Global warming and 21st century drying
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- Cook, B.I., Smerdon, J.E., Seager, R. et al. Clim Dyn (2014) 43: 2607. doi:10.1007/s00382-014-2075-y
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Global warming is expected to increase the frequency and intensity of droughts in the twenty-first century, but the relative contributions from changes in moisture supply (precipitation) versus evaporative demand (potential evapotranspiration; PET) have not been comprehensively assessed. Using output from a suite of general circulation model (GCM) simulations from phase 5 of the Coupled Model Intercomparison Project, projected twenty-first century drying and wetting trends are investigated using two offline indices of surface moisture balance: the Palmer Drought Severity Index (PDSI) and the Standardized Precipitation Evapotranspiration Index (SPEI). PDSI and SPEI projections using precipitation and Penman-Monteith based PET changes from the GCMs generally agree, showing robust cross-model drying in western North America, Central America, the Mediterranean, southern Africa, and the Amazon and robust wetting occurring in the Northern Hemisphere high latitudes and east Africa (PDSI only). The SPEI is more sensitive to PET changes than the PDSI, especially in arid regions such as the Sahara and Middle East. Regional drying and wetting patterns largely mirror the spatially heterogeneous response of precipitation in the models, although drying in the PDSI and SPEI calculations extends beyond the regions of reduced precipitation. This expansion of drying areas is attributed to globally widespread increases in PET, caused by increases in surface net radiation and the vapor pressure deficit. Increased PET not only intensifies drying in areas where precipitation is already reduced, it also drives areas into drought that would otherwise experience little drying or even wetting from precipitation trends alone. This PET amplification effect is largest in the Northern Hemisphere mid-latitudes, and is especially pronounced in western North America, Europe, and southeast China. Compared to PDSI projections using precipitation changes only, the projections incorporating both precipitation and PET changes increase the percentage of global land area projected to experience at least moderate drying (PDSI standard deviation of ≤−1) by the end of the twenty-first century from 12 to 30 %. PET induced moderate drying is even more severe in the SPEI projections (SPEI standard deviation of ≤−1; 11 to 44 %), although this is likely less meaningful because much of the PET induced drying in the SPEI occurs in the aforementioned arid regions. Integrated accounting of both the supply and demand sides of the surface moisture balance is therefore critical for characterizing the full range of projected drought risks tied to increasing greenhouse gases and associated warming of the climate system.
Extreme climate and weather events have caused significant disruptions to modern and past societies (Coumou and Rahmstorf 2012; Ross and Lott 2003; Lubchenco and Karl 2012), and there is concern that anthropogenic climate change will increase the occurrence, magnitude, or impact of these events in the future (e.g., Meehl et al. 2000; e.g., Rahmstorf and Coumou 2011). Drought is one such extreme phenomenon, and is of particular interest because of its often long-term impacts on critical water resources, agricultural production, and economic activity (e.g., Li et al. 2011; e.g., Ding et al. 2011; e.g., Ross and Lott 2003). Focus on drought vulnerabilities has increased due to a series of recent and severe droughts in regions as diverse as the United States (Hoerling et al. 2012, 2013; Karl et al. 2012), east Africa (Lyon and DeWitt 2012), Australia (McGrath et al. 2012), and the Sahel (Giannini et al. 2003). Recent work further suggests that global aridity has increased in step with observed warming trends, and that this drying will worsen for many regions as global temperatures continue to rise with increasing anthropogenic greenhouse gas emissions (Burke et al. 2006; Dai 2013; Sheffield and Wood 2008).
There are significant uncertainties, however, in recent and projected future drought trends, especially regarding the extent to which these trends will be forced by changes in precipitation versus evaporative demand (Hoerling et al. 2012; Sheffield et al. 2012). Drought is generally defined as a deficit in soil moisture (agricultural) or streamflow (hydrologic); as such, it can be caused by declines in precipitation, increases in evapotranspiration, or a combination of the two. In the global mean, both precipitation and evapotranspiration are expected to increase with warming, a consequence of an intensified hydrologic cycle in a warmer world (Allen and Ingram 2002; Huntington 2006). Regional changes in precipitation and evapotranspiration, and the dynamics that drive such changes, are nevertheless more uncertain, despite the fact that these changes are perhaps of greatest relevance to on-the-ground stakeholders.
Precipitation projections in general circulation models (GCMs) have large uncertainties compared to other model variables, such as temperature (e.g., Knutti and Sedlacek 2013). The most confident estimates indicate that precipitation will increase in mesic areas (e.g., the wet tropics, the mid- to high latitudes of the Northern Hemisphere, etc) and decrease in semi-arid regions (e.g., the subtropics). This is generally referred to as the ‘rich-get-richer/poor-get-poorer’ mechanism, and is attributed to thermodynamic (warming and moistening of the atmosphere) and dynamic (circulation) processes (Chou et al. 2009, 2013; Held and Soden 2006; Neelin et al. 2003; Seager et al. 2010).
Evapotranspiration includes both the physical (evaporation) and biological (transpiration) fluxes of moisture from the surface to the atmosphere and can be viewed in terms of actual evapotranspiration (latent heat flux) or evaporative demand (potential evapotranspiration; PET). PET is expected to increase in the future (Scheff and Frierson 2013), forced by increases in both total energy availability at the surface (surface net radiation) and the vapor pressure deficit (the difference between saturation and actual vapor pressure; VPD). Increased radiative forcing from anthropogenic greenhouse gases (GHG) will increase surface net radiation in most areas by inhibiting longwave cooling, while GHG-induced warming of the atmosphere will increase the VPD. Importantly, VPD increases with warming, even at constant relative humidity (e.g., Anderson 1936). Actual evapotranspiration is expected to increase less than PET in areas where latent heat fluxes are, or will become, limited by moisture supply. Indeed, declines in global actual evapotranspiration have been documented over the last two decades (Jung et al. 2010), attributed primarily to soil moisture drying in the Southern Hemisphere.
The idea that increased evaporative demand in a warmer world will enhance drought is not new (e.g., Dai 2011), but it is important to understand where precipitation or evaporation changes will be dominant individual drivers of drought and where they will work in concert to intensify drought. To date, however, little has been done to quantify and explicitly separate the relative contribution of changes in precipitation versus evaporative demand to the magnitude and extent of global warming-induced drying. To address this question, we use output from a suite of twentieth and twenty-first century GCM simulations, available through the Coupled Model Intercomparison Project phase 5 (CMIP5, Taylor et al. 2012), to calculate two offline indices of surface moisture balance: the Palmer Drought Severity Index (PDSI; Palmer 1965) and the Standardized Precipitation Evapotranspiration Index (SPEI; Vicente-Serrano et al. 2009). Both indices provide ideal and flexible estimations of surface moisture balance, allowing us to vary inputs such as model precipitation, temperature, and surface energy availability in order to separate and quantify the influence of specific variables on future drought projections. Our analysis thus addresses three questions: (1) What are the relative contributions of changes in precipitation and evaporative demand to global and regional drying patterns?, (2) Where do the combined effects of changes in precipitation and evaporative demand enhance drying?, and (3) In which regions, if any, are increases in evaporative demand sufficient to shift the climate towards drought when precipitation changes would otherwise force wetter conditions?
2 Data and methods
2.1 CMIP5 model output
Continuous model ensembles from the CMIP5 experiments (historical+RCP8.5) used in this analysis, including the modeling center or group that supplied the output, the number of ensemble members that met our criteria for inclusion, and the approximate spatial resolution
Modeling center (or Group)
2.8° × 2.8°
0.94° × 1.25°
1.4° × 1.4°
1.87° × 1.87°
2.0° × 2.5°
2.0° × 2.5°
2.0° × 2.5°
2.0° × 2.5°
1.5° × 2.0°
1.9° × 3.75°
1.4° × 1.4°
2.8° × 2.8°
2.8° × 2.8°
1.1° × 1.1°
1.9° × 2.5°
2.2 Drought indices
We are interested in long-term (decadal to centennial) trends and changes in moisture availability, rather than shorter-term (month to month) drought events. For this reason, our analysis uses two drought indices that integrate over longer timescales: the PDSI and 12-month SPEI. Understanding the causes, inception, and termination of discrete (and often short and intense) drought events (e.g., Hoerling et al. 2012, 2013) is an important scientific goal. Our focus, however, is on the longer-term drying and wetting responses to GHG warming, the hydroclimatic baseline within which seasonal or annual events will occur in the future.
Simulated soil moisture within the GCMs is not easily separated into contributions from precipitation or PET, making it difficult to identify the extent to which soil moisture trends in the models are driven by changes in supply and/or demand. Moreover, each GCM employs soil models that vary widely in their sophistication (e.g., soil depth, number of layers, etc), tunings, and parameterizations (e.g., soil texture, rooting depths, vegetation types, etc), complicating the meaningful comparison of soil moisture and drought responses across GCMs. PDSI and SPEI provide a flexible framework that allows GCM output to be modified (e.g., detrended) as a means of isolating drought contributions from specific changes, such as trends in precipitation or net radiation. A common offline metric, such as PDSI or SPEI, also provides a standard comparison of soil moisture balance, thus controlling for differences in soil models across the ensemble of CMIP5 GCMs. The PDSI (Palmer 1965) is a normalized index of drought using a simplified soil moisture balance model calculated from inputs of precipitation and losses from evapotranspiration. PDSI is locally normalized, with negative values indicating drier than normal conditions (droughts) and positive values indicating wetter than normal conditions (pluvials), relative to a baseline calibration period for a given location. PDSI has persistence on the order of 12–18 months (Guttman 1998; Vicente-Serrano et al. 2010), integrating moisture gains and losses throughout the calendar year, and providing a useful metric to describe longer term trends and variability in hydroclimate. PDSI has been widely used as a metric to quantify drought using climate model simulations (e.g., Bonsal et al. 2013; Burke and Brown 2008; Coats et al. 2013; Cook et al. 2010, 2013; Dai 2011, 2013; Rosenzweig and Hillel 1993; Seager et al. 2008; Taylor et al. 2013).
Because recent work has suggested that PDSI may be intrinsically too sensitive to changes in PET (e.g., Burke 2011; Seneviratne 2012), we repeat our analysis using an alternative drought index, the SPEI. Like PDSI, SPEI (Vicente-Serrano et al. 2009) is a normalized index of drought, developed from the original Standardized Precipitation Index (SPI, McKee et al. 1993). Whereas the SPI is based on normalized accumulations of precipitation surpluses and deficits over some user-defined interval (typically 1, 3, or 12 months), SPEI uses accumulations of precipitation minus PET. Therefore, SPEI includes in its accounting both supply and demand changes in moisture variability, and can be interpreted similarly to PDSI (i.e., positive values of SPEI indicate wetter than average conditions, negative values indicate drier than average conditions). Unlike PDSI, SPEI does not include an explicit soil moisture balance accounting, and only uses information on precipitation minus PET to curve fit and calculate standardized departures of moisture availability. Similar to PDSI, SPEI has been used previously in GCM based climate projections (e.g., Barrera-Escoda et al. 2013; e.g., Hernandez and Uddameri 2013).
In the PDSI soil moisture calculation, we set the soil moisture capacities for the top and bottom layers to the standard values of 25.4 mm (1 in.) and 127 mm (5 in.). We use the 1931–1990 period from the historical runs as our baseline calibration period for the normalization, the same time interval used by the National Oceanographic and Atmospheric Administration for normalization of their PDSI calculations. In order to maximize comparability with the PDSI, we use a 12-month interval for accumulating precipitation minus PET anomalies in our SPEI calculations, and also use the same standardization interval (1931–1990). PDSI and SPEI are calculated separately for each individual ensemble member at the native resolution of the model.
Diagnostics used from each GCM are monthly values of precipitation, 2-m air temperature, surface pressure and 2-m surface specific humidity (used to calculate vapor pressure), and surface net radiation. Ground heat flux and surface wind speed diagnostics were more difficult to obtain from these models. Relative to changes in energy availability and the VPD, Penman-Monteith PET is relatively insensitive to wind speed (e.g., Scheff and Frierson 2013); we therefore set u2 = 1. Additionally, ground heat fluxes (G) are usually only a small fraction of the total surface energy budget, about 10–15 % (Betts et al. 1996; Sellers et al. 1997). Tests in which we alternately set G to 0 or 15 % of Rn indicated that the PDSI calculation is largely insensitive to this parameter. For the analyses presented herein, we therefore set G = 0.
Description of different versions of the PDSI and SPEI calculations, and the model diagnostics used in their calculation
tsurf, prec, q, rnet
tsurf, q, rnet
tsurf, q, rnet
3.1 Model climate response
The models also show regional changes in summer season (JJA in the Northern Hemisphere; DJF in the Southern Hemisphere) actual evapotranspiration (latent heat fluxes; Fig. 4e) and the ratio of latent heating to the sum of sensible plus latent heating (evaporative fraction or EF, Fig. 4f). Evapotranspiration (Fig. 4e) increases in much of the wet tropics and the Northern Hemisphere high latitudes, where evaporative demand is enhanced (via increased VPD and surface net radiation) and precipitation generally increases. These are areas where evaporation is primarily energy (rather than moisture) limited and where evaporation continues to be energy limited in the future. In the sub-tropics, where evapotranspiration is primarily controlled by surface moisture availability, evapotranspiration decreases as surface moisture is unable to satisfy the increased atmospheric demand.
Changes in EF (Fig. 4f) provide a diagnostic for changing moisture versus energy limitations to evaptranspiration in the future. Areas with declining EF are regions where evapotranspiration rates are increasingly moisture limited. This includes much of the sub-tropics, where evapotranspiration is declining, but also areas of the mid-latitudes where evapotranspiration is projected to increase (e.g., Central Plains of North America and Europe). The fact that EF decreases in areas of both increased and decreased evapotranspiration is suggestive of an overall decline in surface and soil moisture availability in these regions. Increases in EF are confined primarily to areas where precipitation is increasing and evapotranspiration is limited by energy demand, such as the high latitudes of the Northern Hemisphere.
3.2 Model PDSI and SPEI response
Developing and refining projections of hydroclimate, drought, and water resources for the twenty-first century is an active area of research (e.g., Barnett and Pierce 2009; Dai 2013; Seager et al. 2013). Toward this end, significant advances have already been made in key areas, especially in our understanding of regional and seasonal precipitation responses to warming (Chou et al. 2009, 2013; Held and Soden 2006; Neelin et al. 2003; Seager et al. 2010). Precipitation, however, does not represent the only control on ecologically and socially relevant water resources, such as streamflow, reservoir storage, and soil moisture. Evaporative demand from the atmosphere, driven by air temperature, humidity, and energy availability, can also play a critical role. It is generally accepted that a warmer world will increase evaporative demand and drying independent of precipitation changes (Dai 2011). To date, however, few efforts have been made to explicitly separate the relative contributions to future drought trends from changes in supply (precipitation) versus demand (PET).
Using the latest suite of state-of-the-art climate model projections and two indices of surface moisture balance (PDSI and SPEI), we find that robust regional changes in hydroclimate are, to first order, organized around regional changes in precipitation. Increases in precipitation cause wetting in the high northern latitudes and east Africa, and precipitation declines lead to drying in the sub-tropics and Amazon. In areas where declines in precipitation already push the climate towards drought (e.g., Central America, the Amazon, southern Africa, the Mediterranean, etc), increased PET amplifies the precipitation induced drying. Critically, PET also plays a major role in enhancing drying in the midlatitudes and along the margins of the sub-tropics, where precipitation changes are small or even positive. Globally, increased PET nearly triples the fractional land area that will experience drying exceeding one standard deviation of the PDSI index (Fig. 13) by the end of the twenty-first century, from 12 % (precipitation effects only, PDSI-PRE) to 30 % (precipitation+PET effects, PDSI-ALL). In certain regions (e.g., western North America, Europe, and southeast China), PET is in fact the sole or primary driver shifting these areas into drought. Areas dominated by the Asian monsoon (India, Indochina, etc) are some of the few places where there is little change in mean hydroclimate. In these regions, gains in moisture from increased annual and monsoon precipitation (Lee and Wang 2012; Seo et al. 2013) are large enough to compensate for any increases in PET.
Both PDSI and SPEI provide useful metrics of surface moisture balance as it relates to both supply and demand considerations. One factor neglected by these indices as formulated herein, however, is the potential effect of enhanced carbon dioxide concentrations in the atmosphere ([CO2]), which are expected to have a direct physiological effect on plants by reducing stomatal and canopy conductance, increasing the water use efficiency of plants, and thus reducing evapotranspiration and soil moisture losses. Several recent modeling studies suggest this effect could be quite important for projections of soil moisture and water resources (Cao et al. 2010; Wiltshire et al. 2013). We note, however, that empirical evidence for this water use efficiency effect as a large-scale control on the surface moisture balance is still highly uncertain. For example, Domec et al (2009) demonstrated for loblolly pine that the effect of enhanced [CO2] on stomatal conductance manifested only during times of high soil moisture, rather than drought. Naudts et al. (2013), in a simulated drought experiment, found no significant (p < 0.10) additional impact of elevated [CO2] on soil wetness, either before or after a drought manipulation (see their Fig. 4; Table 1). Other experiments have found only modest changes (<15 %) in evapotranspiration fluxes and soil water content with enhanced [CO2] (e.g., Hussain et al. 2013; e.g., Inauen et al. 2013; e.g., Stocker et al. 1997). Large uncertainties in the effect of enhanced [CO2] on future hydroclimate projections, namely through the modification of stomatal resistance, make characterizing the impact of this mechanism on a global scale simply too difficult to quantify for our purposes herein.
This analysis provides a comprehensive accounting of how PET and precipitation changes will each affect global hydroclimate at the end of the twenty-first century. For many regions, focusing on the precipitation response alone will be insufficient to fully capture changes in regional water resources such as soil moisture, runoff, or reservoir storage. Instead, increased evaporative demand will play a critical role in spreading drought beyond the sub-tropics and into the Northern Hemisphere mid-latitudes, regions of globally important agricultural production. China, for example, is the world’s largest rice producer, a crop that serves as the primary nutrition source for more than 65 % of the Chinese population (Peng et al. 2009). North America and much of central Asia are major centers of maize and wheat production; unlike China, they are also important exporters of these crops to the global marketplace (Headey 2011). Increased temperatures, and the associated heat stresses, are already expected to negatively impact crop yields in these regions (Battisti and Naylor 2009; Teixeira et al. 2013), and our analysis suggests that increases in PET due to warming and energy balance changes will have additional impacts through regional drying. Yield losses can be at least partially mitigated through management practices, such as modification of planting and harvest dates (Deryng et al. 2011). However, recent research suggests that climate change over the last 20 years is already having a deleterious impact on agricultural production (Lobell et al. 2011), and the ability of existing water resources to keep pace with future climate impacts is in question (Wada et al. 2013; Zhang et al. 2013). Even with proactive management, our results suggest increased drying, driven primarily by increases in PET, will likely have significant ramifications for globally important regions of agricultural production in the Northern Hemisphere mid-latitudes.
We acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modeling groups (listed in Table 1 of this paper) for producing and making available their model output. For CMIP, the U.S. Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. All derived PDSI and SPEI fields are available for download from http://www.ldeo.columbia.edu/~jsmerdon/2014_clidyn_cooketal_supplement.html Haibo Liu and Naomi Henderson provided computational support at LDEO. RS and JES were supported in part by the NOAA award Global Decadal Hydroclimate Variability and Change (NA10 OAR431037). RS was also supported by DOE award DE-SC0005107. Further support came from NSF award AGS-1243204 and NOAA award NA10OAR4310137. BIC was supported by NASA. LDEO Publication number #7758. We thank two anonymous reviewers for comments that greatly improved the quality of this manuscript.