2005). While the Subpolar Gyre was relatively weak in the period 2000–2009, more warm, salty Mediterranean and Eastern North Atlantic waters flowed poleward around Britain (Lozier and Stewart 2008; Hughes et al. 2012). Negative NAO also correlates with more warm water in the Faroe-Shetland Channel (Chafik 2012). However, observations show no significant longer-term trend in Atlantic Water transport to the north-east past Scotland and Norway (Orvik and Skagseth 2005; Mork and Skagseth 2010; Berx et al. 2013).
Inflow of oceanic waters to the North Sea from the Atlantic Ocean, primarily in the north driven by prevailing south-westerly winds, has been modelled by Hjøllo et al. (2009; 1985–2007), Holt et al. (2009) and using NORWECOM/POM (3-D hydrodynamic model; Iversen et al. 2002; Leterme et al. 2008, for 1958–2003; Albretsen et al. 2012). Relative to the long-term mean, results show weaker northern inflow between 1958 and 1988; within this period, there were increases in the 1960s and early 1970s, a decrease from 1976 to 1980 and an increase in the early and mid-1980s. The northern inflow was greater than the long-term mean in 1988 to 1995 with a maximum in 1989 (McQuatters-Gollop et al. 2007) but smaller again in 1996 to 2003. This inflow is correlated positively with salinity, SST (less strongly) and the NAO (especially in winter), and negatively with discharges from the rivers Elbe and Rhine (less strongly). For the period 1985–2007, Hjøllo et al. (2009) found a weak trend of −0.005 Sv year−1 in modelled Atlantic Water inflows (mean 1.7 Sv, SD 0.41 Sv, correlation with NAO ~0.9). Strong flows into the North Sea (and Nordic Seas) frequently correspond to high-salinity events (Sundby and Drinkwater 2007).
Dover Strait inflow, of the order 0.1 Sv (Prandle et al. 1996), was smaller than the long-term mean from 1958 to 1981 and then greater until 2003 (Leterme et al. 2008). Baltic Sea outflow variations (modelled freshwater relative to salinity 35.0) correlate with winds, resulting sea-surface elevation and NAO index; correlation coefficients with the NAO were 0.57 during the period 1962–2004 and 0.74 during 1980–2004 (Hordoir and Meier 2010; Hordoir et al. 2013). Days-to-months variability O(0.1 Sv) in North Sea—Baltic Sea exchange far exceeds the mean Baltic Sea outflow of the order 0.01 Sv or any trend therein.
North Sea outflows and inflows (plus net precipitation minus evaporation) have to balance on a time scale of just a few days. Off-shelf flow is persistent in the Norwegian Trench and in a bottom layer below the poleward along-slope flow (Holt et al. 2009; Huthnance et al. 2009). A modelled time series for 1958–1997 (Schrum and Sigismund 2001) shows an average outflow of about 2 Sv, little clear trend but consistency with the above interannual variations in inflow.
A MyOcean (project) reanalysis of the region 40°–65°N by 20°W–13°E for the period 1984–2012 was undertaken with the NEMO model version 3.4 (Madec 2008; for details on this application see MyOcean 2014). Transports normal to transects were calculated following NOOS (2010): averaging flow over 24.8 h to give a tidal mean at each model point across the transect; then area-weighting for transports, separating the mean negative and mean positive flows. For the Norway–Shetland transect, flow in the west is dominantly into the North Sea and makes a significant contribution to exchange with the wider Atlantic; circulation is partially density-driven during summer and confined to the coastal waters east of Shetland. Mean inflow is 0.56 Sv with significant seasonality and interannual variability but no obvious trend. In the east sector of the Norway–Shetland transect, flow is both into and out of the North Sea, strongly steered by the Norwegian Trench and includes the Norwegian Coastal Current, resulting in a larger outflow than inflow. Mean net flow is 1.3 Sv (SD 0.97 Sv) representing large seasonal and interannual variability, especially in the outflow.
Net circulation within the North Sea is shown schematically in Fig. 1.7. Tidal currents are important, primarily semi-diurnal with longer-period modulation (Sect. 1.4.4); locally values exceed 1.2 m s−1 in the Pentland Firth, off East Anglia and in Dover Strait. Other important current contributions are due to winds (Sect. 1.4.3 shows representative flow patterns) and to differences in density (Sect. 1.4.2) including estuarine outflows (e.g. van Alphen et al. 1988), varying on time scales from hours to seasons (e.g. Turrell et al. 1992) to decades. Hence flows can be very variable in time; they also vary strongly with location.
Wind forcing is the most variable factor; water transports in one storm (typically in winter; time-scale hours to a day) can be significant relative to a year’s total. 50-year return values for currents in storm surges have been estimated at 0.4–0.6 m s−1 in general, but exceed 1 m s−1 locally off Scottish promontories, in Dover Strait, west of Denmark and over Dogger Bank (Flather 1987). These extreme currents are directed anti-clockwise around the North Sea near coasts, and into the Skagerrak.
In summer-stratified areas (Sect. 3.2.4) cold bottom water is nearly static (velocity tends to zero at the sea bed due to friction). Between stratified and mixed areas, relatively strong density gradients are expected to drive near-surface flows anti-clockwise around the dense bottom water (Hill et al. 2008). These flows, of the order 0.3 m s−1 but sometimes >1 m s−1 in the Norwegian Coastal Current, are liable to baroclinic instability developing meanders, scale 5–10 km (e.g. Badin et al. 2009; their model shows eddy variability increasing in late summer with increased stratification). Such meanders are prominent north of Scotland over the continental slope and off Norway where the fresher surface layer increases stratification.
When a region of freshwater influence (ROFI) is stratified, cross-shore tidal currents may develop; for example, according to de Boer et al. (2009) surface currents rotate clockwise and bottom currents anti-clockwise in the Rhine ROFI when stratified. These authors also found cyclical upwelling there due to tidal currents going offshore at the surface and onshore below.
The winter mean circulation of the North Sea is organised in one anti-clockwise gyre with typical mean velocities of about 10 cm s−1 (Kauker and von Storch 2000). On shorter time scales the circulation is highly variable. Kauker and von Storch (2000) identified four regimes. Two are characterised by a basin-wide gyre with clockwise (15 % of the time) or anti-clockwise (30 % of the time) orientation. The other two regimes are characterised by the opposite regimes of a bipolar pattern with maxima in the southern and northern parts of the North Sea (45 % of the time). For 10 % of the time the circulation nearly ceased. Kauker and von Storch (2000) found that only 40 % of the one-gyre regimes persist for longer than five days while the duration of the bipolar circulation patterns rarely exceeded five days. Accordingly, short-term variability typically dominates transports; tidal flows dominate instantaneous transports (positive and negative volume fluxes across sections) and meteorological phenomena dominate residual (net) transports.
Mean residual transports are generally smaller than their variability. Many transects show strong seasonality as meteorological conditions drive surges, river runoff and ice melt. No trend in transports has been seen in these data: limited duration of available data and large variability in the transports on time scales of days, seasons and interannually makes discerning trends difficult.
In the German Bight, anti-clockwise circulation is about twice as frequent as clockwise, and prevails during south-westerly winds typical of winter storms, giving rapid transports through the German Bight (Thiel et al. 2011, on the basis of Pohlmann 2006). Loewe (2009) associated clockwise flow with high-pressure and north-westerly weather types, anti-clockwise flow with south-westerly weather types, and flow towards the north or north-west with south-easterly weather types. However, Port et al. (2011) found that the wind-current relation changes away from the coast owing to dependence on density effects, the coastline and topography.
In summary, multiple forcings cause currents to vary on a range of time and space scales, including short scales relative to which measurements are sparse. Hence trends are of lesser significance and hard to discern. Moreover, causes of trends in flows are difficult to diagnose; improvements are needed in observational data (quantity and quality). Reliance is placed on models, which need improvement (in formulation, forcing) for currents other than tides and storm surges.
3.4 Mean Sea Level
Changes in mean sea level (MSL) result from different aspects of climate change (e.g. the melting of land-based ice, thermal expansion of sea water) and climate variability (e.g. changes in wind forcing related to the NAO or El Niño–Southern Oscillation) and occur over all temporal and spatial scales. MSL is sea level averaged into monthly or annual mean values, which are the parameters of most interest to climate researchers (Woodworth et al. 2011). The focus in this chapter is on the last 200 years, when direct ‘modern’ measurements of sea level are available from tide gauges and high precision satellite radar altimeter observations. MSL can be inferred indirectly over this period (and thousands of years earlier) using proxy records from salt-marsh sediments and the fossils within them (Gehrels and Woodworth 2013) or archaeology (e.g. fish tanks built by the Romans), and over much longer time scales (thousands to millions of years) using other paleo-data (e.g. geological records, from corals or isotopic methods).
The North Sea coastline has one of the world’s most densely populated tide gauge networks, with many (>15) records spanning 100 years or longer and a few going back almost continuously to the early 19th century. The tide gauges of Brest and Amsterdam also provide some data for parts of the 18th century and are among the longest sea level records in the world. Since 1992, satellite altimetry has provided near-global coverage of MSL. The advantage of altimetry is that it records geocentric sea level (i.e. measurements relative to the centre of the Earth). By contrast, tide gauges measure the relative changes between the ocean surface and the land itself; hence, the term ‘relative mean sea level’ (RMSL), and it is this that is of most relevance to coastal managers, engineers and planners. Calculation from tide gauge records of changes in ‘geocentric mean sea level’ (sometimes referred to as ‘absolute mean sea level’; AMSL) requires the removal of non-climate contributions to sea level change, which arise both from natural processes (e.g. tectonics, glacial isostatic adjustment GIA) and from anthropogenic processes (e.g. subsidence caused by ground water abstraction). Tide gauge records can be corrected using estimates of vertical land motion from (i) models which predict the main geological aspect of vertical motion, namely GIA (e.g. Peltier 2004); (ii) geological information near tide gauge sites (e.g. Shennan et al. 2012); and (iii) direct measurements made at or near tide gauge locations using continuous global positioning system (GPS) or absolute gravity (e.g. Bouin and Wöppelmann 2010). Rates of vertical land movement have also been estimated by comparing trends derived from altimetry data and tide gauge records (e.g. Nerem and Mitchum 2002; Garcia et al. 2007; Wöppelmann and Marcos 2012).
Paleo sea level data from coastal sediments, the few long (pre-1900) tide gauge records and reconstructions of MSL, made by combining tide gauge records with altimetry measurements (e.g. Church and White 2006, 2011; Jevrejeva et al. 2006, 2008; Merrifield et al. 2009), indicate that there was an increase in the rate of global MSL rise during the late 19th and early 20th centuries (e.g. Church et al. 2010; Woodworth et al. 2011; Gehrels and Woodworth 2013). Over the last 2000 to 3000 years, global MSL has been near present-day levels with fluctuations not larger than about ±0.25 m on time scales of a few hundred years (Church et al. 2013) whereas the global average rate of rise estimated for the 20th century was 1.7 mm year−1 (Bindoff et al. 2007). Measurements from altimetry suggest that the rate of MSL rise has almost doubled over the last two decades; Church and White (2011) estimated a global trend of 3.2 ± 0.4 mm year−1 for the period 1993–2009. Milne et al. (2009) assessed the spatial variability of MSL trends derived from altimetry data and found that local trends vary by as much as −10 to +10 mm year−1 from the global average value for the period since 1993, due to regional effects influencing MSL changes and variability (e.g. non-uniform contributions of melting glaciers and ice sheets, density anomalies, atmospheric forcing, ocean circulation, terrestrial water storage). This highlights the importance of regional assessments. Examining whether past MSL has risen faster or slower in certain areas compared to the global average will help to provide more reliable region-specific MSL rise projections for coastal engineering, management and planning.
There have been very few region-wide studies of MSL changes in the North Sea. The first detailed study was by Shennan and Woodworth (1992), who used geological and tide gauge data from sites around the North Sea to infer secular trends in MSL in the late Holocene and 20th century (up until the late 1980s). They concluded that a systematic offset of 1.0 ± 0.15 mm year−1 in the tide gauge trends, compared to those derived from the geological data, could be interpreted as the regional average rate of geocentric MSL change over the 20th century; this is significantly less than global rates over this period. They also showed that part of the interannual MSL variability of the region was coherent, and they represented this as an index, created by averaging the de-trended MSL time series. Like Woodworth (1990), they found no evidence for a statistically significant acceleration in the rates of MSL rise for the 20th century.
Since then many other investigations of MSL changes have been undertaken for specific stretches of the North Sea coastline, mostly on a country-by-country basis, as for example by Araújo (2005), Araújo and Pugh (2008), Wöppelmann et al. (2006, 2008) and Haigh et al. (2009) for the English Channel; by van Cauwenberghe (1995, 1999) and Verwaest et al. (2005) for the Belgian coastline; Jensen et al. (1993) and Dillingh et al. (2010) for the Dutch coastline; Jensen et al. (1993), Albrecht et al. (2011), Albrecht and Weisse (2012) and Wahl et al. (2010, 2011) for the German coastline; Madsen (2009) for the Danish coastline; Richter et al. (2012) for the Norwegian coastline; and by Woodworth (1987) and Woodworth et al. (1999, 2009a) for the United Kingdom (UK). The most detailed analysis of 20th century geocentric MSL changes was undertaken by Woodworth et al. (2009a). They estimated that geocentric MSL around the UK rose by 1.4 ± 0.2 mm year−1 over the 20th century; faster (but not significantly faster at 95 % confidence) than the earlier estimate by Shennan and Woodworth (1992) for the whole North Sea and slower (but not significantly slower at 95 % confidence level) than the global 20th century rate.
However, using correlation analyses, Wahl et al. (2013) showed that part of the variability was coherent throughout the region, with some differences between the Inner North Sea (number 4 anti-clockwise to 26 in Fig. 3.15) and the English Channel. Following Shennan and Woodworth (1992), they represented this coherent part of the variability by means of MSL indices (Fig. 3.16). Geocentric MSL trends of 1.59 ± 0.16 and 1.18 ± 0.16 mm year−1 were obtained for the Inner North Sea and English Channel indices, respectively, for the period 1900–2009 (data sets were corrected for GIA to remove the influence of vertical land movement). For the North Sea region as a whole, the geocentric MSL trend was 1.53 ± 0.16 mm year−1. These results are consistent with those presented by Woodworth et al. (2009a) for the UK (i.e. an AMSL trend of 1.4 ± 0.2 mm year−1 for the 20th century), but were significantly different from those presented by Shennan and Woodworth (1992) for the North Sea region (i.e. a geocentric MSL trend of 1.0 ± 0.15 mm year−1 for the period from 1901 to the late 1980s). For the ‘satellite period’ (i.e. 1993 to 2009) the geocentric MSL trend was estimated to be 4.00 ± 1.53 mm year−1 from the North Sea tide gauge records. This trend is faster but not significantly different from the global geocentric MSL trend for the same period (i.e. 3.20 ± 0.40 mm year−1 from satellite altimetry and 2.80 ± 0.80 mm year−1 from tide gauge data; Church and White 2011). In summary, the observed long-term changes in sea-level rise (SLR) in the North Sea do not differ significantly from global rates over the same period.
In recent years there has also been considerable focus on the issue of ‘acceleration in rates of MSL rise’. Several methods have been applied to examine non-linear changes in long MSL time series from individual tide gauge sites and global or regional reconstructions (see Woodworth et al. 2009b, 2011 for a synthesis of these studies). Wahl et al. (2013) used singular system analysis (SSA) with an embedding dimension of 15 years for smoothing the MSL indices for the Inner North Sea and English Channel (Fig. 3.16). Periods of SLR acceleration were detected at the end of the 19th century and in the 1970s; a period of deceleration occurred in the 1950s. Several authors (e.g. Miller and Douglas 2007; Woodworth et al. 2010; Sturges and Douglas 2011; Calafat et al. 2012) suggested that these periods of acceleration/deceleration are associated with decadal MSL fluctuations arising from large-scale atmospheric changes. The recent rates of MSL rise were found to be faster than on average, with the fastest rates occurring at the end of the 20th century. These rates are, however, still comparable to those observed during the 19th and 20th centuries.
3.5 Extreme Sea Levels
Extreme sea levels pose significant threats (such as flooding and/or erosion) to many of the low-lying coastal areas along the North Sea coast. Two of the more recent examples are the events of 31 January/1 February 1953 and 16/17 February 1962 that caused extreme sea levels along much of the North Sea coastline and that were associated with a widespread failure of coastal protection, mostly in the UK, the Netherlands and Germany (e.g. Baxter 2005; Gerritsen 2005). Since then, coastal defences have been substantially enhanced along much of the North Sea coastline.
Extreme sea levels usually arise from a combination of factors extending over a wide range of spatial and temporal scales comprising high astronomical tides, storm surges (also referred to as meteorological residuals caused by high wind speeds and inverse barometric pressure effects) and extreme sea states (wind-generated waves at the ocean surface) (Weisse et al. 2012). On longer time scales, rising MSL may increase the risk associated with extreme sea levels as it modifies the baseline upon which extreme sea levels act; that is, it tends to shift the entire frequency distribution towards higher values.
The large-scale picture may be modified by local conditions. For example, for given wind speed and direction the magnitude of a storm surge may depend on local bathymetry or the shape of the coastline. Extreme sea states may become depth-limited in very shallow water and effects such as wave set-up (Longuet-Higgins and Stewart 1962) may further raise extreme sea levels. Moreover, there is considerable interaction among the different factors contributing to extreme sea levels, especially in shallow water. For example, for the UK coastline Horsburgh and Wilson (2007) reported a tendency for storm surge maxima to occur most frequently on the rising tide arising primarily from tide-surge interaction. Mean SLR may modify tidal patterns and several authors report changes in tidal range associated with MSL changes. For M2 tidal ranges, estimates vary from a few centimetres increase in the German Bight for a 1-m SLR (e.g. Kauker 1999) to 35 cm in the same area for a 2-m SLR (Pickering et al. 2011). So far, reasons for these differences are not elaborated on in the peer-reviewed literature.
Large sectors of the North Sea coastline are significantly affected by storm surges. A typical measure to assess the weather-related contributions relative to the overall variability is the standard deviation of the meteorological residuals (Pugh 2004). Typically, this measure varies from a few centimetres for open ocean islands hardly affected by storm surges to tens of centimetres for shallow water subject to frequent meteorological extremes (Pugh 2004). For the German Bight, values are in the order of approximately 30–40 cm indicating that storm surges provide a substantial contribution to the total sea level variability (Weisse and von Storch 2009). There is also pronounced seasonal variability with the most severe surges generally occurring within the winter season from November to February reflecting the corresponding cycle in severe weather conditions (Weisse and von Storch 2009).
An alternative approach to analyse changes in extreme sea levels caused by changing meteorological conditions is by using numerical tide-surge models for hindcasting extended periods over past decades. Such hindcasts are usually set up using present-day bathymetry and are driven by observed (reanalysed) atmospheric wind and pressure fields. In such a design any observed changes in extreme sea levels result solely from meteorological changes while contributions from all other effects such as changes in MSL or local construction works are explicitly removed. Generally, and consistent with the results obtained from observations, such studies do not show any long-term trend but pronounced decadal and interannual variability consistent with observed changes in storm activity (e.g. Langenberg et al. 1999; Weisse and Pluess 2006).
In the analysis of von Storch and Reichardt (1997) annual mean high water is used as a proxy to describe changes in the mean. Climatically induced changes in annual mean high water statistics result principally from two different contributions: (i) corresponding changes in MSL and/or (ii) changes in tidal dynamics. Separating both contributions, Mudersbach et al. (2013) found for Cuxhaven from 1953 onwards that, apart from changes in MSL, extreme sea levels have also increased as a result of changing tidal dynamics. Reasons for the observed changes in tidal variation remain unclear. While increasing MSL represents a potential driver discussed by some authors (e.g. Mudersbach et al. 2013) the magnitude of the observed changes is too large compared to expectations from modelling studies (e.g. Kauker 1999; Pickering et al. 2011) and other contributions (such as those caused by local construction works) could not be ruled out (e.g. Hollebrandse 2005). Other potential reasons for changes in tidal constituents are referred to by Woodworth (2010) and Müller (2012) but have not been explored for the North Sea.
Systematic measurements of sea state parameters exist only for periods much shorter than those from tide gauges. In the late 1980s and early 1990s a series of studies analysed changes in mean and extreme wave heights in the North Atlantic and the North Sea (e.g. Neu 1984; Carter and Draper 1988; Bacon and Carter 1991; Hogben 1994). These were typically based on time series of 15 to at most 25 years and, while reporting a tendency towards more extreme sea states, all authors concluded that the time series were too short for definitive statements on longer-term changes. As for storm surges, numerical models are therefore frequently used to make inferences about past long-term changes in wave climate. Such models are either used globally (e.g. Cox and Swail 2001; Sterl and Caires 2005) or regionally for the North Sea and adjacent sea areas (e.g. WASA-Group 1998; Weisse and Günther 2007). For the North Sea, the latter found considerable interannual and decadal variability in the hindcast wave data consistent with existing knowledge on variations in storm activity.
Results from numerical studies should be complemented with those from statistical approaches. While numerical studies may represent variability and changes with fine spatial and temporal detail, the period for which such studies are possible is presently limited to a few decades. Statistical approaches may bridge the gap by providing information for longer time spans, but are usually limited in spatial and/or temporal detail. Such approaches were used by Kushnir et al. (1997), WASA-Group (1998), Woolf et al. (2002) and Vikebø et al. (2003), exploiting different statistical models between sea-state parameters and large-scale atmospheric conditions. Generally these approaches illustrate the substantial interannual and decadal variability inherent in the North Sea and North Atlantic wave climate. While longer periods are covered, the authors described periods of decreases and increases in extreme wave conditions. For example, Vikebø et al. (2003) described an increase in severe wave heights emerging around 1960 and lasting until about 1999 and concluded that this increase is not unusual when longer periods are considered. This indicates that changes extending over several decades, i.e. typical periods covered by numerical or observational based studies, should be viewed in the light of decadal variability obtained by analysing longer time series.
3.6 Carbon Dioxide, pH, and Nutrients
Drivers and consequences of climate change are usually discussed from the perspective of physical processes. As such, Sects. 3.2 and 3.3 focus on aspects of physical water column properties (sea temperature, salinity and stratification) and physical interaction with adjacent water bodies (circulation and currents), and climate-change-driven alterations of these. While biogeochemical properties clearly respond to changes in physical conditions, changes can also be modulated by anthropogenic changes in the chemical conditions. These include increasing atmospheric CO2 levels, ocean acidification as a consequence, and eutrophication/oligotrophication. Relevant time scales can co-vary with those of climate change processes, however they may also be distinctly different (e.g. Borges and Gypens 2010). Furthermore, effects of direct anthropogenic changes (such as nutrient inputs) and feedbacks between anthropogenic and climate changes (atmospheric CO2 and warming, for example) can be synergistic (amplify each other) or antagonistic (diminish each other). Eutrophication and oligotrophication, feedbacks to changes in physical properties and their effects on productivity in the North Sea have been investigated using models (e.g. Lenhart et al. 2010; Lancelot et al. 2011). Results have been used by international bodies and regulations such as OSPAR, the European Water Framework Directive (EC 2000) and the Marine Strategy Framework Directive. A summary was recently given by Emeis et al. (2015).
The main focus of this section is on the carbonate and pH system of the North Sea and its vulnerability to climate and anthropogenic change. To address these issues, large systematic observational studies were initiated in the early 2000s by an international consortium led by the Royal Netherlands Institute of Sea Research (e.g. Thomas et al. 2005b; Bozec et al. 2006). Observational studies have been supplemented by modelling studies (e.g. Blackford and Gilbert 2007; Gypens et al. 2009; Prowe et al. 2009; Borges and Gypens 2010; Kühn et al. 2010; Liu et al. 2010; Omar et al. 2010; Artioli et al. 2012, 2014; Lorkowski et al. 2012; Wakelin et al. 2012; Daewel and Schrum 2013).
The North Sea is one of the best studied and most understood marginal seas in the world and so offers a unique opportunity to identify biogeochemical responses to climate variability and change. To better understand the sensitivity of the North Sea biogeochemistry to climate and anthropogenic change, this section first discusses some of the main responses to variability in the dominant regional climate mode—the NAO—based on observational data for 2001, 2005 and 2008. The effects of long-term perturbations on the major processes regulating biogeochemical conditions in the North Sea are then discussed based on results from multi-decadal ecosystem model runs. Observations on longer time scales exist locally off the Netherlands, Helgoland and elsewhere but are all from sites close to the coast where strong offshore gradients in nutrients and primary productivity (e.g. Baretta-Bekker et al. 2009; Artioli et al. 2014) affect CO2.
3.6.1 Observed Responses to Variable External Forcing
In deeper areas of the North Sea, beyond the 50 m depth contour, primary production and CO2 fixation are supported by seasonal stratification and by nutrients, which are a limiting factor and largely originate from the Atlantic Ocean (Pätsch and Kühn 2008; Loebl et al. 2009). Sinking particulate organic matter facilitates the replenishment of biologically-fixed CO2 by atmospheric CO2. Respiration of particulate organic matter below the surface layer releases metabolic dissolved inorganic carbon (DIC) which is either exported to the deeper Atlantic or mixed back to the surface in autumn and winter (Thomas et al. 2004, 2005b; Bozec et al. 2006; Wakelin et al. 2012). These northern areas of the North Sea act as a net annual sink for atmospheric CO2.
By contrast, in the south (depth <50 m), the absence of stratification causes respiration and primary production to occur within the well-mixed water column. Except during the spring bloom, the effects of particulate organic carbon (POC) production and respiration cancel out and the CO2 system is largely temperature-controlled (Thomas et al. 2005a; Schiettecatte et al. 2006, 2007; Prowe et al. 2009). Total production in this area is high in global terms; terrestrial nutrients contribute, especially in the German Bight, but in the shallow south, primary production is based largely on recycled nutrients with little net fixation of CO2.
Beyond the biologically-mediated CO2 controls, North Atlantic waters, flushing through the North Sea, dominate the carbonate system (Thomas et al. 2005b; Kühn et al. 2010) but may have only small net budgetary effects. The Baltic Sea outflow and river loads constitute net imports of carbon to the North Sea and modify the background conditions set by North Atlantic waters.
Basin-wide observations of DIC, pH, and surface temperature during the summers of 2001, 2005 and 2008 (Salt et al. 2013) reveal the dominant physical mechanisms regulating the North Sea pH and CO2 system. pH and CO2 system responses to interannual variability in climate and weather conditions (NAO, local heat budgets, wind and fluxes to or from the Atlantic, the Baltic Sea and rivers, see also Sects. 3.2 and 3.3) are also considered to be the responses that climate change will trigger. Interannual variability appears generally more pronounced than long-term trends (e.g. Thomas et al. 2008).
The NAO index (Hurrell 1995; Hurrell et al. 2013) is commonly established for the winter months (DJF), although its impacts have been identified at various time scales. Many processes in the North Sea are reported to be correlated with the winter NAO, even if they occur in later seasons. Two aspects may explain an apparent delay between the trigger (i.e. winter NAO) and the response (the timing of the actual process): preconditioning and hysteresis (Salt et al. 2013).
An example of pre-conditioning is the water mass exchange between the North Atlantic Ocean and the North Sea. This exchange is enhanced during years of positive NAO (Winther and Johannessen 2006) and leads to an increased nutrient inventory in the North Sea and to higher annual productivity in spring and summer (Pätsch and Kühn 2008). Hysteresis can be characteristic of the North Sea’s response to the NAO. Stronger westerly winds in winter, correlated with the winter NAO, push North Sea water into the Baltic Sea, a process that in turn leads to an enhanced outflow from the Baltic Sea into the North Sea in subsequent seasons (Hordoir and Meier 2010).
For winter NAO values, 2001 was the most negative (−1.9), 2005 was effectively neutral (0.12) and 2008 was positive (2.1). Weaker winds and circulation in the North Sea are associated with negative NAO (see Sects. 1.4.3 and 3.3.2) and reduce the upward mixing of cold winter water (Salt et al. 2013). Hence, metabolic DIC accumulated in deeper waters during the preceding autumn and winter (Thomas et al. 2004) was mixed into surface waters to a lesser extent in 2001 than in 2005 or 2008 when wind or circulation-driven mixing was stronger (see also Salt et al. 2013), which explained the elevated surface DIC and lower pH in 2005 and 2008 relative to 2001 (Figs. 3.18 and 3.19).
The striking difference between 2001 and 2005 in the northern North Sea (Thomas et al. 2007) was reinforced by the warmer summer with a shallower mixed layer in 2001 (Salt et al. 2013: their Fig. 5). Comparable biological activity caused the shallower mixed layer of 2001 to experience stronger biological DIC drawdown on a concentration basis, resulting in higher pH, than in 2005 (Figs. 3.18 and 3.19).
Such an influence of North Atlantic inflow is supported by strong correlations between changes in the inventories of salinity and corrected DIC (i.e. accounting for biological effects) during the periods 2001–2005 and 2005–2008 (Salt et al. 2013). Mean values of partial pressure of CO2 (pCO2) in the water (331.6 ppm in 2001, 352.5 ppm in 2005, 364.0 ppm in 2008) reflect the large change between 2001 and 2005 and the moderate change between 2005 and 2008. Also, strong NAO-driven anti-clockwise circulation in the North Sea in 2008 intensified the distinct characteristics of the southern and northern North Sea and sharpened the transition between them (e.g. high to low pH, see Salt et al. 2013: their Fig. 2).
Modelling results (Lorkowski et al. 2012) agree with several of these findings: a mixed layer shallower in 2001 and 2008 than in 2005, which had the coolest summer surface waters; central North Sea DIC concentrations about 10 μmol/kg less than average in 2001.
In summary, three factors regulate the North Sea’s CO2 system and thus reveal points of vulnerability to climate change and more direct anthropogenic influences: local weather conditions (including water temperature in the shallower southern North Sea), circulation patterns, and end-member properties of relevant water masses (Atlantic Ocean, German Bight and Baltic Sea). Thus a positive NAO increases Atlantic Ocean and Baltic Sea inflow, the anti-clockwise circulation, carbon export out of the Norwegian Trench below the surface (limiting out-gassing) and hence the effectiveness of the shelf-sea CO2 ‘pump’ (Salt et al. 2013). If the NAO is positive together with higher SST, a shallower mixed layer favours lower surface pCO2 and higher pH in the northern North Sea. These factors can be considered key to regulation of the North Sea’s response to climate change and more direct anthropogenic influences.
3.6.2 Model-Based Interannual Variations in Nitrogen Fluxes
The North Sea is a net nitrogen sink for the Atlantic Ocean, due to efficient flushing by North Atlantic water with strong nitrogen concentrations and to large rates of benthic denitrification in the southern North Sea (Pätsch and Kühn 2008). This is the case despite large nitrogen inputs from the rivers and atmosphere. There is net production of inorganic nitrogen from organic compounds.
Pätsch and Kühn (2008) investigated nitrogen fluxes in 1995 and 1996 as the NAO shifted from very strong positive conditions in winter 1994/1995 to extreme negative conditions in winter 1995/1996. Due to enhanced ocean circulation on the Northwest European Shelf, the influx of total nitrogen from the North Atlantic was much stronger in 1995 (NAO positive) than in 1996. River input of nitrogen was also larger in 1995 than 1996. While the import of organic nitrogen was similar for both years, the import of inorganic nitrogen was larger in 1995 than in 1996. The ecosystem response was stronger dominance of remineralisation over production of organic nitrogen in 1996 with negative NAO conditions.
According to this simulation, in 1996 (with extreme negative winter NAO) the net-heterotrophic state of the North Sea was stronger than in 1995. As a result, the biologically-driven air-to-sea flux of CO2 was larger in 1995 than in 1996 (Kühn et al. 2010). In other words, in positive NAO years stronger fixation of inorganic nitrogen and inorganic carbon facilitates stronger biological CO2 uptake. This carbon is exported into the adjacent North Atlantic in positive NAO years, as reported above. The balance between respiration and production in regulating DIC and pCO2 conditions thus acts in synergy with the processes discussed in Sect. 3.6.1. At regional and sub-regional scales, modelling studies have investigated the concurrent impacts of eutrophication, increases in atmospheric CO2 and climate change on the Southern Bight of the North Sea (Gypens et al. 2009; Borges and Gypens 2010; Artioli et al. 2014). The studies clearly highlight the complex effects of the individual drivers, as well as the different time scales of impact. Eutrophication, oligotrophication and temperature variability affect the CO2 system at interannual to decadal time scales. Long-term trends of increases in atmospheric CO2 and rising temperature have begun to cause tangible effects (e.g. Artioli et al. 2014) although, to date, these have been much less pronounced than effects at shorter time scales.
3.6.3 Ocean Acidification and Eutrophication
The interplay of the different anthropogenic and climate change processes, as well as their different, obviously overlapping time scales, can be exemplified with respect to the long-term effects of ocean acidification and the shorter-term effects of eutrophication/oligotrophication. Effects of eutrophication are closely related to the trend of ocean acidification, since both affect DIC concentrations and the DIC/AT ratio (AT: total alkalinity) in coastal waters, and thus CO2 uptake capacity. Increased nutrient loads may lead to enhanced respiration of organic matter, which releases DIC and thus lowers pH. On shorter time scales, enhanced respiration overrides ocean acidification, which acts at centennial time scales (e.g. Borges and Gypens 2010; Artioli et al. 2014). (Surface-ocean pH has declined by 0.1 over the industrial era, in the North Sea as well as globally, and a hundred times faster in recent decades than during the previous 55 million years; EEA 2012).
If eutrophication-enhanced respiration of organic matter exhausts available oxygen, respiration then takes place through anaerobic pathways. Denitrification is crucial here; the biogeochemical consequences of depleted oxygen are many. Under eutrophic conditions, release of nitrate (NO3) by enhanced respiration is controlled by the amount of available oxygen. If oxygen is depleted, NO3 is converted to nitrogen gas (N2). Any further input of NO3 stimulates denitrification. The lost NO3 is not available for biological production, thus the system is losing reactive nitrogen (Pätsch and Kühn 2008) as with eutrophication in the Baltic Sea (Vichi et al. 2004). A transition from aerobic to anaerobic processes has consequences for CO2 uptake capacity and pH regulation: denitrification driven by allochthonous NO3 releases alkalinity in parallel with the metabolic DIC, with a DIC/AT ratio of 1:1.
Compared with aerobic respiration, which gives a DIC/AT ratio of −6.6, the release of alkalinity in denitrification increases the CO2 and pH buffer capacity of the waters, in turn buffering ocean acidification. Since denitrification is irreversible, the increased CO2 and pH buffer capacity will persist on time scales relevant for climate change. In other words, if eutrophication yields anaerobic metabolic pathways, this constitutes a negative feedback to climate change, since more CO2 can be absorbed from the atmosphere, which in turn dampens the CO2 greenhouse gas effect.
Other anaerobic pathways such as sulphate or iron reduction give even lower DIC/AT release ratios (Chen and Wang 1999; Thomas et al. 2009); those may be reversible, however. Reduced (nitrogen-) nutrient input (i.e. oligotrophication) thus comes with a negative feedback with regard to ocean acidification: a desirable reduction in NO3 release enhances vulnerability of the coastal ecosystem to ocean acidification, since most organic matter respiration is on or in shallow surface sediments (Thomas et al. 2009; Burt et al. 2013, 2014).
3.6.4 Variability on Longer Time Scales
Climate, CO2 and more direct anthropogenic drivers also determine the variability of carbon fluxes in the North Sea. They can all be indicated as negative or positive feedback mechanisms for CO2 exchange with the atmosphere and thus as feedbacks on climate change. The main direct anthropogenic impact on the carbon cycle, mostly for the southern North Sea, is the input of bio-reactive tracers, namely nutrients, via the atmosphere and rivers. Indirect anthropogenic drivers include acidification due to the ongoing increase in atmospheric pCO2. Climate change processes (rising SST and changes in salinity distribution due to changes in circulation and winds) also induce shifts in the carbonate system and thus changes in carbon fluxes.
The ‘standard’ simulation showed a decrease in CO2 uptake from the atmosphere in the last decade (Fig. 3.22), an increase in SST by 0.027 °C year−1 and a decrease in winter pH by 0.002 year−1 (Lorkowski et al. 2012). Thus climate change alone (i.e. rising sea temperature) thermodynamically raises the pCO2 and reduces CO2 uptake in the North Sea. Furthermore, warming waters cause a lower pH, thus increased surface water acidity (Fig. 3.22).
Increasing atmospheric pCO2 during the ‘standard’ simulation increases the gradient between seawater and atmospheric pCO2 and increases the (net-) CO2 uptake. To investigate this, the standard simulation is compared with a simulation using a repeated 1970 annual cycle of atmospheric pCO2 (Fig. 3.22). 1970 pCO2 (with rising temperature in common) leads to a smaller air-sea flux and less CO2 uptake. pH decreases less than in the standard simulation (Fig. 3.22). Thus the simulations show enhanced CO2 uptake in the North Sea as a consequence of rising atmospheric pCO2, in turn increasing North Sea acidification as a ‘local’ process. This experiment also shows that for today’s carbonate-system-status the increase in atmospheric CO2 has a stronger impact on air-sea flux of CO2 than the reduction in the buffer capacity by the ongoing acidification. This trend in acidification might be overlain on shorter time scales by advective processes (Thomas et al. 2008; Salt et al. 2013) as discussed in Sect. 3.6.1, by eutrophication (Gypens et al. 2009; Borges and Gypens 2010; Artioli et al. 2014) or by variability in biological activity.
Climate change enhances the hydrologic cycle, which means enhanced precipitation and river runoff, which drive changes in surface water salinity. Salinity decrease generally represents a dilution of DIC and AT, with the DIC-effect dominating the AT-effect on pCO 2 and pH (e.g. Thomas et al. 2008). Changes in salinity also alter the equilibrium conditions of the carbonate system (a minor effect): on addition of freshwater, pCO2 decreases and pH increases. In coastal areas, precipitation-evaporation effects are confounded by changes in the mixing ratios of the dominant water masses, i.e., runoff and the oceanic end-member; higher salinity can mean a larger proportion of oceanic water relative to river runoff and vice versa. A sensitivity study, with salinity reduced by 1 (compared with the standard setup) and no biological processes, showed 10 % less outgassing, slightly counteracting the effect of rising temperature. In summary, rising temperature reduces uptake of atmospheric CO2; increasing atmospheric pCO2 or reduced salinity increases net uptake of atmospheric CO2.
Oxygen is of concern because depletion (hypoxia) adversely affects ecosystem functioning and can lead to fish mortality. Air-sea exchange and photosynthesis tend to keep upper waters oxygenated; oxygen concentrations can be strongest in the thermocline associated with a sub-surface chlorophyll maximum (Queste et al. 2013). However, oxygen concentration near the sea bed can be reduced by organic matter respiration below stable stratification, breakdown of detrital organic matter in the sediment and lack of oxygen supply (by advection or vertical mixing). Temperature is also a factor; warmer waters can contain less oxygen but increase metabolic rates. Extra nutrients from rivers and estuaries can increase the amount of respiring organic matter. In the North Sea, most areas are well-oxygenated but some areas are prone to low oxygen concentrations near the bottom—the Oyster Grounds (central North Sea), off the Danish coast (Karlson et al. 2002) and locally near some estuaries, as in the German Bight. Climate change may influence oxygen concentrations through changes in absolute water temperature as well as through changes in temperature gradient, storm intensity and frequency, and related changes in mixing.
Climate change, for example raised water temperatures, is expected to have a negative impact on oxygen concentrations in surface waters, and deepening of the thermocline will reduce the bottom mixed layer and may cause further depletion of oxygen concentrations in deeper layers. However, quantifying temperature effects is difficult, owing to climate-related effects on nutrient inputs to the North Sea as well as on local mixing characteristics and the duration of reduced oxygen concentrations.
3.8 Suspended Particulate Matter and Turbidity
Suspended particulate matter (SPM) is a significant agent of change in morphology, it also transports pollutants, redistributes nutrients and modifies the light climate (Capuzzo et al. 2013), hence its role in modulating primary production. Suspended particulate matter includes plankton.
3.8.1 Sources in the North Sea
The seabed is an important source of SPM in the North Sea. Rivers and cliffs are also important sources in certain areas. Cliff sources are very variable interannually.
Long-term measurements of annual mean amounts of SPM eroded from English cliffs imply the following average rates: Suffolk, 50 kg s−1; Norfolk, 45 kg s−1; and Holderness, 58 kg s−1. SPM loads eroded from cliffs are dependent on whether storm or calm conditions occur (Fig. 3.27). According to Gayer et al. (2006) cliff erosion appears to start when significant wave heights near the coast exceed 2 m. Sediment composition suggests that SPM for alongshore transport off the Belgian and Dutch coasts is largely supplied by sediment transported through Dover Strait from the erosion of the French and British cliff coasts (Irion and Zöllmer 1999; Fettweis and van den Eynde 2003). The transport through Dover Strait largely exceeds the fluvial input by rivers such as the Rhine-Meuse estuary (de Nijs 2012). The annual sediment influx shows large interannual variations which appear to reflect differences in number and duration of storms (van Alphen 1990).
3.8.2 Overall Distribution
Sediment is transported either as bed load (typically for coarse material) or suspended load (SPM). Advances in observational techniques, from water samples to in situ instruments (transmissometers, optical backscatter and Laser In Situ Scattering and Transmissometery—LISST) and reliable use of optical remote sensing (e.g. AVHRR, SeaWiFS, recent MODIS and MERIS) have increased understanding of SPM distribution. Remote sensing techniques provide a synoptic view of the sea surface at fine temporal (daily) and spatial (kilometre) resolution, providing information on variability in SPM distribution. The ability to estimate near-surface SPM loads, relatively continuously and at synoptic scale, has allowed study of surface SPM over seasonal cycles: for example it has been observed that high SPM concentrations evolve during winter, with much lower values in summer (Eleveld et al. 2004, 2006, 2008). However, it should be noted that satellites provide information concerning the sea surface only. When the water column is well mixed SPM can be remotely observed at the sea surface; when it is stratified (as in the Rhine ROFI) SPM can only be observed remotely for high discharge events and close to the mouth (of the Rotterdam Waterway), see Sect. 3.8.5.
Suspension of particles off the bed needs stronger currents (including waves and turbulence) than the limit for the same particles to settle. If the currents allow settling, there is still transport until the particles reach the bed. These biases, tidal straining and current asymmetries cause net transport of SPM. Moored instruments have allowed better understanding of tidal, spring-neap and vertical variability (Jones et al. 1996).
The waters exiting the rivers Rhine, Tees, Humber and Wash are deflected to the right under the Coriolis influence, forming classic river plumes of which the Rhine ROFI dominates the southernmost North Sea. A zone of high turbidity extends along the Belgian and Dutch coasts, primarily controlled by the Rhine ROFI which transports SPM northwards along the Dutch coast (Dronkers et al. 1990; Visser et al. 1991; de Kok 1992; de Ruijter et al. 1992; McCandliss et al. 2002). The Flemish Banks turbidity maximum off Belgium (Fig. 3.28) is present throughout the year but is much reduced in summer; studies have disagreed about its cause. Off the Dutch coast, Visser et al. (1991) and Suijlen and Duin (2001, 2002) found a local SPM minimum about 30 km offshore. In the German Bight, SPM from the Rhine ROFI appears to merge with SPM from the Weser and Elbe, before arriving in the Skagerrak (Simpson et al. 1993).
Plume currents can be enhanced by the thermohaline circulation (jets associated with tidal mixing fronts; Hill et al. 1993). The East-Anglia Plume eventually joins the northward flow from Dover Strait and the Rhine ROFI (Prandle 1978b; Prandle et al. 1993). The plume carries an estimated annual SPM flux of 6.6 million tonnes, from English rivers and cliffs (Sündermann 1993), eastwards across the southern North Sea (Howarth et al. 1993). Both the East-Anglia Plume and the seasonal thermocline have a large impact on the transport of SPM across the southern North Sea. Holt and James (1999) found that deposition typically occurs along the 40 m depth contour. Their results are consistent with those of Eisma (1981) and Eisma and Kalf (1987), who found that the main areas of fine sediment accumulation are the Oyster Grounds and the strip along the German Bight (see also Fig. 3.26).
3.8.3 Tidal Influence
Deposition and erosion of sediment are related to critical values of the bed shear stress. Fine sediments are deposited when the bed shear stress is less than critical (0.1–0.2 N m−2) and are typically eroded if the stress exceeds 0.4–0.5 N m−2 (Puls and Sündermann 1990; Holt and James 1999; Souza et al. 2007). In principle the tidal bed stress can be used to characterise regions where the tides can resuspend bed material. For example, in the Belgium coastal zone, the water column is always well mixed by tidal currents which also cause the SPM maxima in this zone (Lacroix et al. 2004).
3.8.4 Wave Effects
Data from the northern Rhine ROFI after storms show a sudden increase in SPM over the entire coastal zone, suggesting local resuspension of sediment (Suijlen and Duin 2001, 2002) and an important effect of waves. Waves are strongly seasonal in the North Sea. Significant wave heights (Hs) in the winter half year are usually much higher than in the summer half year over the entire North Sea. For example, Dobrynin et al. (2010) showed that during 2002–2003 seasonally-averaged Hs was up to 1.5 m higher for the winter season.
The largest values of Hs are usually found in the open North Sea (e.g. Weisse and Günther 2007; Dobrynin et al. 2010), owing to a combination of topography effects with predominant wind direction and storms coming from the North Atlantic. In shallow regions, effects such as bottom boundary dissipation of waves can be significant even during the (calmer) summer, although they are usually more important during winter. For example, in winter over Dogger Bank, Dobrynin et al. (2010) found enhanced combined wave-current stress resulting in very high SPM concentrations (>50 mg l−1).
3.8.5 Impact of Stratification
Stratification (Sect. 3.2) has an impact on the vertical distribution of SPM through reduced turbulence at the pycnocline. The effect of thermal stratification is particularly noticeable in the East-Anglia Plume around the stratified areas close to the Frisian Front. Moreover, at the 55° 30′N station shown in Fig. 3.31, as thermal stratification develops the SPM concentrates in the lower 40 m of the water column, with values up to 15 g m−3, but decreases to zero at the surface. Haline stratification appears to be significant especially within the Rhine ROFI and other adjacent ROFI areas such as the German Bight. The Rhine ROFI switches between well-mixed and stratified within a tidal cycle and through the spring-neap cycle (de Boer et al. 2006); haline stratification can develop at neap tides throughout the year and thermal stratification can be important in summer. As found in field studies of the Rotterdam Waterway, with stratification and reduced turbulence, any SPM advected over the salt wedge settles (de Nijs et al. 2010). Observations (e.g. Joordens et al. 2001) and numerical simulations (Fig. 3.31) show how SPM is trapped beneath the pycnocline when stratified. When the water column is mixed, turbulence and SPM can reach the surface; when a pycnocline develops, it inhibits turbulence, preventing upward flux of SPM and turbulence to the surface layer. Although de Boer et al. (2006) found differences to Heaps’ model for the Rhine ROFI, cross-shore flow associated with estuarine-type circulation (Heaps 1972) tends to be offshore near the surface and onshore near the bed, giving a bias to onshore transport of settling SPM.
3.8.6 Seasonal Variability
Pleskachevsky et al. (2005) found that most SPM transport occurs in winter when cliff erosion along the English coasts is greatest. Numerical simulations of Holt and James (1999) highlighted seasonal variability of SPM. They found that SPM is only measured in time series of the water column in a series of discrete events associated with stronger winds, which resuspend bed material and mix the water column. Likewise, Souza et al. (2007) found their modelled water column to be well-mixed with very similar surface and bed SPM distributions in the East-Anglia Plume in February. In contrast, during summer the water column is stratified, almost all SPM settles out and an increase in net deposition correlates with decreased wind stress; moreover, less SPM is supplied by erosion of Holderness cliffs.
Satellite images of surface SPM show significant annual and seasonal variability (Eleveld et al. 2004, 2006, 2008; Pietrzak et al. 2011; see Fig. 3.28). Large values of surface SPM were observed in winter in Southern Bight coastal waters, especially in the Rhine ROFI, the East-Anglia Plume and Frisian Front. Summer minima occur throughout the southern North Sea, with low SPM values from April to August; in August almost all surface SPM in the Dutch coastal zone has gone. Pietrzak et al. (2011) showed the influence of the Rhine ROFI, East-Anglia Plume and fronts on the intra-annual distribution of SPM.
De Nijs (2012) found siltation rates in the Dutch waters to vary over the year and correlate with variations in sediment supply; the availability of sediment at Dover Strait and the river boundaries is typically greater between late autumn and spring. Verlaan and Spanhoff (2000) found massive siltation events, caused by storms, to occur near the mouth of the Rotterdam Waterway; a few such events determine annual siltation rates.
3.8.7 Interannual and Long-Term Variability
Strong year-to-year variation in sediment supply and required dredging near the mouth of the Rotterdam Waterway (de Nijs 2012) highlight the importance of interannual meteorological forcing.
Most long-term records of SPM are from satellites or sea-surface water samples collected along the Dutch coast (Suijlen and Duin 2001, 2002). However, near surface values vary in relation to stratification, making it difficult to use these data to infer trends or study impacts of climate change. Nevertheless, recent work continuing that of Pietrzak et al. (2011) indicates that interannual variability in wind stress, river discharge and heating due to variations in the NAO may have a pronounced impact on SPM distribution in the North Sea. Fettweis et al. (2012) classified surface SPM distributions according to 11 weather types, emphasising dependence in different locations on different hydrodynamic and wave conditions. Thus Southern Bight SPM is strongly influenced by advection, while exposure to waves favours resuspension in the central North Sea and German Bight and there is some overall positive correlation with the NAO.
Many coasts around the southern North Sea, notably the Dutch coast, are highly engineered, making it difficult to assess climate change impacts on sediment dynamics. However, supply of SPM for transport along the Belgian and Dutch coast, from French and English cliff coasts (Sect. 3.8.1), is determined by prevailing meteorological conditions and associated periods of large waves. This explains (seasonal and) interannual variations in the transport of SPM into the southern North Sea through Dover Strait. Climate-change effects on river discharge, storm tracks and associated winds and waves (intensity) are likely to affect the supply and distribution of SPM in the coastal zone and thus sediment distribution within the North Sea.
3.9 Coastal Erosion, Sedimentation and Morphology
3.9.1 Historical Perspective
Coastal erosion is a key element of coastal behaviour and a useful—though imperfect—indicator of climate change. It is the active removal of sediment from various environments that marks the transition from sea to land, generally forcing the coastline landward. For cliffs and bluff coasts, erosion is irreversible. For sandy or muddy coasts, accretion and erosion may alternate. Coastal erosion may result in loss of land, destruction of sea defences and flooding. It has been measured for many centuries.
Coastal erosion takes place on different time scales. Long-term changes are commonly driven by relative sea-level rise and the associated creation of ‘accommodation space’ available for potential sediment accumulation. Shorter-term changes show regular patterns and irregular, partly short-lived effects of waves, tides, storm surges, slope processes and local or regional sediment dynamics. Long-term measurements enable distinction between the effects of these short- and long-term drivers, both natural and human-induced.
Systematic observation of coastal erosion, by periodically mapping or measuring the North Sea water lines and frontal dunes, started in earnest during the 17th century.
Annual coastline monitoring started in 1843, in the western Netherlands. Between 1840 and 1857, 124 oak poles were driven about 3 m into the beach sand, initiating a network of beach poles with 1000-m spacing that spans the entire Dutch coast and still operates today. For each pole, cross-shore distances to the low- and high-water lines and dune foot have been logged annually ever since. Starting in 1965, some 1450 transects at 250-m spacing have been profiled near-annually from at least the frontal dune to about 1000 m seaward of the dune foot. Below low water, data come primarily from single-beam echo sounding. Above low-water, photogrammetry was used before 1996 and laser altimetry since.
Monitoring records for other countries bordering the North Sea are shorter, commonly limited to local or regional studies, and used mainly to assess the need for coastal maintenance and protection measures. In Denmark, soundings and levelling for coastal monitoring started in 1874 in the Thyborøn area along lines spaced 600–1000 m apart (Thyme 1990). Since 1957, the entire west coast of Jutland has been monitored for cross-shore coastline migration at least several times per decade, at similar spacing (Kystdirektoratet 2008). In Belgium, coastline changes have been monitored nationally since the late 1970s, using echo-sounding, topographic surveying, photogrammetry and laser altimetry (since the late 1990s). In the United Kingdom, one of the longest records concerns cliff behaviour at Holderness, Yorkshire, from 1951 to the present day (Brown 2008) using erosion posts, on average 500 m apart along the cliff edge. More extensive regional surveys have been conducted since 1992, when the Environment Agency started annual monitoring of winter and summer cliff and beach profile change at 1-km intervals between the Humber and Thames estuaries.
Where systematic monitoring networks to quantify behaviour of the entire coastline do not exist, useful information on local or regional coastal erosion is provided by analyses of historical to recent maps, aerial photographs (extensively used in mapping since the Second World War) and satellite images. In Germany, series of maps were analysed by Mroczek (1980) to deduce rates of erosion or accretion for the North Sea coast over more than one hundred years. In Britain, maps from the late 16th century provide the oldest former positions of coastal bluffs and cliffs.
Detail on evidence for (possibly varying) coastal erosion rates is given in E-Supplement Sect. S3.4.
3.9.2 Understanding Coastal Erosion: State, Variability and Trends
Erosion is widespread along the central and southern parts of the North Sea coastline, with about 25 % of the Danish-to-Scottish coastline eroding. Farther north, erosion is rare. Erosion percentages for the North Sea part of the coastline of countries bordering the North Sea vary widely. On a country-by-country basis, the percentage eroding is as follows: UK (22 %), France (76 %), Belgium (40 %), Netherlands (30 %), Germany (14 %), Denmark, north to Skagen (57 %), Sweden, south to Marstrand (0 %), and Norway (0 %). The data were calculated from EUrosion/EMODnet data (http://onegeology-europe.brgm.fr/geoportal/viewer.jsp) and the values are approximate. They exclude areas with no data (e.g. back-barrier shorelines are excluded for some countries like the Netherlands) and the Skagerrak is included.
In Norway, the coastline is mostly rocky. Combined with limited and only recent relative sea-level rise, there is no significant coastline change. In Sweden, Rydell et al. (2004) showed that erosion is limited to very few small areas with pocket beaches and bluffs in Västra Götaland County.
In Denmark, coastal erosion affects most of the North Sea shoreline, as summarised by Sørensen (2013). Much of the northern Jutland headland coast has eroded about 2–4 m year−1 over the last 20 years, with maximum erosion in central bays and maximum deposition on the north-western side of eroding headlands (Christiansen and Bowman 1990). The central west coast, which is dominated by barrier beaches and bounded by glacial bluffs, has been the most vulnerable, with natural erosion rates of 2–8 m year−1. Only the southernmost barrier beaches around Vejers, between Nymindegab and Blaavands Huk, are accreting naturally (Aagaard 2011). Horns Rev, which marks the southern end of the barrier-beach coast, acts as a natural groyne, protecting the island coast farther south from wave attack (Meesenburg 1996). This coast is in overall sediment balance, with parts of the barrier islands growing seaward, supplied by sediment from north and south (Sørensen 2013). Here, the southern ends of the islands are the most vulnerable to erosion.
Along the German North Sea coast, erosion has had the most impact on the barrier-island coasts in the north and west, away from the central estuaries. At present, erosion occurs only along the 100 km of the coastal length that is not protected by dikes. Rates vary between <1 and 8 m year−1. Most erosion has resulted from storm surges (Kelletat 1992) which have also reshaped the overall barrier and back-barrier morphology. Eroded sediments are partially regained during subsequent fair-weather periods, after temporary storage in the nearshore zone, and may be redistributed on neighbouring coasts. Through spit progradation, Sylt (one of the North Frisian Islands) has been growing northwards and southwards.
The west-Frisian islands fringing the Wadden Sea show large spatial and temporal differences in coastal erosion and accretion, similar to those observed along the German coast; local processes clearly overprint the long-term regional trend in coastal behaviour (Oost et al. 2012). Island growth and dune development reflect sand supply from the ebb deltas and shoreface, with episodes of rapid accretion linked to bars merging with the islands. Coastal erosion and dune scarping are associated with exposure to high-energy waves, as ebb-tidal deltas shift, or to strong currents, as marginal flood channels are forced toward the coast. The adjacent barrier islands of Ameland and Schiermonnikoog showed opposite behaviour during the last 150 years: Ameland receded and extended only slightly in a longshore direction, whereas Schiermonnikoog shifted seawards and accreted eastwards by more than 6 km (Oost 1995).
In Belgium, beaches and dunes showed long-term erosion until human intervention intensified (Charlier 2013). Currently, the dune foot is growing seaward in most places, aided by sand nourishments and hard coastal-protection structures (De Wolf 2002). Beaches behave more heterogeneously. West of Oostende, most are stable or show slight accretion. East of Oostende, erosive, stable and accretionary stretches alternate (De Wolf 2002).
The North Sea coasts of England and Scotland are dominated by cliffs and bluffs, with shorter stretches of pebble beaches, several estuaries, dwindling tidal flats and few dunes. The English areas at risk of erosion are mostly those where the North Sea is fringed by beaches or bluffs (Blott et al. 2013). Coastal erosion percentages are estimated as 27 % for north-eastern England, 56 % for Yorkshire and Humber, 9 % for Lincolnshire, 13 % for Norfolk to Essex and 31 % for south-eastern England (EUROSION 2004). Intertidal zones of saltmarsh and mudflat have been disappearing at a typical loss rate of 1–1.5 % per year over a period of more than 50 years. This loss is linked, though not solely attributable, to coastal squeeze. The lateral recession rates of coastal cliffs and bluffs vary with rock type (French 2001). In Scotland, most cliffs consist of resistant rock; significant erosion is limited to beaches.
Frontal dunes along beaches show average erosion rates of 1 m year−1 (Pye et al. 2007). Bluffs are particularly common in east Yorkshire, Humberside and East Anglia; they show very variable spatial and temporal erosion patterns, a function of complex glacial geology. Changes are episodic with no discernible trend. At Dunwich, for example, phases of accelerated coastal retreat (e.g. 1863–1880, 2.57 m year−1; 1903–1919, 3.53 m year−1) have alternated with periods of relative stability (e.g. 1826–1863, 0.06 m year−1; 1882/3–1903, 0.08 m year−1; Carr 1979). Landslides, protection by shingle beaches, cliff or bluff material, pore-water pressure and hydrodynamics all play a role (Brooks and Spencer 2010). These factors interact with longer-term drivers, including shifts in dominant weather patterns and changes in the rate of relative sea-level rise. As receding bluffs and cliffs expose new material, they may steepen, flatten or even disappear (Brooks and Spencer 2010). Feedbacks also operate. Large landslides provide the steep coast with temporary protection from the sea. Longshore drift of eroded sand and gravel leads to net accumulation at nearby beaches.
3.9.3 Offshore Morphology
220.127.116.11 History and Evidence
Long bathymetric time series (many decades to centuries) are rare and generally localised in the context of port approaches and dredging (for navigation) or aggregate extraction. 200-year records of an ebb-tidal delta in the Deben estuary, eastern England, and a 180-year record in the outer Thames estuary, have been analysed (Burningham and French 2006, 2011). Horrillo-Caraballo and Reeve (2008) interpreted sandbank configurations off Great Yarmouth using historic charts over a 150-year period. There are no field studies relating such series to climate-dependent factors (e.g. wave heights).
Van der Molen et al. (2004) numerically modelled millennial-scale morphodynamics of an idealised, semi-enclosed, energetic tidal shelf sea with dimensions and tidal characteristics resembling the Southern Bight of the North Sea. Several local process studies have related morphological change to wider-scale sediment transport and to tides, wind-driven flows and waves, for example eight years of current profile monitoring at Marsdiep inlet, Netherlands (Buijsman and Ridderinkhof 2008a, b) and a 1977–2003 sidescan sonar and multibeam backscatter record in the German Bight (Diesing et al. 2006).
The English Channel is shallow and tidally dominated with waves mainly from the west-southwest during storms; it has limited sediment sources and is marked by extensive reworking of a relatively thin sediment cover (Paphitis et al. 2010). The central Channel seabed is covered by coarse-grained material. Wide areas are occupied by sand-sized sediments with various bedforms: ripples, sandwaves, longitudinal bedforms and sandbanks. Fine-grained sediments are confined to coastal embayments, rias, estuaries and open-coast intertidal flats (Paphitis et al. 2010).
Sandbanks occur widely in the North Sea (Knaapen 2009); they dominate the southern North Sea except near the mouth of the Rhine. In some areas, bank crests comprise fine sand whereas troughs comprise material too coarse to be moved by local tidal currents alone: such as shoreface-connected ridges off the East Frisian barrier-island coast (Son et al. 2012) and banks off Thorsminde (west Denmark; Anthony and Leth 2002). Sand-waves also occur widely, most commonly around sand banks. There is an extensive field of large sand-waves off the Netherlands.
Van der Molen (2002) modelled the influence of tides, wind and waves on net sand transport in the present southern North Sea. Results showed that wind-driven flow and waves only contribute significantly to net sand transport by tides when acting together where tidal currents are small. However, various combinations of forcing dominate net sand transport in different regions of the southern North Sea. Tides dominate in the southern, middle and north-western parts of the Southern Bight and in the region of The Wash. Tides, wind-driven flow and waves are all important in the north-eastern part of the Southern Bight. Wind-driven flow and waves dominate north of the Frisian Islands, in the German Bight and on Dogger Bank. In the Channel, Reynaud et al. (2003) inferred predominant control by tidal dynamics, with mobile sediments in the central and eastern Channel, and longer-term influence of sea-level rise.
Contrasts in sand-wave character are related to differences in the relative importance of suspended load transport and of tidal currents and waves near the bed (Van Dijk and Kleinhans 2005; stronger tidal residual currents tend to cause faster sand-wave migration, waves tend to flatten sand-wave crests). In Marsdiep inlet, sand-waves typically migrate in the flood direction and not necessarily with predicted bedload and suspended load transport; Buijsman and Ridderinkhof (2008b) hypothesised that advection of suspended sand and lag effects may govern sand-wave migration.
Tidal sandbank height (60–90 % of water depth) and shape are controlled by the mode of sediment transport and hydrodynamic conditions (Roos et al. 2004). Bedload transport under symmetrical tidal conditions leads to high spiky banks. Profiles are lowered and smoothed by relaxation of suspended sediment, wind-wave stirring and tidal asymmetry; this last factor also causes profiles to be asymmetric. Thus strong tidal currents and their residuals, with enhancement of bed stress by waves, are the main hydrodynamic agents for (long-term changes in) sandbank morphology. Examples are off Great Yarmouth (Horrillo-Caraballo and Reeve 2008), Westhinder sandbank (Deleu et al. 2004) and Kwinte Bank (Giardino et al. 2010) off Belgium, the outer troughs of shoreface-connected ridges off the East Frisian barrier-island coast (Son et al. 2012). Near the mouth of the river Rhine, freshwater outflow affects the direction of tidal ellipses and residual flow, suppressing the formation of open ridges (Knaapen 2009). Other studies of local system dynamics (E-Supplement Sect. S3.4.1) emphasise the role of tidal asymmetries with wave-enhanced transport, and show that shoals evolve, in some cases cyclically.
18.104.22.168 Evolution in Relation to Climate
Idealised simulations (Van der Molen et al. 2004) suggest that a basin resembling the Southern Bight may be expected to export sediment, deepen and expand by accumulation of eroded sediment in the deeper waters to the north, owing to asymmetry in the amphidromic tidal velocities. Sea-level changes affect tidal wavelength, hence this sediment distribution, and deepening reduces evolution rate. However, changes in sea level and tides are small relative to average water depths on time scales of decades to a century (Sect. 3.4).
In practice, repeat surveys tend to show stability of larger (kilometre-scale) features or patterns over periods of nine months (off Thorsminde on the Danish west coast; Anthony and Leth 2002), some years (e.g. Son et al. 2012) or even decades (Diesing et al. 2006). There is no clear evidence of any regionally coherent response to large-scale historical forcing such as sea-level rise. However, harbour or dike works and large-scale dredging induce wider change, such as to Texel inlet (Elias et al. 2006) and Kwinte Bank (Degrendele et al. 2010). Detecting abiotically-induced climate-related changes in morphological evolution could only be expected where the influence of storms and waves is significant in net sediment transport, namely in the north-east of the Southern Bight (notably the German Bight) and on Dogger Bank (van der Molen 2002).
Climate-related change is more likely where benthic organisms play an important role in the development and dynamics of bedforms, as so-called ‘ecosystem engineers’. Although no field studies exist that link changes in benthic habitats and indicator species to bedform development and mobility, an increasing number of modelling studies suggest that such a link exists. Borsje et al. (2009) showed that including the abundance of three dominant eco-engineers (Lanice conchilega, Tellina fabula, Echinocardium cordatum) gives a more accurate prediction of sand-wave occurrence on the Dutch continental shelf than modelling without biology. Very little work has been done on eco-engineers, climate change and seabed morphology; this is an important subject for future research.
3.10 Sea Ice
In the North Sea, ice does not form in every winter season. Ice formation generally depends on the weather regimes prevailing over Europe and their temporal stability, and on the morphological characteristics of the North Sea.
Typically, in autumn and winter, prevailing westerly weather regimes bring relatively mild air masses from the Atlantic Ocean into the North Sea area. These weather regimes also cause inflows of relatively warm, high-salinity Atlantic water into the North Sea, preventing or delaying the seasonal cooling of North Sea water and hence ice formation. By contrast, easterly weather regimes cause rapid cooling of the water.
Large stationary high-pressure zones over northern Scandinavia and the European polar seas, and stable anti-cyclonic regimes over eastern Europe, are particularly effective in this respect. The extent and duration of ice cover in the North Sea are governed by the number, intensity, and length of freezing periods and by the timing of their occurrence. The North Sea comprises open sea areas, Wadden Sea areas, and tributaries; these play an important role in the development of ice conditions. Peer-reviewed literature on long-term changes in sea-ice conditions in the North Sea is sparse.
In the past 52 years (1961–2012), 26 (50 %) winter seasons on the North Sea coast were very weak or weak, 18 (35 %) were moderate and eight (15 %) were strong, very strong or extremely strong. In comparison with the 116-year time span shown in Fig. 3.38, the occurrence of extremely strong and very strong ice winters has decreased while there has been a simultaneous increase in winters with low ice-cover conditions.
Ice formation on the tidal flats of the Wadden Sea normally begins in mid- to late January (BSH 2008). Ice-cover duration varies widely, in space and time. In moderate ice winters, ice occurs on 10–20 days in the sheltered inner coastal waters of the North Frisian Wadden Sea, and on up to 10 days in open navigation channels, which is comparable to ice formation in the East Frisian Wadden Sea.
In strong and very strong ice winters, ice cover in the sheltered navigation channels of the North Frisian Wadden Sea lasts from 55 to 75 days on average, and in open navigation channels from 45 to 55 days, similar to the East Frisian Wadden Sea. In near-shore tidal flats, the most common type of ice is fast ice or rafted/ridged ice; in outer tidal flats, ice floes and slash or shuga predominate, kept in motion by wind and tidal forces.
In the open part of the German Bight, the remaining heat content of North Sea water in early winter is so large that ice rarely forms. Shuga and ice floes occurred in offshore waters to the west and northwest of Helgoland in about 8 % of all winter seasons, being last observed in late January 1970 and before that in February and March 1963; they do not originate near Helgoland but are carried there from the coastal area by tidal currents and by easterly winds persisting for long periods. In the offshore waters of the German Bight off the North and East Frisian islands, ice forms only in very strong or extremely strong ice winters.
Level ice thicknesses reach 10–15 cm in most winters, 15–30 cm in strong ice winters, and as much as 30–50 cm in very strong ice winters. The higher ice thickness categories are most likely to occur in February and early March. Ice thickness data refer to level ice, which occurs primarily in the form of ice cakes and small- to medium-sized floes. A typical phenomenon caused by tidal influences in areas of the Wadden Sea is rafting and ridging of initially level ice, which may cause ice walls several metres high. In such areas, floebits of about 1–3 m thickness comprising frozen ice cakes and small to medium-sized floes of coarse compacted ice are also likely to occur, especially in winters with long freezing periods.
Thawing causes rapid retreat of ice in south-eastern North Sea coastal areas. Westerly winds push warmer, high-salinity North Sea water toward the coast, accelerating the melting process started by meteorological influences. Ice in south-eastern North Sea coastal waters normally melts completely by the end of February. In very strong to extremely strong ice winters during which maximum ice formation is not reached until mid-February, the last remnants of ice may melt as late as the end of March.
The frequency of ice occurrence has decreased since 1961, analogous to the development of ice conditions in the western Baltic Sea (Schmelzer and Holfort 2012). Although there is presently a trend toward milder ice winters, strong to extremely strong ice winters in the area of the western Baltic Sea and North Sea are still likely in the future (Vavrus et al. 2006; Kodra et al. 2011).
3.11 Wadden Sea
The Wadden Sea, fringing the coast of the south-eastern North Sea, is the largest coherent tidal flat system of the temperate world. Its outstanding geological and biological importance makes it a world nature heritage site (Reise et al. 2010). The Wadden Sea is the result of the intricate interplay of tidal forces, sediment supply and moderate sea-level rise (Reise 2013). Since its beginning some 8000 years ago, the Wadden Sea has been increasingly affected by humans (Lotze et al. 2005). Around 1000 years ago, coastal people started to transform the coast, ultimately embanking about half of the original Wadden Sea (Reise 2005).
Owing to this coastal squeeze, hydrodynamic forces increased leading to a loss of fine material (Flemming and Nyandwi 1994). Accelerated sea-level rise may also have contributed to increased hydrodynamics and a loss of mudflats (Dolch and Hass 2008). Also, the estuaries intersecting the Wadden Sea have dramatically changed due to diking and dredging (Reise 2005).
The Wadden Sea receives large amounts of riverine fresh water, either directly as (branches of) rivers debouch into it (e.g. IJsselmeer, Ems, Weser, Elbe, Eider, Varde A) or indirectly (especially from the Rhine and Maas). River water from the Rhine and Maas is transported to the Wadden Sea along with residual North Sea currents flowing predominantly anti-clockwise around the North Sea. Thus, nutrient and contaminant loads can be imported into the Wadden Sea and large amounts may be present. Given this background, three interacting aspects of environmental quality are addressed in this section: SPM, nutrients and contaminants.
3.11.2 Suspended Matter Dynamics
The Wadden Sea is characterised by relatively high SPM concentrations compared to the North Sea. To maintain these gradients, some type of accumulation process must be active (Postma 1954). Earlier explanations focusing on Wadden Sea processes included the biases described at the end of Sect. 3.8 (see also Postma 1954; van Straaten and Kuenen 1957; Burchard et al. 2008). Biological interactions such as filter-feeding or microphytobenthic microfilms (Staats et al. 2001; Andersen et al. 2010) increase the retention efficiency of the Wadden Sea. Thus the Wadden Sea proper can be regarded as an effective ‘keeper’ of SPM (van Beusekom et al. 2012), at least on a seasonal scale.
The North Sea, on the other hand, can be seen as the ‘pusher’ (van Beusekom et al. 2012) of suspended organic matter into the Wadden Sea via estuarine-type circulation (see Sect. 3.8.5; Postma 1981). The Wadden Sea is heterotrophic: locally-produced and imported organic matter is remineralised (Postma 1984) as supported by carbon budgets (van Beusekom et al. 1999). The ecological importance of organic matter import from the North Sea for the productivity of the Wadden Sea was discussed by Verwey (1952).
Changes have occurred in SPM dynamics in the Wadden Sea and its intersecting estuaries: De Jonge et al. (2014) showed that dredging of the Ems estuary caused a regime shift in hydrodynamics leading to hyperturbid conditions. Dredging of the Rhine estuary and subsequent dumping along the Dutch coast ultimately led to increased SPM concentrations in the western Dutch Wadden Sea (de Jonge and de Jong 2002). SPM dynamics were related to riverine runoff and de Jonge and de Jong (2002) suggested that increased runoff due to global climate change will enhance SPM dynamics in the Dutch Wadden Sea.
3.11.3 Changes in Nutrient and Organic Matter Dynamics
Riverine nutrient discharges are the main drivers of Wadden Sea eutrophication. Historic nutrient concentrations for the Rhine, compiled by van Bennekom and Wetsteijn (1990), show a clear increase in concentrations after the Second World War and a decrease since the mid-1980s due to the implementation of wastewater treatment and better agricultural practice (e.g. de Jong 2000) (see also Chaps. 11 and 13).
Increased nutrient availability has led to increased primary production and increased turnover of organic matter and nutrients. Changes in primary production are demonstrated by the Marsdiep time series (western Dutch Wadden Sea) initiated by Cadée (Cadée and Hegeman 2002), showing an increase in primary production until the 1990s and then a decrease. Present levels are about 120–200 g C m−2 year−1 in the western and northern Wadden Sea (Loebl et al. 2007; Philippart et al. 2007). Most monitoring programmes started in the 1980s or later, thus only documenting changes after the maximum eutrophication had occurred. Summer chlorophyll levels are a good proxy for Wadden-Sea-wide eutrophication, correlating with riverine nutrient discharges (van Beusekom et al. 2009a) and demonstrating a gradual decrease in eutrophication.
Since the mid-1980s, several monitoring programmes have covered the entire Wadden Sea. These data document large regional differences in eutrophication. In general, levels are higher in the southern Wadden Sea than in the northern Wadden Sea, but some overlap exists (van Beusekom et al. 2009a). These differences are also captured by summer chlorophyll levels (average May–September) and autumn ammonium and nitrite levels (average September–November) suggesting that both parameters are useful proxies for describing Wadden Sea eutrophication. At a few stations only, dissolved nutrients are monitored and show the same spatial differences: high values in the Dutch Wadden Sea and low values in the northern Wadden Sea (Sylt; van Beusekom and de Jonge 2012).
The factors responsible for the regional differences are not yet known. Van Beusekom et al. (2012) suggested two possibilities: differences in the amount of imported organic matter or the size of the receiving tidal basins. Evidence was presented to show that the size of the receiving tidal basin, in particular the distance between barrier islands and the mainland, may lead to a dilution of the import signal, whereas in narrow basins with a small distance between the islands the imported organic matter is concentrated.
The general decrease in eutrophication has resulted in lower chlorophyll levels and, in the northern Wadden Sea, to a decrease in green macroalgae (van Beusekom et al. 2009a) and has possibly contributed to an increase in seagrass (Reise and Kohlus 2008).
Within the framework of the Wadden Sea Quality Status Reports, Bakker et al. (2009) presented an overview of hazardous substances. Dissolved heavy metal concentrations in the Wadden Sea (mercury, cadmium, copper, zinc and lead) are not monitored and the focus is on sediment contamination. The main reduction in riverine heavy metal loads—the principal source for the Wadden Sea—was during the 1980s and 1990s. Notable decreases were observed in the river Elbe after the end of the German Democratic Republic. Since then heavy metal concentrations have remained similar or decreased slightly. Sediment concentrations of mercury and lead still pose a risk in a majority of Wadden Sea sub-regions.
Xenobiotic compounds in the Wadden Sea are a major concern because most are persistent, bio-accumulative and toxic (Bakker et al. 2009). In general, riverine inputs and environmental concentrations have decreased. For instance, a ban on tributyltin (TBT) has proved very successful, but effects can still be observed, for example on snails. Polychlorinated biphenyls (PCBs) are still widespread but concentrations are decreasing. Levels of lindane and DDT are also decreasing, but occasional erosion of old deposits leads to fluctuating concentrations in the Wadden Sea. Of particular future concern (as possible hormone disruptors) are newly developed xenobiotics, which include flame retardants, perfluorinated sulfonates and phthalates. These substances are not regularly monitored and little is known about their ecological effects.
3.11.5 Relevance of Climate Change
Climate change is expected to affect the sediment composition of the Wadden Sea: directly by sea-level rise (e.g. Dolch and Hass 2008) and indirectly by altered wind regimes (de Jonge and van Beusekom 1995). It is an open question whether sediment import into the Wadden Sea can compensate for increased sea-level rise or whether the Wadden Sea will ultimately drown (CPSL 2010). Nevertheless, sea-level rise will necessitate new strategies for coastal protection with yet unknown consequences for dikes, hydrodynamics and morphology (Reise 2013).
Climate change will drive complex and interacting effects on Wadden Sea water quality. One aspect of climate change is weather extremes. Extreme high river flows may transport large amounts of toxic substances to the Wadden Sea affecting filter feeders and fish (Einsporn et al. 2005). Likewise, increased nutrient fluxes during high river flows or extreme wet years lead to greater organic matter turnover (Fig. 3.40). De Jonge and de Jong (2002) suggested that increased river runoff will increase SPM levels in the Wadden Sea.
Higher temperatures will have complex and interacting effects on organic matter turnover in the Wadden Sea (Reise and van Beusekom 2008). For instance, higher temperatures enhance zooplankton dynamics (Martens and van Beusekom 2008) but suppress spring phytoplankton blooms through enhanced grazing (van Beusekom et al. 2009a). In general, higher temperatures are expected to enhance organic matter turnover with as yet unknown effects on the Wadden Sea food web.
In general, temperature variability on all time scales to multi-decadal tends to obscure longer-term trends. This variability is probably a greater source of uncertainty than lack of surface temperature data. Nevertheless, evidence of exceptional warming, especially since the 1980s, is very strong. In adjacent Atlantic waters (Faroe-Shetland Channel) and the northern North Sea, there has been a positive temperature anomaly of more than one standard deviation in most years since the mid-1990s; more than two standard deviations in a majority of years between 2002 and 2010 (Hughes et al. 2011). The temperature rise is not uniform in space, with largest rises (exceeding 1 °C since the end of the 19th century) in the south-east. Models provide some evidence of increasing duration of summer stratification away from estuarine outflow regions.
Shorter-term variations in salinity exceed any climate-related changes.
The Atlantic Meridional Overturning Circulation is very variable with no clear trend to date. However, changes in northern inflow to the North Sea correlate with changes in the NAO. Otherwise currents are highly variable on various time scales (tides, winds, seasonal density), and one storm can be significant compared to a year’s integrated transport. The evidence is strong, coming from models as much as from measurements; however, observations are sparse at any one time and brief relative to climate-change time scales.
Over the past 100 to 120 years, absolute mean sea level in the North Sea rose by about 1.6 mm year−1, comparable to the rates of global mean sea-level rise. Extreme sea levels have increased over the past 100–150 years in the North Sea, mainly due to a rise in mean sea level. Evidence for changes in sea level is very strong. Waves and storm surges (resulting from the weather) show pronounced variation on time scales of years and decades but no substantial long-term trend.
Carbon dioxide, pH and nutrients basin-wide are influenced by the circulation pattern, especially inflow from the Atlantic, local weather conditions (correlating with the NAO) and properties of component water masses. However, measurements on long time scales relating to climate change are only local, made close to the coast and affected by strong offshore gradients. There is net CO2 uptake from the atmosphere, attributable to areas stratified in summer. The North Sea is a net nitrogen sink for the Atlantic. Model results suggest a long-term decrease in pH with relatively large variability due to shorter-term changes of circulation.
Higher temperatures tend to reduce oxygen concentrations near the surface; at depth, concentrations depend on vertical mixing for which variable weather is an important driver where depletion is a concern.
Suspended matter and turbidity are very variable, influenced by river inputs, seasons, tidal resuspension and advection (spring-neap modulation), waves and stratification. The North Sea is generally turbid in the unstratified south with transport to the north-east.
Coastal erosion is extensive but irregular; where it occurs, long-term rates are often 1 m year−1 or more. Some sectors accrete. Climate-related change in evolving morphology is not yet known.
Ice occurrence is restricted to shallow waters of the southern and eastern North Sea and has decreased over the last 50 years. Nevertheless, some severe ice winters are still expected in future.
In the Wadden Sea, higher temperatures are expected to enhance organic matter turnover.
Much work has focused on detecting long-term change in the North Sea region, either from measurements or model results. In other regions, there have been attempts to attribute such changes to, for example, anthropogenic forcing (e.g. Barkhordarian et al. 2012). However, comparable studies are still missing for the North Sea. Such studies are urgently needed to assess consistency between observed changes and current expectations, in order to increase the level of confidence in projections of expected future conditions.
Figures 3.6, 3.9, 3.10 are reproduced under the Open Government Licence v3.0: www.nationalarchives.gov.uk/doc/open-government-licence/version/3.
- Aagaard T (2011) Sediment transfer from beach to shoreface: The sediment budget of an accreting beach on the Danish North Sea Coast. Geomorphology 135:143–157Google Scholar
- Aagaard T, Davidson-Arnott R, Greenwood B, Nielsen J (2004) Sediment supply from shoreface to dunes: Linking sediment transport measurements and long-term morphological evolution. Geomorphology 60:205–224Google Scholar
- Albrecht F, Weisse R (2012) Pressure effects on past regional sea level trends and variability in the German Bight. Ocean Dyn 62:1169–1186Google Scholar
- Albrecht F, Wahl T, Jensen J, Weisse R (2011) Determining sea level change in the German Bight. Ocean Dyn 61:2037–2050Google Scholar
- Albretsen J, Aure J, Sætre R, Danielssen DS (2012) Climatic variability in the Skagerrak and coastal waters of Norway. ICES J Mar Sci 69:758–763Google Scholar
- Alheit J, Pohlmann T, Casini M, Greve W, Hinrichs R, mathis M, O’driscoll K, Vorberg R, Wagner C (2012) Climate variability drives anchovies and sardines into the North and Baltic Seas. Prog Oceanogr 96:128–139Google Scholar
- Allen JI, Holt JT, Blackford J, Proctor R (2007) Error quantification of a high-resolution coupled hydrodynamic-ecosystem coastal-oceanmodel: Part 2. Chlorophyll-a, nutrients and SPM. J Mar Sys 68:381–404Google Scholar
- Andersen TJ, Lanuru M, van Bernem C, Pejrup M, Riethmueller R (2010) Erodibility of a mixed mudflat dominated by microphytobenthos and Cerastoderma edule, East Frisian Wadden Sea, Germany. Estuar, Coastal Shelf Sci 87:197–206.Google Scholar
- Anthony D, Leth JO (2002) Large-scale bedforms, sediment distribution and sand mobility in the eastern North Sea off the Danish west coast. Mar Geol 182:247–263Google Scholar
- Araújo I (2005) Sea level variability: Examples from the Atlantic coast of Europe. Ph.D. Thesis, University of Southampton, United KingdomGoogle Scholar
- Araújo I, Pugh DT (2008) Sea levels at Newlyn 1915–2005: Analysis of trends for future flooding risks. J Coast Res 24(sp3):203–212Google Scholar
- Artioli Y, Blackford JC, Butenschön M, Holt JT, Wakelin SL, Thomas H, Borges AV, Allen JI (2012) The carbonate system in the North Sea: sensitivity and model validation. J Mar Sys 102–104:1–13Google Scholar
- Artioli Y, Blackford JC, Nondal G, Bellerby RGJ, Wakelin SL, Holt JT, Butenschön M, Allen JI (2014) Heterogeneity of impacts of high CO2 on the North Western European Shelf. Biogeosciences 11:601–612Google Scholar
- Bacon S, Carter D (1991). Wave climate changes in the North Atlantic and the North Sea. Int J Climatol 11:545–558Google Scholar
- Badin G, Williams RG, Holt JT, Fernand LJ (2009) Are mesoscale eddies in shelf seas formed by baroclinic instability of tidal fronts? J Geophys Res 114:C10021, doi: 10.1029/2009JC005340
- Bakker J, Lüerßen G, Marencic H, Jung K (2009) Hazardous substances. In: Thematic Report 5.1 In: Marencic H, de Vlas J (eds) Quality Status Report 2009, Wadden Sea Ecosystem. No 25. Common Wadden Sea Secretariat, Trilateral Monitoring and Assessment Group, Wilhelmshaven, GermanyGoogle Scholar
- Baretta-Bekker JG, Baretta JW, Latuhihin MJ, Desmit X, Prins TC (2009) Description of the long-term (1991–2005) temporal and spatial distribution of phytoplankton carbon biomass in the Dutch North Sea. J Sea Res 61:50–59Google Scholar
- Barkhordarian A, Bhend J, von Storch H (2012) Consistency of observed near surface temperature trends with climate change projections over the Mediterranean region. Clim Dyn 38:1695–1702Google Scholar
- Baxter P (2005) The East Coast Great Flood, 31 January–1 February 1953: A summary of the human disaster. Phil Trans R Soc A 363:1293–1312Google Scholar
- Becker GA, Frohse A, Damm P (1997) The Northwest European shelf temperature and salinity variability. German J. Hydr. (Ocean. Dynamics) 49:135–151Google Scholar
- Berx B, Hansen B, Østerhus S, Larsen KM, Sherwin T, Jochumsen K (2013) Combining in-situ measurements and altimetry to estimate volume, heat and salt transport variability through the Faroe Shetland Channel. Ocean Sci 9:639–654Google Scholar
- Besch H-W (1987) Sylt-Naturräumliche Gliederung und Umwandlung durch Meer und Mensch. In: Hofmeister B, Voss F (eds), Beiträge zur Geographie der Küsten und Meere - Sylt 1986 und Berlin 1987. Berliner Geogr. Studien 25Google Scholar
- Beszczynska-Möller A, Dye SR (eds) (2013) ICES Report on Ocean Climate 2012. ICES Coop Res Rep 321Google Scholar
- Bindoff NL, Willebrand J, Artale V, Cazenave A, Gregory J, Gulev S, Hanawa K, Le Quéré C, Levitus S, Nojiri Y, Shum CK, Talley LD, Unnikrishnan A (2007) Observations: Oceanic climate change and sea level. In: Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor M., Miller H.L (eds) Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University PressGoogle Scholar
- Blackford JC, Gilbert FJ (2007) pH variability and CO2 induced acidification in the North Sea. J. Mar Sys 64:229–241Google Scholar
- Blott SJ, Duck RW, Phillips MR, Pontee NI, Pye K, Williams A (2013) Great Britain. In: Pranzini E, Williams A (eds) Coastal Erosion and Protection in Europe. Routledge, Abingdon, pp. 173–208Google Scholar
- Borges AV, Gypens N (2010) Carbonate chemistry in the coastal zone responds more strongly to eutrophication than to ocean acidification. Limnol Oceanogr 55:346–353Google Scholar
- Borsje BW, Hulscher SJMH, Herman PMJ, De Vries MB (2009) On the parameterization of biological influences on offshore sand wave dynamics. Ocean Dyn 59:659–670Google Scholar
- Bouin MN, Wöppelmann G (2010) Land motion estimates from GPS at tide gauges: A geophysical evaluation. Geophys J Int 180:193–209Google Scholar
- Bozec Y, Thomas H, Schiettecatte L-S, Borges AV, Elkalay K, de Baar HJW (2006) Assessment of the processes controlling the seasonal variations of dissolved inorganic carbon in the North Sea. Limnol Oceanogr 51:2746–2762Google Scholar
- Brooks SM, Spencer T (2010) Temporal and spatial variations in recession rates and sediment release from soft rock cliffs, Suffolk coast, UK. Geomorphology 124:26–41Google Scholar
- Brown S (2008) Soft cliff retreat adjacent to coastal defences, with particular reference to Holderness and Christchurch Bay, UK. PhD Thesis University of SouthamptonGoogle Scholar
- Bryche A, de Putter B, De Wolf P (1993) French and Belgian Coast from Dunkirk to De Panne: a case study of transborder cooperation in the framework of the Interreg initiative of the European community. In: Coastal Zone: Proceedings of the Symposium on Coastal and Ocean ManagementGoogle Scholar
- BSH (2008) Umweltbericht zum Raumordnungsplan für die deutsche ausschließliche Wirtschaftszone (AWZ) Teil Nordsee. Bundesamt fűr Seeschiffahrt und Hydrographie (BSH)Google Scholar
- Buijsman MC, Ridderinkhof H (2008a) Long-term evolution of sand waves in the Marsdiep inlet. I: High-resolution observations. Cont Shelf Res 28:1190–1201Google Scholar
- Buijsman MC, Ridderinkhof H (2008b) Long-term evolution of sand waves in the Marsdiep inlet. II: Relation to hydrodynamics. Cont Shelf Res 28:1202–1215Google Scholar
- Burchard H, Flöser G, Staneva J, Badewien TH, Riethmüller R (2008) Impact of density gradients on net sediment transport into the Wadden Sea. J Phys Oceanogr 38:566–587Google Scholar
- Burningham H, French J (2006) Morphodynamic behaviour of a mixed sand-gravel ebb-tidal delta: Deben estuary, Suffolk, UK. Mar Geol 225:23–44Google Scholar
- Burningham H, French J (2011) Seabed dynamics in a large coastal embayment: 180 years of morphological change in the outer Thames estuary. Hydrobiologia 672:105–119Google Scholar
- Burt WJ, Thomas H, Fennel K, Horne E (2013) Sediment-water column fluxes of carbon, oxygen and nutrients in Bedford Basin, Nova Scotia, inferred from 224Ra measurements. Biogeosci 10:53–66Google Scholar
- Cadée GC, Hegeman J (2002) Phytoplankton in the Marsdiep at the end of the 20th century; 30 years monitoring biomass, primary production, and Phaeocystis blooms. J Sea Res 48:97–110Google Scholar
- Cambers G (1976) Temporal scales in coastal erosion systems. Trans Inst British Geographers 1:246–256Google Scholar
- Capuzzo E, Painting SJ, Forster RM, Greenwood N, Stephens DT, Mikkelsen OA (2013) Variability in the sub-surface light climate at ecohydrodynamically distinct sites in the North Sea. Biogeochemistry 113:85–103Google Scholar
- Carr AP (1979) Sizewell-Dunwich Banks Field Study. Topic Report: 2. Long-term changes in the coastline and offshore banks. Report 89, Institute of Oceanographic Sciences, TauntonGoogle Scholar
- Carter D, Draper L (1988). Has the north-east Atlantic become rougher? Nature 332:494Google Scholar
- Charlier RH (2013) Belgium. In: Pranzini E, Williams A (eds) Coastal Erosion and Protection in Europe. Routledge, Abingdon, pp. 158–172Google Scholar
- Chaverot S, Héquette A, Cohen O (2005) Evolution of climatic forcings and potentially eroding events on the coast of Northern France. Proceedings 5th International Conference on Coastal Dynamics, Barcelona, Spain, April 2005, Cd-Rom, 11 p.Google Scholar
- Chen C-TA, Wang S-L (1999) Carbon, alkalinity and nutrient budgets on the East China Sea continental shelf. J Geophys Res 104:20675–20686Google Scholar
- Christiansen C, Bowman D (1990) Long-term beach and shoreface changes, NW Jutland, Denmark: effects of a change in wind direction. In: Beukema JJ et al. (eds) Expected Effects of Climatic Change on Marine Coastal Ecosystems, pp. 113–122Google Scholar
- Church JA, White NJ (2011) Sea-level rise from the late 19th to the early 21st Century. Surveys in Geophysics 32:585–602Google Scholar
- Church J, Aarup T, Woodworth P, Wilson W, Nicholls R, Rayner R, Lambeck K, Mitchum G, Steffen K, Cazenave A, Blewitt G, Mitrovica J, Lowe J (2010). Sea-level rise and variability: synthesis and outlook for the future. In: Church J, Woodworth P, Aarup T, Wilson S (eds) Understanding Sea-level Rise and Variability. Wiley & SonsGoogle Scholar
- Church JA, Clark PU, Cazenave A, Gregory JM, Jevrejeva S, Levermann A, Merrifield MA, Milne GA, Nerem RS, Nunn PD, Payne AJ, Pfeffer WT, Stammer D, Unnikrishnan AS (2013) Sea level change. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Stocker TF, Qin D, Plattner G-K, Tignor M, Allen SK, Boschung J, Nauels A, Xia Y, Bex V, Midgley PM (eds.). Cambridge University PressGoogle Scholar
- Clabaut P, Chamley H, Marteel H (2000) Évolution récente des dunes littorales à l’est de Dunkerque (Nord de la France) [Recent coastal dunes evolution, East of Dunkirk, Northern France]. Géomorphologie: relief, processus, environnement 6:125–136Google Scholar
- Corten A, van de Kamp G (1996) Variation in the abundance of southern fish species in the southern North Sea in relation to hydrography and climate. ICES J Mar Sci 53:1113–1119Google Scholar
- Cox AT, Swail VR (2001) A global wave hindcast over the period 1958–1997: Validation and climate assessment. J Geophys Res 106C:2313–2329Google Scholar
- CPSL (2010) Third Report. The role of spatial planning and sediment in coastal risk management. Wadden Sea Ecosystem No. 28. Common Wadden Sea Secretariat, Trilateral Working Group on Coastal Protection and Sea Level Rise (CPSL), Wilhelmshaven, Germany: 1-51Google Scholar
- Cunningham S, Marsh R, Wood R, Wallace C, Kuhlbrodt T, Dye S (2010) Atlantic Heat Conveyor (Atlantic Meridional Overturning Circulation). In MCCIP Annual Report Card 2010-11, MCCIP Science Review, 14 pp, www.mccip.org.uk/arc
- Daewel U, Schrum C (2013) Simulating long-term dynamics of the coupled North Sea and Baltic Sea ecosystem with ECOSMO II: Model description and validation. J Mar Sys 119–120:30–49Google Scholar
- de Boer GJ, Pietrzak JD, Winterwerp JC (2006) On the vertical structure of the Rhine region of freshwater influence. Ocean Dyn 56:198–216Google Scholar
- de Boer GJ, Pietrzak JD, Winterwerp JC (2009) SST observations of upwelling induced by tidal straining in the Rhine ROFI. Cont Shelf Res 29:263–277Google Scholar
- de Jong F (2000) Marine Eutrophication in Perspective. On the Relevance of Ecology for Environmental Policy. SpringerGoogle Scholar
- de Jonge VN, de Jong DJ (2002) ‘Global change’ Impact of inter-annual variation in water discharge as a driving factor to dredging and spoil disposal in the river Rhine system and of turbidity in the Wadden Sea. Est Coast Shelf Sci 55:969–991Google Scholar
- de Jonge VN, Postma H (1974) Phosphorus compounds in the Dutch Wadden Sea. Netherland J Sea Res 8:139–153Google Scholar
- de Jonge VN, van Beusekom JEE (1995) Wind-induced and tide-induced resuspension of sediment and microphytobenthos from tidal flats in the Ems estuary. Limnol Oceanogr 40:766–778Google Scholar
- de Jonge VN, Schuttelaars HM, van Beusekom JEE, Talke SA, de Swart HE (2014) The influence of channel deepening on estuarine turbidity levels and dynamics, as exemplified by the Ems estuary. Est Coast Shelf Sci 139:46–59Google Scholar
- de Kok JM (1992) A three-dimensional finite difference model for computation of near- and far-field transport of suspended matter near a river mouth. Cont Shelf Res 12:625–642Google Scholar
- de Nijs MAJ (2012) On sedimentation processes in a stratified estuarine system. PhD Thesis, Delft University of TechnologyGoogle Scholar
- de Nijs MAJ, Winterwerp JC, Pietrzak JD (2010) The effects of internal flow structure on sediment entrapment in the Rotterdam Waterway. J Phys Oceanogr 40:2357–2380Google Scholar
- de Ruijter WPM, Van der Giessen A, Groenendijk FC (1992) Current and density structure in the Netherlands coastal zone. In: Prandle D (ed) Coast Estuar Studies 40: Dynamics and exchanges in estuaries and the coastal zone. AGU, Washington, USAGoogle Scholar
- De Wolf P (2002) Kusterosie en verzanding van het Zwin. In: Van Lancker V et al. (eds) Colloquium Kustzonebeheer vanuit geo-ecologische en economische invalshoek. Oostende (B), 16–17 May 2002. Genootschap van Gentse Geologen (GGG)- Vlaams lnstituut van de Zee (VLIZ). VLIZ Special Publication 10: Oostende, BelgiumGoogle Scholar
- Degrendele K, Roche M, Schotte P, Van Lancker V, Bellec V, Bonne W (2010) Morphological evolution of the Kwinte Bank central depression before and after the cessation of aggregate extraction. J Coast Res SI 51:77–86Google Scholar
- Deleu S, Van Lancker V, Van den Eynde D, Moerkerke G (2004) Morphodynamic evolution of the kink of an offshore tidal sandbank: the Westhinder Bank (Southern North Sea). Cont Shelf Res 24:1587–1610Google Scholar
- Dette HH, Gärtner J (1987) Erfahrungen mit der Versuchssandvorspülung vor Hörnum im Jahre 1983. Die Küste 45:209–258Google Scholar
- Dickson RR, Meincke J, Malmberg SA, Lee AJ (1988) The “Great Salinity Anomaly” in the northern North Atlantic 1968–1982. Prog Oceanogr 20:103–151Google Scholar
- Diesing M, Kubicki A, Winter C, Schwarzer K (2006) Decadal scale stability of sorted bedforms, German Bight, southeastern North Sea. Cont Shelf Res 26:902–916Google Scholar
- Dillingh D, Fedor B, de Ronde J (2010) Definitiezeespiegelstijgingvoorbepalingsuppletiebehoefte. Deltares Report 1201993-002, DelftGoogle Scholar
- Dobrynin M, Gayer G, Pleskachevsky A, Günther H (2010) Effect of waves and currents on the dynamics and seasonal variations of suspended particulate matter in the North Sea. J Mar Sys 82:1–20Google Scholar
- Dolch T, Hass HC (2008) Long-term changes of intertidal and subtidal sediment composition in a tidal basin in the northern Wadden Sea (SE North Sea). Helgol Mar Res 62:3–11Google Scholar
- Dronkers J, Van Alphen JSLJ, Borst JC (1990) Suspended sediment transport processes in the southern North Sea. In: Cheng RT (ed) Residual Currents and Long Term Transport. SpringerGoogle Scholar
- Dye SR, Hughes SL, Tinker J, Berry DI, Holliday NP, Kent EC, Kennington K, Inall M, Smyth T, Nolan G, Lyons K, Andres O, Beszczynska-Möller A (2013a) Impacts of climate change on temperature (air and sea). MCCIP Science Review 2013:1–12. doi:10.14465/2013.arc01.001-012Google Scholar
- Dye SR, Holliday NP, Hughes SL, Inall M, Kennington K, Smyth T, Tinker J, Andres O, Beszczynska-Möller A (2013b) Climate change impacts on the waters around the UK and Ireland: Salinity. MCCIP Science Review 2013:60–66Google Scholar
- Dyer KR, Moffat TJ (1998) Fluxes of suspended matter in the East Anglian plume Southern North Sea. Cont Shelf Res 18:1311–1331Google Scholar
- EC (2000) Directive of the European Parliament and of the council 2000/60/EC establishing a framework for community action in the field of water policy, European Union. The European Parliament, The Council, PE-CONS 3639/1/00 REV 1 ENGoogle Scholar
- EEA (2012) Climate Change, Impacts and Vulnerability in Europe 2012. European Environment Agency, CopenhagenGoogle Scholar
- Einsporn S, Broeg K, Koehler A (2005) The Elbe flood 2002 – toxic effects of transported contaminants in flatfish and mussels of the Wadden Sea. Mar Poll Bull 50:423–429Google Scholar
- Eisma D (1981) Supply and deposition of suspended matter in the North Sea. In: Nio S-D, Shüttenhelm RTE, Van Weering TJCE (eds), Holocene marine sedimentation in the North Sea basin. Special Publication 5 of the International Association of Sedimentologists. Blackwell Scientific PublishersGoogle Scholar
- Eisma D, Kalf J (1987) Dispersal, concentration and deposition of suspended matter in the North Sea. J Geol Soc London 144:161–178Google Scholar
- Eleveld MA, Pasterkamp R, Van der Woerd HJ (2004) A survey of total suspended matter in the southern North Sea based on the 2001 SeaWiFS data. EARSeL eProceedings 3:166–178, www.eproceedings.org
- Eleveld MA, Pasterkamp R, van der Woerd HJ, Pietrzak J (2006) Suspended particulate matter from SeaWiFS data: statistical validation, and verification against the physical oceanography of the southern North Sea. Ocean Optics OOXVIII, (9–14 Oct 2006, Montreal)Google Scholar
- Eleveld MA, Pasterkamp R, van der Woerd HJ, Pietrzak JD (2008) Remotely sensed seasonality in the spatial distribution of sea-surface suspended particulate matter in the southern North Sea. Estuarine Coast Shelf Sci 80:103–113Google Scholar
- Elias EPL, Cleveringa J, Buijsman MC, Roelvink JA, Stive MJF (2006) Field and model data analysis of sand transport patterns in Texel Tidal inlet (the Netherlands). Coast Eng 53:505–529Google Scholar
- Ellett DJ, Martin JHA (1973) The physical and chemical oceanography of the Rockall Channel. Deep-Sea Res. 20:585–625Google Scholar
- Emeis K-C, van Beusekom J, Callies U, Ebinghaus R, Kannen A, Kraus G, Kröncke I, Lenhart H, Lorkowski I, Matthias V, Möllmann C, Pätsch J, Scharfe M, Thomas H, Weisse R, Zorita E (2015) The North Sea – a shelf sea in the Anthropocene. J Mar Sys 141:18-33Google Scholar
- EUROSION (2004) Living with Coastal Erosion in Europe: Sediment and Space for Sustainability. Part II – Maps and Statistics. Online at: www.eurosion.org/reports-online/part2.pdf
- Fettweis M, van den Eynde D (2003) The mud deposits and the high turbidity in the Belgian-Dutch coastal zone, southern bight of the North Sea. Cont Shelf Res 23:669–691Google Scholar
- Fettweis M, Monbaliu J, Baeye M, Nechad B, van den Eynde D (2012) Weather and climate induced spatial variability of surface suspended particulate matter concentration in the North Sea and the English Channel. Method Oceanogr 3–4:25–39Google Scholar
- Flather RA (1987) Estimates of extreme conditions of tide and surge using a numerical model of the north-west European continental shelf. Estuarine, Coast Shelf Sci 24:69–93Google Scholar
- Flemming B, Nyandwi N (1994) Land reclamation as a cause of fine-grained sediment depletion in backbarrier tidal flats (southern North Sea). Netherlands J Aquat Ecol 28:299–307Google Scholar
- Folland CK, Parker DE (1995) Correction of instrumental biases in historical sea surface temperature data. Quart J Roy Met Soc 121:319–367Google Scholar
- French PW (2001) Coastal Defences - Processes, Problems and Solutions. RoutledgeGoogle Scholar
- Fruergaard M, Andersen TJ, Johannessen PN, Nielsen LH, Pejrup M (2013) Major coastal impact induced by a 1000-year storm event. Scientific Reports 3: no. 1051, doi: 10.1038/srep01051
- Garcia D, Vigo I, Chao BF, Martinez MC (2007) Vertical crustal motion along the Mediterranean and Black Sea coast derived from ocean altimetry and tide gauge data. Pure Appl Geophys 164:851–863Google Scholar
- Gayer G, Dick S, Pleskachevsky A, Rosental W (2004) Modellierung von Schwebstofftransporten in Nordsee und Ostsee. Berichte des BSH 36. BSH, HamburgGoogle Scholar
- Gayer G, Dick S, Pleskachevsky A, Rosenthal W (2006) Numerical modelling of suspended matter transport in the North Sea Ocean Dyn 56:62–77Google Scholar
- Gehrels WR, Woodworth PL (2013) When did modern rates of sea-level rise start? Global and Plan Change 100:263–277Google Scholar
- Gerritsen H (2005) What happened in 1953? The big flood in the Netherlands in retrospect. Phil Trans R Soc A 363:1271–1291Google Scholar
- Giardino A, Van den Eynde D, Monbaliu J (2010) Wave effects on the morphodynamic evolution of an offshore sand bank. J Coast Res 51:127–140Google Scholar
- Greenwood N, Parker ER, Fernand L, Sivyer DB, Weston K, Painting SJ, Kröger S, Forster RM, Lees HE, Mills DK, Laane RWPM (2010) Detection of low bottom water oxygen concentrations in the North Sea; implications for monitoring and assessment of ecosystem health. Biogeosciences 7:1357–1373Google Scholar
- Gypens N, Borges AV, Lancelot C (2009) Effect of eutrophication on air-sea CO2 fluxes in the coastal Southern North Sea: a model study of the past 50 years. Glob Change Biol 15:1040–1056Google Scholar
- Haekkinen S, Rhines PB, Worthen DL (2011). Warm and saline events embedded in the meridional circulation of the northern North Atlantic. J Geophys Res 116: C03006, doi: 10.1029/2010JC006275
- Haigh ID, Nicholls RJ, Wells NC (2009) Mean sea-level trends around the English Channel over the 20th century and their wider context. Cont Shelf Res 29:2083–2098Google Scholar
- Hátún H, Sando A, Drange H, Bentsen M (2005) Seasonal to decadal temperature variations in the Faroe–Shetland inflow waters. In: Drange H, Dokken T, Furevik T, Gerdes R, Berger W (eds) The Nordic Seas: an Integrated Perspective: Oceanography, Climatology, Biogeochemistry, and Modeling. American Geophysical Union, pp. 239–250Google Scholar
- Heaps NS (1972) Estimation of density currents in Liverpool Bay. Geophys J Roy Astron Soc 30:415–432Google Scholar
- Henriksen P (2009) Long-term changes in phytoplankton in the Kattegat, the Belt Sea, the Sound and the western Baltic Sea. J Sea Res 61:114–123Google Scholar
- Heyen H, Dippner JW (1998) Salinity variability in the German Bight in relation to climate variability. Tellus 50A:545–556Google Scholar
- Hill AE, James ID, Linden PF, Matthews JP, Prandle D, Simpson JH, Gmitrowicz EM, Smeed DA, Lwiza KMM, Durazo R, Fox AD, Bowers DG (1993) Dynamics of tidal mixing fronts in the North Sea. Phil Trans Roy Soc London A 343:431–446Google Scholar
- Hill AE, Brown J, Fernand L, Holt J, Horsburgh KJ, Proctor R, Raine R, Turrell WR (2008) Thermohaline circulation of shallow tidal seas. Geophys Res Lett 35:L11605, doi: 10.1029/2008GL033459
- Hjøllo SS, Skogen MD, Svendsen E (2009) Exploring currents and heat within the North Sea using a numerical model. J Mar Sys 78:180–192Google Scholar
- Hogben N (1994) Increases in wave heights over the North Atlantic: a review of the evidence and some implications for the naval architect. Trans Roy Inst Naval Arch W5:93–101.Google Scholar
- Hollebrandse FAP (2005) Temporal development of the tidal range in the Southern North Sea. Ph.D. Thesis, Delft University of Technology, NetherlandsGoogle Scholar
- Holliday NP (2003) Air-sea interaction and circulation changes in the northeast Atlantic. J Geophys Res 108:C3259, doi: 10.1029/2002JC001344
- Holliday NP, Cunningham SA (2013) The Extended Ellett Line: Discoveries from 65 years of marine observations west of the UK. Oceanography 26:156–163Google Scholar
- Holliday NP, Hughes SL, Bacon S, Beszczynska-Möller A, Hansen B, Lavín A, Loeng H, Mork KA, Østerhus S, Sherwin T, Walczowski W (2008) Reversal of the 1960s – 1990s freshening trend in the North Atlantic and Nordic Seas. Geophys Res Lett 35:L03614, doi: 10.1029/2007GL032675
- Holliday NP, Hughes SL, Beszczynska-Möller A (eds) (2009) ICES Report on Ocean Climate 2008. ICES Coop Res Rep 298Google Scholar
- Holliday NP, Hughes SL, Dye S, Inall M, Read J, Shammon T, Sherwin T, Smyth T (2010) Salinity. In: MCCIP Annual Report Card 2010-11, MCCIP Science Review, www.mccip.org.uk/arc
- Holliday NP, Hughes SL, Borenäs K, Feistel R, Gaillard F, Lavìn A, Loeng H, Mork KA, Nolan G, Quante M, Somavilla R (2011) Long-term physical variability in the North Atlantic Ocean. In: Reid PC, Valdés L (eds) ICES Status Report on Climate Change in the North Atlantic. ICES Coop Res Rep 310: 21–46.Google Scholar
- Holt JT, James ID (1999) A simulation of the southern North Sea in comparison with measurements from the North Sea Project Part 2, Suspended Particulate Matter. Cont Shelf Res 19:1617–1642Google Scholar
- Holt J, Proctor R (2008) The seasonal circulation and volume transport on the northwest European continental shelf: a fine-resolution model study. J Geophys Res 113:C06021, doi: 10.1029/2006JC004034
- Holt JT, Allen JI, Proctor R, Gilbert F (2005) Error quantification of a high resolution coupled hydrodynamic–ecosystem coastal–ocean model: part 1 model overview and assessment of the hydrodynamics. J Mar Sys 57:167–188Google Scholar
- Holt J, Wakelin SL, Huthnance JM (2009) The down-welling circulation of the northwest European continental shelf: a driving mechanism for the continental shelf carbon pump. Geophys Res Lett 36:L14602, doi: 10.1029/2009GL038997
- Holt J, Hughes S, Hopkins J, Wakelin SL, Holliday NP, Dye S, González-Pola C, Hjøllo SS, Mork KA, Nolan G, Proctor R, Read J, Shammon T, Sherwin T, Smyth T, Tattersall G, Ward B, Wiltshire K (2012) Multi-decadal variability and trends in the temperature of the northwest European continental shelf: A model-data synthesis. Prog Oceanogr 106:96–117Google Scholar
- Hordoir R, Meier HEM (2010) Freshwater fluxes in the Baltic Sea: A model study. J Geophys Res 115:C08028, doi: 10.1029/2009JC005604
- Hordoir R, Dieterich C, Basu C, Dietze H, Meier HEM (2013) Freshwater outflow of the Baltic Sea and transport in the Norwegian Current: A statistical correlation analysis based on a numerical experiment. Cont Shelf Res 64:1–9Google Scholar
- Horrillo-Caraballo JM, Reeve DE (2008) Morphodynamic behaviour of a nearshore sandbank system: The Great Yarmouth Sandbanks, UK. Mar Geol 254:91–106Google Scholar
- Horsburgh K, Wilson C (2007) Tide-surge interaction and its role in the distribution of surge residuals in the North Sea. J Geophys Res 112:C08003, doi: 10.1029/2006JC004033
- Howarth MJ, Dyer KR, Joint IR, Hydes DJ, Purdie DA, Edmunds H, Jones JE, Lowry RK, Moffat TJ, Pomroy AJ, Proctor R (1993) Seasonal cycles and their spatial variability. Phil Trans Roy Soc Lond A 343:383–403Google Scholar
- Hughes SL, Holliday NP, Beszczynska-Möller A (eds) (2011) ICES Report on Ocean Climate 2010. ICES Coop Res Rep 309Google Scholar
- Hughes SL, Holliday NP, Gaillard F, ICES Working Group on Oceanic Hydrography (2012) Variability in the ICES/NAFO region between 1950 and 2009: observations from the ICES Report on Ocean Climate. ICES J Mar Sci 69:706–719Google Scholar
- Hurrell JW (1995) Decadal trends in the North Atlantic Oscillation: Regional temperatures and precipitation. Science 269:676–679Google Scholar
- Hurrell JW, National Center for Atmospheric Research Staff (eds) (2013) The Climate Data Guide: Hurrell North Atlantic Oscillation (NAO) Index (station-based). https://climatedataguide.ucar.edu/climate-data/hurrell-north-atlantic-oscillation-nao-index-station-based
- Huthnance JM, Holt JT, Wakelin SL (2009) Deep ocean exchange with west-European shelf seas. Ocean Sci 5:621–634Google Scholar
- Irion G, Zöllmer V (1999) Clay mineral associations in fine-grained surface sediments of the North Sea. J Sea Res 41:119–128Google Scholar
- Ivchenko V, Wells N, Aleynik D, Shaw A (2010) Variability of heat and salinity content in the North Atlantic in the last decade. Ocean Sci 6:719–735Google Scholar
- Iversen SA, Skogen MD, Svendsen E (2002) Availability of horse mackerel (Trachurus trachurus) in the north-eastern North Sea, predicted by the transport of Atlantic water. Fish Oceanogr 11:245–250Google Scholar
- Jago CF, Bale AJ, Green MO, Howarth MJ, Jones SE, McCave IN, Millward GE, Morris AW, Rowden AA, Williams JJ, Hydes D, Turner A, Huntley D, Van Leussen W (1993) Resuspension processes and seston dynamics, southern North Sea [and discussion]. Phil Trans Roy Soc London A 343:475–491Google Scholar
- Janssen F (2002) Statistische Analyse mehrjähriger Variabilität der Hydrographie in Nord- und Ostsee. Dissertation Fachbereich Geowissenschaften, Universität Hamburg, GermanyGoogle Scholar
- Jensen J, Hofstede J, Kunz H, De Ronde J, Heinen P, Siefert W (1993) Long-term water level observations and variations. Coastal Zone ‘93, Coastlines of the Southern North SeaGoogle Scholar
- Jevrejeva S, Grinsted A, Moore JC, Holgate S (2006) Nonlinear trends and multiyear cycles in sea level records. J Geophys Res 111:C09012, doi.org/ 10.1029/2005JC003229
- Jevrejeva S, Moore JC, Grinsted A, Woodworth PL (2008) Recent global sea level acceleration started over 200 years ago? Geophys Res Lett 35:L08715, doi: 10.1029/2008GL033611
- Johnson GC, Gruber N (2007) Decadal water mass variations along 20 degrees W in the Northeastern Atlantic Ocean. Prog Oceanogr 73:277–295Google Scholar
- Jones SE, Jago CF, Simpson JH (1996) Modelling suspended sediment dynamics in tidally stirred and periodically stratified waters: Progress and pitfalls. In: Pattiaratchi C (ed), Mixing in Estuaries and Coastal Seas, AGU, Coast Estuar Studies 50Google Scholar
- Joordens JCA, Souza AJ, Visser AW (2001) The Influence of tidal straining and wind on suspended matter and phytoplankton dynamics in the Rhine outflow region. Cont Shelf Res 21:301–325Google Scholar
- Karlson K, Rosenberg R, Bonsdorff E (2002) Temporal and spatial large-scale effects of eutrophication and oxygen deficiency on benthic fauna in Scandinavian and Baltic waters – a review. Oceanogr Mar Biol 40:427–489Google Scholar
- Kauker F (1999) Regionalization of climate model results for the North Sea. Ph.D. Thesis, Universität Hamburg, GermanyGoogle Scholar
- Kauker F, von Storch H (2000) Statistics of “synoptic circulation weather” in the North Sea as derived from a multiannual OGCM simulation. J Phys Oceanogr 30:3039–3049Google Scholar
- Kelletat D (1992) Coastal erosion and protection measures at the German North Sea coast. J Coast Res 8:699–711Google Scholar
- Kennedy JJ, Rayner NA, Smith RO, Saunby M, Parker DE (2011a) Reassessing biases and other uncertainties in sea-surface temperature observations since 1850 part 1: measurement and sampling errors. J Geophys Res 116:D14103, doi: 10.1029/2010JD015218
- Kennedy JJ, Rayner NA, Smith RO, Saunby M, Parker DE (2011b) Reassessing biases and other uncertainties in sea-surface temperature observations since 1850, part 2: biases and homogenisation. J Geophys Res 116:D14104, doi: 10.1029/2010JD015220
- Kent EC, Taylor PK, Truscott BS, Hopkins JS (1993) The accuracy of voluntary observing ship’s meteorological observations – Results of the VSOP-NA. J Atmos Oceanic Tech 10:591–608Google Scholar
- Kent EC, Kennedy JJ, Berry DI, Smith RO (2010) Effects of instrumentation changes on ocean surface temperature measured in situ. Climate Change 1:718–728Google Scholar
- Kirby RR, Beaugrand G, Lindley JA, Richardson AJ, Edwards M, Reid PC (2007) Climate effects and benthic-pelagic coupling in the North Sea. Mar Ecol-Prog Ser 330:31–38Google Scholar
- Knaapen MAF (2009) Sandbank occurrence on the Dutch continental shelf in the North Sea. Geo-Mar Lett 29:17–24Google Scholar
- Koslowski G (1989) Die flächenbezogene Eisvolumensumme, eine neue Maßzahl für die Bewertung des Eiswinters an der Ostseeküste Schleswig-Holsteins und ihr Zusammenhang mit dem meteorologischen Charakter des meteorologischen Winters. Dt Hydrogr Z 42:61–80Google Scholar
- Kühn W, Pätsch J, Thomas H, Borges AV, Schiettecatte L-S, Bozec Y, Prowe F (2010) Nitrogen and carbon cycling in the North Sea and exchange with the North Atlantic - a model study Part II. Carbon budget and fluxes. Cont Shelf Res 30:1701–1716Google Scholar
- Kushnir Y, Cardone VJ, Greenwood JG, Cane MA (1997) The recent increase in North Atlantic wave heights. J. Climate 10:2107–2113Google Scholar
- Kystdirektoratet (2008) Vestkysten 2008. Kystdirektoratet, LemvigGoogle Scholar
- Lacroix G, Ruddick K, Ozer J, Lancelot C (2004) Modelling the impact of the Scheldt and Rhine/Meuse plumes on the salinity distribution in Belgian waters (southern North Sea). J Sea Res 52:149–163Google Scholar
- Lancelot C, Thieu V, Polard A., Garnier J, Billen G, Hecq W, Gypens N (2011) Ecological and economic effectiveness of nutrient reduction policies on coastal Phaeocystis colony blooms in the Southern North Sea: an integrated modeling approach. Sci Tot Env 409:2179–2191Google Scholar
- Langenberg H, Pfizenmayer A, von Storch H, Sündermann J (1999) Storm-related sea level variations along the North Sea coast: natural variability and anthropogenic change. Cont Shelf Res 19:821–842Google Scholar
- Latif M, Böning C, Willebrand J, Biastoch A, Dengg J, Keenlyside N, Schweckendiek U, Madec G (2006) Is the thermohaline circulation changing? J Climate 19:4631–4637Google Scholar
- Lee AJ (1980) North Sea: Physical oceanography. In: Banner FT, Collins MB, Massie KS (eds) The North-West European Shelf Seas: The seabed and the sea in motion. 2. Physical and chemical oceanography, and physical resources. Elsevier Oceanography Series, 24B, pp. 467–493Google Scholar
- Lenhart H-J, Mills DK, Baretta-Bekker H, van Leeuwen SM, van der Molen J, Baretta JW, Blaas M, Desmit X, Kűhn W, Lacroix G, Los HJ, Ménesguen A, neves R, Proctor R, Ruardij P, Skogen MD, Vanhoutte-Brunier A, Villars MT, Wakelin S (2010) Predicting the consequences of nutrient reduction on the eutrophication status of the North Sea. J Mar Sys 81:148–170Google Scholar
- Leterme SC, Pingree RD, Skogen MD, Seuront L, Reid PC, Attrill MJ (2008) Decadal fluctuations in North Atlantic water inflow in the North Sea 1958–2003: impacts on temperature and phytoplankton populations. Oceanologia 50:59–72Google Scholar
- Liu KK, Atkinson L, Quinones R, Talaue-McManus L (eds) (2010) Carbon and Nutrient Fluxes in Continental Margins. SpringerGoogle Scholar
- Loebl, M., Dolch T, van Beusekom JEE (2007) Annual dynamics of pelagic primary production and respiration in a shallow coastal basin. J Sea Res 58:269–282Google Scholar
- Loebl M, Colijn F, Beusekom JEE, Baretta-Bekker JG, Lancelot C, Philippart CJM, Rousseau V (2009) Recent patterns in potential phytoplankton limitation along the NW European continental coast. J Sea Res 61:34–43Google Scholar
- Loewe P (ed) (2009) System Nordsee – Zustand 2005 im Kontext langzeitlicher Entwicklungen. Report 44, Bundesamt für Seeschifffahrt und Hydrographie, Hamburg and RostockGoogle Scholar
- Longuet-Higgins M, Stewart R (1962) Radiation stress and mass transport in gravity waves, with application to ‘surf beats’. J Fluid Mech 13:481–503Google Scholar
- Lorkowski I, Pätsch J, Moll A, Kühn W (2012) Interannual variability of carbon fluxes in the North Sea from 1970 to 2006 – Competing effects of abiotic and biotic drivers on the gas-exchange of CO2. Estuar Coast Shelf Sci 100:38–57Google Scholar
- Lotze HK, Reise K, Worm B, van Beusekom JEE, Busch M, Ehlers A, Heinrich D, Hoffmann RC, Holm P, Jensen C, Knottnerus OS, Langjanki N, Prummel W, Vollmer M, Wolff WJ (2005) Human transformation of the Wadden Sea ecosystem through time: a synthesis. Helgol Mar Res 59:84–95Google Scholar
- Lozier MS, Stewart NM (2008) On the temporally varying northward penetration of Mediterranean Overflow Water and eastward penetration of Labrador Sea Water. J Phys Oceanogr 38:2097–2103Google Scholar
- Madec G (2008) NEMO reference manual, ocean dynamics component, Institut Pierre-Simon Laplace, technical reportGoogle Scholar
- Madsen KS (2009) Recent and future climatic changes in temperature, salinity, and sea level of the North Sea and the Baltic Sea. PhD Thesis, Niels Bohr Institute, University of CopenhagenGoogle Scholar
- Martens P, van Beusekom JEE (2008) Zooplankton response to a warmer northern Wadden Sea. Helg Mar Res 62:67–75Google Scholar
- Mathis M, Elizalde A, Mikolajewicz U, Pohlmann T (2015) Variability patterns of the general circulation and sea water temperature in the North Sea. Prog Oceanogr 135:91–112Google Scholar
- McCandliss RR, Jones SE, Hearn M, Latter R, Jago CF (2002) Dynamics of suspended particles in coastal waters (southern North Sea) during a spring bloom. J Sea Res 47:285–302Google Scholar
- McCave IN (1987) Fine sediment sources and sinks around the East Anglian Coast. J Geol Soc London 144:149–152Google Scholar
- McQuatters-Gollop A, Raitsos DE, Edwards M, Pradhan Y, Mee LD, Lavender SJ, Attrill MJ (2007) A long-term chlorophyll data set reveals regime shift in North Sea phytoplankton biomass unconnected to nutrient trends. Limnol Oceanogr 52:635–648Google Scholar
- Meesenburg H (1996) Man’s role in changing the coastal landscapes in Denmark. GeoJournal 39:143–151Google Scholar
- Meire L, Soetaert KER, Meysman FJR (2013) Impact of global change on coastal oxygen dynamics and risk of hypoxia. Biogeosciences 10:2633–2653Google Scholar
- Merrifield M, Merrifield S, Mitchum G (2009) An anomalous recent acceleration of global sea level rise. J Clim 22:5772–5781Google Scholar
- Meyer EMI, Pohlmann T, Weisse R (2011) Thermodynamic variability and change in the North Sea (1948–2007) derived from a multidecadal hindcast. J Mar Sys 86:35–44Google Scholar
- Miller L, Douglas BC (2007) Gyre-scale atmospheric pressure variations and their relation to 19th and 20th century sea level rise. Geophys Res Lett 34:L16602, http://dx.doi.org/10.1029/2007GL030862
- Milne GA, Gehrels WR, Hughes CW, Tamisiea ME (2009) Identifying the causes of sea-level change. Nat Geosci 2:471–478Google Scholar
- Mork KA, Skagseth Ø (2010) A quantitative description of the Norwegian Atlantic Current by combining altimetry and hydrography. Ocean Sci 6:901–911Google Scholar
- Mroczek P (1980) Zu einer Karte der Veränderungen der Uferlinie der deutschen Nordseeküste in den letzten 100 Jahren im Maßstab 1:200.000Google Scholar
- Mudersbach C, Wahl T, Haigh I, Jensen J (2013) Trends in high sea levels of German North Sea gauges compared to regional mean sea level changes. Cont Shelf Res, 65:111–120Google Scholar
- Müller M (2012) The influence of changing stratification conditions on barotropic tidal transport and its implications for seasonal and secular changes of tides. Cont Shelf Res 47:107–118Google Scholar
- MyOcean (2014) Atlantic-European North West Shelf – Ocean Physics Reanalysis from Met Office (1985-2012). http://www.myocean.eu/web/69-myocean-interactive-catalogue.php?option=com_csw&view=details&product_id=NORTHWESTSHELF_REANALYSIS_PHYS_004_009
- Nedwell DB, Parkes RJ, Upton AC, Assinder DJ (1993) Seasonal fluxes across the sediment-water interface, and processes within sediments. Phil Trans Roy Soc London A 343:519–529Google Scholar
- Neu H (1984) Interannual variations and longer-term changes in the sea state of the North Atlantic. J Geophys Res 89:6397–6402Google Scholar
- NOOS (2010) NOOS Activity: Exchange of computed water, salt and heat transport across selected transects. North West European Shelf Operational Oceanographic System (NOOS). www.noos.cc/fileadmin/user_upload/exch_transports_NOOS-BOOS_2010-11-11.pdf
- Omar AM, Olsen A, Johannessen T, Hoppema M, Thomas H, Borges AV (2010) Spatio-temporal variations of fCO2 in the North Sea. Ocean Sci 6:77–89Google Scholar
- Oost AP (1995) Dynamics and sedimentary development of the Dutch Wadden Sea with emphasis on the Frisian Inlet; a study of the barrier islands, ebb-tidal deltas and drainage basins. Thesis, Utrecht, Geologica Ultraiectina, 126Google Scholar
- Oost AP, Hoekstra P, Wiersma A, Flemming B, Lammerts EJ, Pejrup M, Hofstede J, van der Valk B, Kiden P, Bartholdy J, van der Berg MW, Vos PC, de Vries S, Wang ZB (2012) Barrier island management: Lessons from the past and directions for the future. Ocean Coast Manage 68:18–38Google Scholar
- Otto L, Zimmerman JTF, Furnes GK, Mork M, Saetre R, Becker G (1990) Review of the physical oceanography of the North Sea. Netherlands J Sea Res 26:161–238Google Scholar
- Paphitis D, Bastos AC, Evans G, Collins MB (2010) The English Channel (La Manche): evolution, oceanography and sediment dynamics – a synthesis. In: Whittaker JE, Hart MB (eds) Micropalaeontology, Sedimentary Environments and Stratigraphy: a tribute to Dennis Curry (1921–2001). pp. 99–132Google Scholar
- Pätsch J, Kühn W (2008) Nitrogen and carbon cycling in the North Sea and exchange with the North Atlantic – a model study. Part I. Nitrogen budget and fluxes. Cont Shelf Res 28:767–787Google Scholar
- Pawlowicz R, McDougall T, Feistel R, Tailleux R (2012) Preface: An historical perspective on the development of the Thermodynamic Equation of Seawater – 2010. Ocean Sci 8:161–174Google Scholar
- Peltier WR (2004) Global glacial isostasy and the surface of the ice-age earth: The ICE-5G(VM2) model and GRACE. Ann Rev Earth Planet Sci 32:111–149Google Scholar
- Philippart CJM, Beukema JJ, Cadée GC, Dekker R, Goedhart PW, van Iperen JM, Leopold MF, Herman PNJ (2007) Impact of nutrient reduction on coastal communities. Ecosystems 10:96–119Google Scholar
- Pickering M, Wells N, Horsburgh K, Green J (2011) The impact of future sea-level rise on the European Shelf tides. Cont Shelf Res 35:1–15Google Scholar
- Pietrzak JD, de Boer GJ, Eleveld M (2011) Mechanisms controlling the intra-annual mesoscale variability of SST and SPM in the southern North Sea. Cont Shelf Res 31:594–610Google Scholar
- Pleskachevsky A, Gayer G, Horstmann J, Rosenthal W (2005) Synergy of satellite remote sensing and numerical modeling for monitoring of suspended particulate matter. Ocean Dyn 55:2–9Google Scholar
- Pohlmann T (2006) A meso-scale model of the central and southern North Sea: consequences of an improved resolution. Cont Shelf Res 26:2367–2385Google Scholar
- Port A, Gurgel K-W, Stanev J, Schulz-Stellenfleth J, Stanev EV (2011) Tidal and wind-driven surface currents in the German Bight: HFR observations versus model simulations. Ocean Dyn 61:1567–1585Google Scholar
- Postma H (1954) Hydrography of the Dutch Wadden Sea. Archives néerlandaises de Zoologie 10:405–511Google Scholar
- Postma H (1966) The cycle of nitrogen in the Wadden Sea and adjacent areas. Netherlands J Sea Res 3:186–221Google Scholar
- Postma H (1981) Exchange of materials between the North Sea and the Wadden Sea. Mar Geol 40:199–215Google Scholar
- Postma H (1984) Introduction to the symposium on organic matter in the Wadden Sea. Neth Inst S 10:15–22Google Scholar
- Poulton CVL, Lee JR, Jones LD, Hobbs PRN, Hall M (2006) Preliminary investigation into monitoring coastal erosion using terrestrial laser scanning: case study at Happisburgh, Norfolk, UK. Bull Geol Soc Norfolk 56:45–65Google Scholar
- Prandle D (1978a) Monthly-mean residual flows through Dover Strait, 1949–1972. J Mar Biol Assoc UK 58:965–973Google Scholar
- Prandle D (1978b) Residual flows and elevations in the Southern North Sea. Proc Roy Soc Lond A 259:189–228Google Scholar
- Prandle D, Player R (1993) Residual current through the Dover Strait measured by HF radar. Estuar Coast Shelf Sci 37:635–653Google Scholar
- Prandle D, Loch SG, Player RJ (1993) Tidal flow through the Straits of Dover. J Phys Oceanogr 23:23–37Google Scholar
- Prandle D, Ballard G, Flatt D, Harrison AJ, Jones SE, Knight PJ, Loch S, McManus J, Player R, Tappin A (1996) Combining modelling and monitoring to determine fluxes of water, dissolved and particulate metals through the Dover Strait. Cont Shelf Res 16:237–257Google Scholar
- Prowe F, Thomas H, Pätsch J, Kühn W, Bozec Y, Schiettecatte L-S, Borges AV, de Baar HJW (2009) Mechanisms controlling the air–sea CO2 flux in the North Sea. Cont Shelf Res 29:1801–1808Google Scholar
- Pugh D (2004) Changing Sea Levels: Effects of Tides, Weather and Climate. Cambridge University PressGoogle Scholar
- Puls W, Sündermann J (1990) Simulation of suspended sediment dispersion in the North Sea. Coast Estuar Studies 38:356–372Google Scholar
- Puls W, Pohlmann T, Sündermann J (1997) Suspended particulate matter in the southern North Sea: application of a numerical model to extend NERC North Sea project data interpretation. Deutsche Hydrografische Zeitschrift 49:307–327Google Scholar
- Pye K, Saye SE, Blott SJ (2007) Sand Dune Processes and Management for Flood and Coastal Defence. Part 2. Sand Dune Processes and Morphology. Joint Defra/Environment Agency Flood and Coastal Erosion Risk Management R & D Programme, R & D Technical report FD1302/TRGoogle Scholar
- Queste BY, Fernand L, Jickells TD, Heywood KJ (2013) Spatial extent and historical context of North Sea oxygen depletion in August 2010. Biogeochemistry 113:53–68Google Scholar
- Rayner NA, Parker DE, Horton EB, Folland CK, Alexander LV, Rowell DP, Kent EC, Kaplan A (2003) Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. J Geophys Res 108(D14):4407, doi: 10.1029/2002JD002670
- Reise K (2005) Coast of change: habitat loss and transformations in the Wadden Sea. Helgoland Mar Res 59:9–21Google Scholar
- Reise K (2013) A Natural History of the Wadden Sea. Riddled by Contingencies. Wadden Academy, Leeuwarden, NLGoogle Scholar
- Reise K, Kohlus J (2008) Seagrass recovery in the Northern Wadden Sea? Helgoland Mar Res 62:77–84Google Scholar
- Reise K, van Beusekom JEE (2008) Interactive effects of global and regional change on a coastal ecosystem. Helgoland Mar Res 62:85–91Google Scholar
- Reise K, Baptist M. Burbridge P, Dankers NMJA, Fischer L, Flemming BW, Oost AP, Smit C (2010) The Wadden Sea – A Universally Outstanding Tidal Wetland. Wadden Sea Ecosystem 29:7–24Google Scholar
- Reynaud JY, Tessier B, Auffret JP, Berné S, de Baptist M, Marsset T, Walker P (2003) The offshore Quaternary sediment bodies of the English Channel and its Western Approaches. J Quat Sci 18:361–371Google Scholar
- Robinson IS (1976) Theoretical analysis of use of submarine cables as electromagnetic oceanographic flowmeters. Phil Trans Roy Soc Lond A 280:355–396Google Scholar
- Rosenhagen G, Schatzmann M (2011) Das Klima der Metropolregion auf Grundlage meteorologischer Messungen und Beobachtungen. In: von Storch H, Claussen M (eds) Klimabericht für die Metropolregion Hamburg. SpringerGoogle Scholar
- Ruz MH, Meur-Férec C (2004) Influence of high water levels on aeolian sand transport: upper-beach/dune evolution on a macrotidal coast, Wissant Bay, Northern France. Geomorphology 60:73–87Google Scholar
- Rydell B, Angerud P, Hågeryd A-C (2004) Omfattning av stranderosion i Sverige Översiktlig kartläggning av erosionsförhållanden: Metodik och redovisning. Varia 543, Statens Geotekniska Institut, LinköpingGoogle Scholar
- Salman A, Lombardo S, Doody P (2004) Living with coastal erosion in Europe: Sediment and Space for Sustainability. EUROSION.Google Scholar
- Salt L, Thomas H, Prowe AEF, Borges AV, De Baar HJW (2013) Variability of North Sea pH and CO2 in response to North Atlantic Oscillation forcing. J Geophys Res (Biogeosciences) 118:1584–1592Google Scholar
- Schiettecatte L-S, Thomas H, Bozec Y, Borges AV (2007) High temporal coverage of carbon dioxide measurements in the Southern Bight of the North Sea. Mar Chem 106:161–173Google Scholar
- Schmelzer N, Holfort J (eds) (2012) Climatological Ice Atlas for the western and southern Baltic Sea (1961–2010). BSH Hamburg und RostockGoogle Scholar
- Schrum C, Sigismund F (2001) Modellkonfiguration des Nordsee/Ostsee-Modells. 40-Jahres NCEP Integration. Ber Zent Meeres- Klimaforsch Univ Hamburg B(4)Google Scholar
- Sharples J, Ross ON, Scott BE, Greenstreet SPR, Fraser H (2006) Inter-annual variability in the timing of stratification and the spring bloom in the North-western North Sea. Cont Shelf Res 26:733–751Google Scholar
- Sharples J, Holt J, Dye S (2010) Shelf sea stratification. In: MCCIP Annual Report Card 2010-11, MCCIP Science Review, www.mccip.org.uk/arc
- Shennan I, Woodworth PL (1992) A comparison of late Holocene and twentieth century sea level trends from the UK and North Sea region. Geophys J Int 109:96–105Google Scholar
- Shennan I, Milne G, Bradley S (2012) Late Holocene vertical land motion and relative sea-level changes: lessons from the British Isles. J Quart Sci 27:64–70Google Scholar
- Sherwin TJ, Read JF, Holliday NP, Johnson C (2012) The impact of changes in North Atlantic Gyre distribution on water mass characteristics in the Rockall Trough. ICES J Mar Sci 69:751–757Google Scholar
- Simpson JH, Hunter JR (1974) Fronts in the Irish Sea. Nature 250:404–406Google Scholar
- Simpson JH, Bos WG, Schirmer F, Souza AJ, Rippeth TP, Jones SE, Hydes D (1993) Periodic stratification in the Rhine ROFI in the North Sea. Oceanol Acta 16:23–32Google Scholar
- Smeed DA, McCarthy GD, Cunningham SA, Frajka-Williams E, Rayner D, Johns WE, Meinen CS, Baringer MO, Moat BI, Duchez A, Bryden HL (2014) Observed decline of the Atlantic meridional overturning circulation 2004–2012. Ocean Sci 10:29–38Google Scholar
- Smith TM, Reynolds RW (2002) Bias corrections for historical sea surface temperatures based on marine air temperatures. J Climate 15:73–87Google Scholar
- Smyth TJ, Fishwick JR, Al-Moosawi L, Cummings DG, Harris C, Kitidis V, rees A, Martinez-Vicente V, Woodward EMS (2010) A broad spatio-temporal view of the Western English Channel observatory. J Plankton Res 32:585–601Google Scholar
- Son CS, Flemming BV, Bartholomae A, Chun SS (2012) Long-term changes in morphology and textural sediment characteristics in response to energy variation on shoreface-connected ridges off the East Frisian barrier-island coast, southern North Sea. J Sed Res 82:385–399Google Scholar
- Sørensen P (2013) Denmark. In: Pranzini E, Williams A (eds) Coastal erosion and protection in Europe. RoutledgeGoogle Scholar
- Southward AJ (1960) On changes of sea temperature in the English Channel. J Mar Biol Assoc UK 42:275–375Google Scholar
- Souza AJ (2013) On the use of the Stokes number to explain frictional tidal dynamics and water column structure in shelf seas. Ocean Sci 9:391–398Google Scholar
- Souza A, Holt J, Proctor R (2007) Modelling SPM on the NW European Shelf Seas. In: Balson PS, Collins MB (eds) Coastal and Shelf Sediment Transport. Geol Soc London, Spec Publ 274:147–158Google Scholar
- Staats N, de Deckere EMGT, de Winder B, Stal LJ (2001) Spatial patterns of benthic diatoms, carbohydrates and mud on a tidal flat in the Ems-Dollard estuary. Hydrobiologia 448:107–115Google Scholar
- Sterl A, Caires S (2005) Climatology, variability and extrema of ocean waves. The web-based KNMI/ERA-40 Wave Atlas. Int J Climatol 25:963–977Google Scholar
- Sturges W, Douglas BC (2011) Wind effects on estimates of sea level rise. J Geophys Res 116:C06008, http://dx.doi.org/10.1029/2010JC006492
- Suijlen JM, Duin RNM (2001) Variability of near-surface total suspended-matter concentrations in the Dutch coastal zone of the North Sea: Climatological study on the suspended matter concentrations in the North Sea. Report RIKZ/OS/2001.150X. RIKZ, The Hague, NetherlandsGoogle Scholar
- Suijlen JM, Duin RNM (2002) Atlas of near-surface total suspended matter concentrations in the Dutch coastal zone of the North Sea. Report RIKZ/2002.059. RIKZ, The Hague, NetherlandsGoogle Scholar
- Sundby S, Drinkwater K (2007). On the mechanisms behind salinity anomaly signals of the northern North Atlantic. Progr Oceanogr 73:190–202Google Scholar
- Sündermann J (1993) Suspended particulate matter in the North Sea: Field observations and model simulations. Phil Trans Roy Soc Lond A 343:423–430Google Scholar
- Thiel M, Hinojosa IA, Joschko T, Gutow L (2011) Spatio-temporal distribution of floating objects in the German Bight (North Sea). J Sea Res 65:368–379Google Scholar
- Thomalla F, Vincent CE (2003) Beach response to shore-parallel breakwaters at Sea Palling, Norfolk, UK. Estuar, Coast Shelf Sci 56:203–212Google Scholar
- Thomas H, Bozec Y, Elkalay K, deBaar HJW (2004) Enhanced open ocean storage of CO2 from shelf sea pumping. Science 304:1005–1008Google Scholar
- Thomas H, Bozec Y, Elkalay K, deBaar HJW, Borges AV, Schiettecatte L-S (2005a) Controls of the surface water partial pressure of CO2 in the North Sea. Biogeosciences 2:323–334Google Scholar
- Thomas H, Bozec Y, de Baar HJW, Elkalay K, Frankignoulle M, Schiettecatte L-S, Kattner G, Borges AV (2005b) The carbon budget of the North Sea. Biogeosciences 2:87–96Google Scholar
- Thomas H, Prowe F, van Heuven S, Bozec Y, deBaar HJW, Schiettecatte L-S, Suykens K, Koné K, Borges AV, Lima ID, Doney SC (2007) Rapid decline of the CO2 buffering capacity in the North Sea and implications for the North Atlantic Ocean. Global Biogeochem Cy 21:GB4001, doi: 10.1029/2006GB002825 Google Scholar
- Thomas H, Schiettecatte L-S, Suykens K, Koné YJM, Shadwick EH, Prowe F, Bozec Y, de Baar HJW, Borges AV (2009) Enhanced ocean carbon storage from anaerobic alkalinity generation in coastal sediments. Biogeosciences 6:267–274Google Scholar
- Thyme F (1990) Beach nourishment on the west coast of Jutland. J Coast Res 6:201–210Google Scholar
- Tsimplis MN, Shaw AGP, Flather RA, Woolf DK (2006) The influence of the North Atlantic Oscillation on the sea-level around the northern European coasts reconsidered: the thermosteric effects. Phil Trans Roy Soc London A 364:845–856Google Scholar
- Turrell WR, Henderson EW, Slesser G, Payne R, Adams RD (1992) Seasonal changes in the circulation of the northern North Sea. Cont Shelf Res 12:257–286Google Scholar
- UKMMAS (2010) Charting Progress 2 Feeder Report: Ocean Processes. Huthnance J (ed.). Department for Environment Food and Rural Affairs on behalf of UKMMAS (UK Marine Monitoring and Assessment Strategy)Google Scholar
- Valentin H (1954) Land loss at Holderness. Reprinted in 1971: Steers JA (ed) Applied coastal geomorphology. Macmillan, LondonGoogle Scholar
- van Aken HM (2008) Variability of the water temperature in the western Wadden Sea on tidal to centennial time scales. J Sea Res 60:227–234Google Scholar
- van Aken HM (2010) Meteorological forcing of long-term temperature variations of the Dutch coastal waters. J Sea Res 63:143–151Google Scholar
- van Alphen JSLJ (1990) A mud balance for Belgian-Dutch coastal waters between 1969 and 1986. Netherlands J Sea Res 25:19–30Google Scholar
- van Alphen JSLJ, de Ruijter WPM, Borst JC (1988) Outflow and three-dimensional spreading of Rhine river water in the Netherlands coastal zone. In: Dronkers J, van Leussen W (eds) Physical Processes in Estuaries. SpringerGoogle Scholar
- van Bennekom AJ, Wetsteijn FJ (1990) The winter distribution of nutrients in the southern bight of the North Sea (1961–1978) and in the estuaries of the Scheldt and the Rhine/Meuse. Netherlands J Sea Res 25:75–87Google Scholar
- van Beusekom JEE, de Jonge VN (2002) Long-term changes in Wadden Sea nutrient cycles: importance of organic matter import from the North Sea. Hydrobiologia 475/476:185–194Google Scholar
- van Beusekom JEE, de Jonge VN (2012) Dissolved organic phosphorus: An indicator of organic matter turnover? Estuar Coast Shelf Sci 108:29–36Google Scholar
- van Beusekom JEE, Brockmann UH, Hesse KJ, Hickel W, Poremba K, Tillmann U (1999) The importance of sediments in the transformation and turnover of nutrients and organic matter in the Wadden Sea and German Bight. German J Hydrogr 51:245–266Google Scholar
- van Beusekom JEE, Bot P, Carstensen J, Goebel J, Lenhart H, Pätsch J, Petenati T, Raabe T, Reise K, Wetsteijn B (2009a) Eutrophication. Thematic Report 6 in: Marencic H, de Vlas J (eds) Quality Status Report 2009, Wadden Sea Ecosystem. No 25. Common Wadden Sea Secretariat, Trilateral Monitoring and Assessment Group, Wilhelmshaven, GermanyGoogle Scholar
- van Beusekom JEE, Loebl M, Martens P (2009b) Distant riverine nutrient supply and local temperature drive the long-term phytoplankton development in a temperate coastal basin. J Sea Res 61:26–33Google Scholar
- van Beusekom JEE, Buschbaum C, Reise K (2012) Wadden Sea tidal basins and the mediating role of the North Sea in ecological processes: scaling up of management? Ocean Coast Manage 68:69–78Google Scholar
- van Cauwenberghe C (1995) Relative sea level rise: further analyses and conclusions with respect to the high water, the mean sea and the low water levels along the Belgian coast. Report 37, HydrografischeDienst der KustGoogle Scholar
- van Cauwenberghe C (1999) Relative sea level rise along the Belgian coast: analyses and conclusions with respect to the high water, the mean sea and the low water levels. Report 46, HydrografischeDienst der KustGoogle Scholar
- van der Meulen F, Van der Valk B, Arens B (2013) The Netherlands. In Pranzini E, Williams A (eds) Coastal Erosion and Protection in Europe. Routledge, AbingdonGoogle Scholar
- van der Molen J (2002) The influence of tides, wind and waves on the net sand transport in the North Sea. Cont Shelf Res 22:2739–2762Google Scholar
- van der Molen J, Gerrits J, de Swart HE (2004) Modelling the morphodynamics of a tidal shelf sea. Cont Shelf Res 24:483–507Google Scholar
- van Hal R, Smits K, Rijnsdorp AD (2010) How climate warming impacts the distribution and abundance of two small flatfish species in the North Sea. J Sea Res 64:76–84Google Scholar
- van Straaten LMJU, Kuenen PH (1957) Accumulation of fine-grained sediments in the Dutch Wadden Sea. Geol Mijnb 19:319–354Google Scholar
- Vasseur B, Héquette A (2000) Storm surges and erosion of coastal dunes between 1957 and 1988 near Dunkerque (France), southwestern North Sea. Geol Soc Lond, Spec Publ 175:99–107Google Scholar
- Vavrus S, Walsh JE, Chapman WL, Portis D (2006) The behavior of extreme cold air outbreaks under greenhouse warming. Int J Climatol 26:1133–1147Google Scholar
- Verlaan PAJ, Spanhoff R (2000) Massive sedimentation events at the mouth of the Rotterdam Waterway. J Coast Res 16:458–469Google Scholar
- Verwaest T, Viaene P, Verstraeten J, Mostaert F (2005) De zeespiegelstijgingmeten, begrijpen en afblokken. De Grote Rede 15:15–25Google Scholar
- Verwey J (1952) On the ecology of cockle and mussel in the Dutch Wadden Sea. Archives néerlandaises de Zoologie 10:171–239Google Scholar
- Vichi M, Ruardij P, Baretta JW (2004) Link or sink: a modelling interpretation of the open Baltic biogeochemistry. Biogeosciences 1:79–100Google Scholar
- Vikebø F, Furevik T, Furnes G, Kvamstø NG, Reistad M (2003) Wave height variations in the North Sea and on the Norwegian Continental Shelf, 1881–1999. Cont Shelf Res 23:251–263Google Scholar
- Visser M, de Ruijter WPM, Postma L (1991) The distribution of suspended matter in the Dutch coastal zone. Netherlands J Sea Res 27:127–143Google Scholar
- von Storch H, Reichardt H (1997) A scenario of storm surge statistics for the German Bight at the expected time of doubled atmospheric carbon dioxide concentration. J Clim 10:2653–2662Google Scholar
- von Storch H, Zwiers F (1999) Statistical Analysis in Climate Research. Cambridge University PressGoogle Scholar
- Wahl T, Jensen J, Frank T (2010) On analysing sea level rise in the German Bight since 1844. Nat Hazards Earth Syst Sci 10:171–179Google Scholar
- Wahl T, Jensen J, Frank T, Haigh ID (2011) Improved estimates of mean sea level changes in the German Bight over the last 166 years. Ocean Dyn 61:701–715Google Scholar
- Wahl T, Haigh I, Woodworth P, Albrecht F, Dillingh D, Jensen J, Nicholls RJ, Weisse R, Wöppelmann G (2013) Observed mean sea level changes around the North Sea coastline from 1800 to present. Earth-Sci Rev 124:51–67Google Scholar
- WASA-Group (1998) Changing waves and storms in the Northeast Atlantic? Bull Am Met Soc 79:741–760Google Scholar
- Watson AJ, Schuster U, Bakker DCE, Bates N, Corbiere A, Gonzalez-Davila M, Friedrich T, Hauck J, Heinze C, Johannessen T, Kortzinger A, Metz N, Olaffson J, Olsen A, Oschlies A, Pfeil B, Santano-Casiano JM, Steinhoff T, Telszewski M, Rios A, Wallace DWR, Wanninkhof R (2009) Tracking the variable North Atlantic sink for atmospheric CO2. Science 326:1391–1393Google Scholar
- Weisse R, Günther H (2007) Wave climate and long-term changes for the Southern North Sea obtained from a high-resolution hindcast 1958–2002. Ocean Dyn 57:161–172Google Scholar
- Weisse R, Pluess A (2006) Storm-related sea level variations along the North Sea coast as simulated by a high-resolution model 1958–2002. Ocean Dyn 56:16–25Google Scholar
- Weisse R, von Storch H (2009) Marine Climate and Climate Change: Storms, Wind Waves and Storm Surges. SpringerGoogle Scholar
- Weisse R, von Storch H, Niemeyer H, Knaack H (2012) Changing North Sea storm surge climate: An increasing hazard? Ocean Coast Manag 68:58–68Google Scholar
- Wiltshire KH, Malzahn AM, Wirtz K, Greve W, Janisch S, Mangelsdorf P, Manly BFJ, Boersma M (2008) Resilience of North Sea phytoplankton spring bloom dynamics: an analysis of long-term data at Helgoland Roads. Limnol Oceanogr 53:1294–1302Google Scholar
- Winther NG, Johannessen JA (2006) North Sea circulation: Atlantic inflow and its destination. J Geophys Res 111:C12018, doi: 10.1029/2005JC003310
- Woodruff SD, Worley SJ, Lubker SJ, Ji Z, Freeman JE, Berry DI, Brohan P, Kent EC, Reynolds RW, Smith SR, Wilkinson C (2011) ICOADS Release 2.5 and Data Characteristics. Int J Climatol 31:951–967Google Scholar
- Woodworth PL (1987) Trends in U.K. mean sea level. Mar Geod 11:57–87Google Scholar
- Woodworth PL (1990) A search for accelerations in records of European mean sea level. Int J Climatol 10:129–143Google Scholar
- Woodworth PL (2010) A survey of recent changes in the main components of the ocean tide. Cont Shelf Res 30:1680–1691Google Scholar
- Woodworth PL, Tsimplis MN, Flather RA, Shennan I (1999) A review of the trends observed in British Isles mean sea level data measured by tide gauges. Geophys J Int 136:651–670Google Scholar
- Woodworth PL, Teferle FN, Bingley RM, Shennan I, Williams SDP (2009a) Trends in UK mean sea level revisited. Geophys J Int 176:19–30Google Scholar
- Woodworth PL, White NJ, Jevrejeva S, Holgate SJ, Gehrels WR (2009b) Evidence for the accelerations of sea level on multi-decade and century timescales. Int J Climatol 29:777–789Google Scholar
- Woodworth PL, Pouvreau N, Wöppelmann G (2010) The gyre-scale circulation of the North Atlantic and sea level at Brest. Ocean Sci 6:185–190Google Scholar
- Woodworth PL, Menéndez M, Gehrels WR (2011) Evidence for century-timescale acceleration in mean sea levels and for recent changes in extreme sea levels. Surv in Geophys 32:603–618Google Scholar
- Woolf DK, Challenor PG, Cotton PD (2002) Variability and predictability of the North Atlantic wave climate. J Geophys Res 107:C3145, doi: 10.1029/2001JC001124
- Wöppelmann G, Marcos M (2012) Coastal sea level rise in southern Europe and the non-climate contribution of vertical land motion. J Geophys Res 117:C01007, doi: 10.1029/2011JC007469
- Wöppelmann G, Pouvreau N, Simon B (2006). Brest sea level record: A time series construction back to the early eighteenth century. Ocean Dyn 56:487–497Google Scholar
- Wöppelmann G, Pouvreau N, Coulomb A, Simon B, Woodworth PL (2008) Tide gauge datum continuity at Brest since 1711: France’s longest sea-level record. Geophys Res Lett 35:L22605, doi: 10.1029/2008GL035783
- Young EF, Holt JT (2007) Prediction and analysis of long-term variability of temperature and salinity in the Irish Sea. J Geophys Res 112:C01008, doi: 10.1029/2005JC003386
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