During the past decade, the Arctic has experienced its highest temperatures of the instrumental record, even exceeding the warmth of the 1930s and 1940s. Recent paleo-reconstructions also show that recent Arctic summer temperatures are higher than at any time in the past 2000 years. The geographical distribution of the recent warming points strongly to an influence of sea ice reduction. The spatial pattern of the near-surface warming also shows the signature of the Pacific Decadal Oscillation in the Pacific sector as well as the influence of a dipole-like circulation pattern in the Atlantic sector. Areally averaged Arctic precipitation over the land areas north of 55°N shows large year-to-year variability, superimposed on an increase of about 5% since 1950. The years since 2000 have been wetter than average according to both precipitation and river discharge data. There are indications of increased cloudiness over the Arctic, especially low clouds during the warm season, consistent with a longer summer and a reduction of summer sea ice. Storm events and extreme high temperature show signs of increases. The Arctic Ocean has experienced enhanced oceanic heat inflows from both the North Atlantic and the North Pacific. The Pacific inflows evidently played a role in the retreat of sea ice in the Pacific sector of the Arctic Ocean, while the Atlantic water heat influx has been characterized by increasingly warm pulses. Recent shipboard observations show increased ocean heat storage in newly sea-ice-free ocean areas, with increased influence on autumn atmospheric temperature and wind fields.
KeywordsArctic climate Climate change Arctic temperature Precipitation Arctic Ocean changes
The cryospheric changes described in Callaghan et al. (2011a [this issue]) and AMAP (2011) on which it is based are part of a broader suite of interrelated variations in the Arctic climate system. A thorough review of Arctic climate, its relation to external forcing, and its variations through 2004 is presented in Chap. 2 of the Arctic Climate Impact Assessment (ACIA 2005). Accordingly, we preface both the AMAP (2011) SWIPA chapters and the Callaghan et al. (2011a [this issue]) papers with a brief overview of more recent Arctic climate variations, updating the ACIA climate chapter by summarizing new findings that have emerged in the post-2004 period. This article will emphasize the primary climate drivers (temperature, precipitation, storminess, clouds, and the ocean) of cryospheric change. A section on each variable contains a review of the recent literature (as well as additional results for temperature and precipitation). Recent changes of the primary cryospheric variables (snow, sea ice, glaciers, and permafrost) are presented in other articles in Callaghan et al. (2011a [this issue]) and the AMAP (2011) SWIPA report on which it is based.
The role of the sea ice feedbacks in the Arctic temperatures of the past 5 years is further supported by the Online Supplementary Material, Figs. A and B which show the spatial patterns of the annual and seasonal temperature anomalies of the past 5 years relative to the mean temperatures for 1951–2000. In contrast to the Arctic warming in the 1930s, which was centered in the Atlantic Arctic, recent temperature increases are Arctic-wide. The annual pattern in Fig. A contains anomalies exceeding 2°C over much of the Arctic Ocean, and it shows a very apparent polar amplification. It should be noted that reduced ice concentrations and thicknesses, in addition to reduced ice extent, can contribute to the feedback between sea ice and temperature over the central Arctic Ocean. Equatorward of the Arctic Ocean, the warming is generally stronger over the continents than over the oceans. These spatial features are also consistent with greenhouse-driven projections of change simulated by climate models.
The seasonal patterns in the Online Supplementary Material (Fig. B) highlight the maximum warming in autumn and winter, as noted in the latitude-season depiction in Fig. 3. The seasonal patterns, especially for winter, contain more spatial variability, consistent with advective influences arising from anomalies of the atmospheric circulation. Differences in the seasonal patterns relative to the corresponding patterns in ACIA (2005) include the warming maxima over the marginal ice zone during autumn and winter, as well as some warming over the Arctic Ocean during summer—despite the large thermal capacity of the ocean. The summertime warming of the Arctic Ocean is consistent with thinner sea ice and/or an earlier retreat of sea ice during summer, as documented in Meier et al. (2011).
To some extent, these recent spatial patterns of the temperature change are shaped by the phase of low-frequency (decadal or multidecadal) variations of the atmospheric circulation. Two large-scale modes for which there are documented effects on regional Arctic air temperatures are (1) the Arctic Oscillation, which drives temperature anomalies from eastern Canada across the North Atlantic to northern Eurasia (Thompson and Wallace 2000) and (2) the Pacific Decadal Oscillation (PDO), which has a strong influence on sub-arctic temperatures in the Pacific sector (Mantua and Hare 2002). The Arctic warming of the late 1980s and early 1990s has been attributed (e.g., Comiso 2003; Overland et al. 2008) to a predominantly positive phase of the Arctic Oscillation (Online supplementary material, Fig. C). The warming of the 1980s–1990s was indeed stronger over northern Eurasia than over many other sectors of the Arctic. In contrast, the more recent Arctic warming cannot be attributed to the Arctic Oscillation. First, the Arctic Oscillation has been in a generally neutral state, oscillating between positive and negative phases, since 1997—yet the Arctic’s warmest years in the instrumental record have occurred since 2004 (Fig. 2). Second, the Arctic Oscillation Index reached the most negative values ever recorded in Dec 2009–Jan 2010. At the same time, the high Arctic was relatively warm while northern Europe and Asia suffered from extreme cold. These concurrent anomalies of opposite sign point to the perils of using spatially aggregated temperatures as proxies for temperatures in particular sub-regions.
The PDO has been shown to be a primary determinant of wintertime temperature anomalies in northwestern North America (Hartmann and Wendler 2005). The PDO index (evaluated from Pacific sea surface temperatures) indeed has a multidecadal character. The increase of the PDO index from the middle 1970s to the early 1980s corresponds with a substantial increase in temperatures over Alaska and northwestern Canada. The negative PDO index of 2008–2009 coincides with an episode of below-normal temperatures in 2008–2009. The influence of the PDO extends westward to far eastern Siberia, where temperature anomalies are out of phase with those of Alaska and the Yukon, largely as a result of the intensification cycles of the Aleutian low pressure system in conjunction with the PDO. Indeed, the couplet of temperature anomalies of opposite sign in the winter pattern of Fig. B (Online Supplementary Material) and also in the multidecadal trends shown in the Arctic Climate Impact Assessment’s Sect. 184.108.40.206 (ACIA 2005), are driven, to a large extent, by the PDO and associated wind anomalies in the vicinity of the Aleutian low. A major challenge in anticipation of temperature changes in the Pacific Sub-arctic is related to our inability to predict phase transitions of the PDO. For purposes of our concluding discussion (Conclusion), however, we note that neither the PDO nor the Arctic Oscillation, the two major modes of northern hemispheric low-frequency climate variability, has been in a phase conducive to Arctic warming during the last several years—despite the anomalous pan-Arctic warmth of these years (Fig. 2).
The relatively high Arctic temperatures of recent years have also been associated with atmospheric circulation patterns conducive to the export of older, thicker sea ice from the Arctic Ocean to the North Atlantic. Several post-ACIA studies have pointed to the prominent role of similar circulation patterns, which have been assigned names ranging from the Dipole Anomaly (Wu et al. 2006) to the Arctic Rapid-Change pattern (Zhang et al. 2008). Current consensus is to refer to this newly dominant atmospheric circulation pattern as the “Arctic Dipole (AD)”. These patterns are best developed in the winter half of the year but can affect sea ice export in all seasons, and they are argued to have preconditioned the Arctic sea ice cover for the rapid summer retreat of the late 2000s (Smedsrud et al. 2008). Overland and Wang (2005) and Overland et al. (2008) highlight the meridional (across-pole) character of this atmospheric pattern which, in addition to affecting sea ice export, advects warmth into the Arctic Ocean in a pattern distinct from the PDO and the Arctic Oscillation.
Recently, Overland and Wang (2010) have presented evidence that the loss of sea ice and additional ocean heat storage (Jackson et al. 2010) have become sufficient to influence the atmospheric heat budget and circulation pattern in the autumn and early winter months. Honda et al. (2009) predicted this sea ice-atmospheric circulation connectivity from modeling studies, and Francis et al. (2009) arrive at a similar conclusions based on a data analysis encompassing a larger sample of years.
A major topic of attention in the past few years has been the vertical structure of the recent Arctic warming, since the vertical structure provides clues to the nature (drivers) of the warming. Graversen et al. (2008) argued that an elevated maximum of the warming precludes a major role of surface heating, although several subsequent studies (Grant et al. 2008; Bitz and Fu 2008; Alexeev et al. 2009) have provided evidence of a surface-based warming. The apparent discrepancy between the different analyses is due to the use of different datasets (reanalyses) and time periods. As shown in the Online Supplementary Material (Fig. D), latitude-height cross-sections depict a warming that is clearly strongest at the surface during autumn in the NCEP/NCAR reanalysis. The European ERA-40 database used by Graversen et al. (2008) does not show the near-surface warming seen in the NCEP/NCAR reanalysis. Moreover, the data on which the cross-section in Fig. D is based include the years of extreme ice minima that were not in the Graversen et al.’s study, giving credence to the argument that the ice-albedo feedback to Arctic temperatures is just now emerging in the post-ACIA period (Serreze et al. 2008).
Finally, the studies of variations and trends of Arctic temperatures have focused almost exclusively on monthly, seasonal, or annual mean temperatures. There has been little work on systematic changes in variability or extremes. Among the few studies of this kind, Walsh et al. (2005) found little evidence of increased variance of daily temperatures in Alaska and western Canada over the 50 years ending in 2000. However, there were indications of an increased frequency of daily extreme temperatures from the 1950s to the 1990s. Whether this trend has continued into the past 5 years of record Arctic warmth (Fig. 2) is unknown. On a more local scale, Weatherhead et al. (2010) report a decrease of the persistence of daily springtime temperatures at Baker Lake, Canada. This decrease of persistence corresponds with reports from indigenous residents that their weather has become less predictable. Model projections do indicate increasing frequencies of record high daily temperatures in twenty-first century scenario simulations (Timlin and Walsh 2007). Given the impacts of extreme events on other parts of the cryosphere and on humans and ecosystems, a priority for research is a determination of the relationship between changes in means and extremes of Arctic climate variables.
Evidence for trends of Arctic precipitation is complicated by inadequacies in both in situ measurements and remote-sensing-derived estimates of precipitation in cold climates. While these deficiencies were highlighted in ACIA (2005) and in the SWIPA report (AMAP 2011), we reiterate here the challenges created by changing station distributions and gauge undercatch. Both these factors impede attempts to construct temporally homogeneous records of areally averaged precipitation. Partly for this reason, variations of Arctic precipitation have been examined using atmospheric reanalysis output, either as directly simulated by the reanalysis models of the European Center for Medium-range Weather Forecasts and the US National Centers for Environmental Prediction (Serreze et al. 2005) or as moisture flux convergences (e.g., Peterson et al. 2006).
The increasing frequency of wet years in high latitudes is supported by increases in river discharge amounts, which are shown in the Online Supplementary Material (Fig. E). For Eurasia, the discharge of the largest rivers has increased by about 10% since 1935, despite the large interannual variations that are apparent (Peterson et al. 2002). The rate of increase for North America is similar, although the record length of river discharge is shorter for North America. The discharge curves for the two continents show a positive correlation, and their extreme years also show some correspondence with the annual precipitation amounts in Fig. E.
In addition to their highly publicized impacts on coastal regions and their residents, storms impact other components of the cryosphere through their associated precipitation (affecting glaciers, ice sheets, snow cover and even permafrost), winds (affecting sea ice motion and the distribution of snow on land and sea ice), and waves (affecting coastal permafrost). While storms have received increased diagnostic analyses through case studies (e.g., Roberts et al. 2008), there have been few rigorous evaluations of variations and trends of storminess in the Arctic, particularly the central Arctic. Wang et al. (2006) reported a northward shift of cyclone activity, primarily during winter, over Canada during 1953–2002, while Mesquita et al. (2010) found that temporal trends of cyclones in the North Pacific have generally been weak over the 60-year period ending 2008, although the US Global Change Research Program (Karl et al. 2009) points to increased impacts of storms on the northern Alaskan coast. Since, any increases of coastal flooding and erosion are also related to retreating sea ice, the role of storminess per se can be difficult to unravel. Nevertheless, it is apparent from the absence of a comprehensive (pan-Arctic) evaluation of recent variations in storminess that there is a need for systematic assessments of storminess in the Arctic. Such assessments should include both historical variations and their diagnosis, together with more substantive attempts to project changes into the future.
Through their large contributions to the surface energy budget, Arctic clouds can have important impacts on the surface energy budget and the cryosphere. These impacts can be manifested in interannual variations as well as trends. For example, Kay et al. (2008) show that the extreme retreat of sea ice in the summer of 2007 was accompanied by unusually clear skies over much of the Arctic Ocean. Trends and other longer-term variations have been addressed in several post-ACIA studies, although one must be cognizant of the observational challenges posed by Arctic clouds, both for remote sensing and for in situ measurements.
Wang and Key (2005) used Advanced Very High Resolution Radiometer (AVHRR) infrared satellite imagery to compute trends of −6, +3, +2, and −2% per decade during 1982–1999 for winter, spring, summer, and autumn, respectively. Eastman and Warren (2010), on the other hand, used surface-based observations from 1991 to 2007 to obtain small positive trends in all seasons. Low clouds were primarily responsible for these trends. Perhaps more importantly for cryospheric changes, clouds over sea ice showed a tendency to increase with warming temperatures and decreasing sea ice in all seasons except summer. Particularly in autumn, there was a positive low-cloud response to reduced sea ice, indicating that recent cloud changes may be enhancing the warming of the Arctic and accelerating the decline of sea ice (Eastman and Warren 2010). This suggestion is consistent with the recent model-based results of Vavrus et al. (2010), who found that, in ensembles of twenty-first century projections by the Community Climate System Model (CCSM3), clouds increased in autumn and decreased in summer during periods of rapid sea ice loss. This seasonality of the sea ice/cloud associations is not inconsistent with the loss of sea ice in recent years such as 2007, and it could amplify the loss of sea ice in the future.
Ocean Temperature Variations
A key driver of cryospheric change is the temperature variability of the high-latitude oceans. For example, the heat content of the polar oceans directly affects sea ice, tidewater glaciers and ice shelves, snowfall over the high latitudes, and perhaps even the large-scale atmospheric circulation. Salinity variations affect the stratification and control the locations of deep mixing of the oceans, while high-latitude ocean currents contribute to the driving of sea ice motion and the advection of heat and freshwater anomalies. Historically the high-latitude oceans have been woefully undersampled by observations, especially below the surface. However, during the past decade, and especially in the post-ACIA period of the International Polar Year (IPY, 2007–2009), there have been unprecedented opportunities to monitor the Arctic Ocean and its exchanges with middle latitudes—precisely during a period of unprecedented change in the cryosphere.
The preceding review of recent Arctic climate variations, with an emphasis on temperature and precipitation, serves two purposes. First, Arctic temperature and precipitation variations are key drivers of recent Arctic cryospheric change. Taken together with the cryospheric changes presented in Callaghan et al. (2011a) a picture emerges of Arctic changes that are generally consistent, in fact interconnected, across the Arctic system. Second, the preceding summary shows that Arctic climate has entered a unique period relative to the instrumental record and, in the case of summer temperatures, relative to 2000-year reconstructions of past variations. The unprecedented warmth of the past 5 years reinforces the urgency of an ongoing assessment of combined cryospheric/atmospheric/oceanic changes in the North. From a general climate perspective, the results point to the emergence of the ice-albedo and ice-insulation feedbacks in the seasonal and spatial patterns of the recent temperature anomalies in the Arctic. This emergence is the most fundamentally important development in high-latitude climate since the publication of the Arctic Climate Impact Assessment in 2005, because these processes promote further Arctic amplification and increase the potential for connectivity between the Arctic and mid-latitudes. Given the absence of strong anomalies of large-scale circulation drivers such as the Arctic Oscillation and the PDO in the past 5 years, the recent events support the changes anticipated in ACIA (2005), and this echoes the statement of Serreze and Francis (2006, p. 241) that “Given the general consistency (of ongoing changes) with model projections, we are likely near the threshold when absorption of solar radiation during summer limits ice growth the following autumn and winter, initiating a feedback leading to a substantial increase in Arctic Ocean surface air temperatures”. The cryospheric and atmospheric changes of the past 5 years indicate that we may well have crossed this threshold.
The results presented here also point to observational needs that are discussed in more detail in Key et al. (2011). There is a major lack of mid-tropospheric data over the Arctic to support the quality of atmospheric reanalysis products from major climate centers. These reanalysis products, in turn, are the basis for understanding ongoing Arctic climate changes. In addition, the precipitation estimates summarized in “Arctic Precipitation” are, by necessity, for Arctic land areas only. There are no systematically compiled sources of precipitation over the Arctic Ocean and its marginal seas, although it should be noted that Peterson et al. (2006) deduced recent increases of precipitation over the sub-arctic North Atlantic on the basis of computed moisture flux convergences in atmospheric reanalyses. Moreover, the station-derived estimates of precipitation for land areas in the Arctic have uncertainties arising from measurement errors (gauge undercatch of snow, for which only approximate correction procedures exist) and from the preferential siting of precipitation gauges in low-elevation areas. Even temperatures over the Arctic Ocean are subjected to uncertainties, as the estimates of surface air temperatures over ice-covered seas are generally based on extrapolation of temperature anomalies from nearby land areas. Satellite-derived estimates of Arctic surface (skin) temperatures are generally biased toward cloud-free conditions.
Despite the uncertainties in variables such as Arctic cloudiness and precipitation, the trends in other parts of the Arctic climate system are the largest observed in the historical record, and even over the past 2000 years. Moreover, these changes are consistent with the trends projected by global climate models as described in the modeling chapter of the AMAP (2011) SWIPA report. While natural variations may result in interannual to decadal-scale deviations from the recent trends, the trends described here should continue and should dominate Arctic environmental change by the later decades of the twenty-first century.
This paper benefitted from helpful comments and editorial assistance from Margareta Johansson, Terry Callaghan and Terry Prowse. The authors also acknowledge valuable scientific input provided by the contributing and lead author teams of the Snow, Water, Ice and Permafrost in the Arctic (SWIPA) assessment report. The Secretariat of the Arctic Monitoring and Assessment Programme provided the logistical support that enabled the SWIPA activity to come to fruition.
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