16 Drought Severity in a Changing Climate Sergio M. Vicente-Serrano Pyrenean Institute of Ecology, Higher Council for Scientific Research
Santiago Beguería Experimental Station of Aula Dei, Council Superior of Investigations Scientific
Jesús Julio Camarero Pyrenean Institute of Ecology, Higher Council of Scientific Research
16.1 Introduction ......................................................................................280 16.2 Climate Impacts on Drought Severity ...........................................280 16.3 Drought Frequency and Severity under a Changing Climate ..... 283 16.4 Drought Impacts in a Changing Climate .....................................287 16.5 Summary and Conclusions .............................................................294 Authors...........................................................................................................295 Acknowledgments ........................................................................................296 References ......................................................................................................296
Abstract This chapter discusses the climatic factors that affect drought severity at the global scale and also current drought variability and trends under climate change conditions. Quantifying drought is a difficult task in a changing climate, mainly as a consequence of the uncertain contribution of water input (precipitation) and the atmospheric water demand on drought severity. The contribution of the atmospheric water demand is poorly known since to have an accurate assessment, it is necessary to consider both radiative and aerodynamic components of evapotranspiration processes, for which a high amount of data is required. The evolution of drought severity at the global scale suggests an intensification of droughts associated with enhanced atmospheric evaporative demand. Nevertheless, data uncertainty is important, and it prevents drawing definitive conclusions about this issue. The best and less uncertain alternative to quantify trends in drought severity is to focus on the recent impacts observed in water-limited natural systems. There are many evidences suggesting that the decrease of water resources, desertification processes, forest dieback and tree mortality, and lower crop yields are enhanced by recent drought trends, for which temperature rise and increased vapor pressure deficit are playing an important role.
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16.1 Introduction Drought is a natural phenomenon that affects all the regions of the world. It is a result of climate variability processes in both humid and arid regions [150]. There are several evidences of drought occurrence from paleoclimatic records [118,107]. Thus, it is largely hypothesized that droughts have played an important role to explain some societal crisis and human migrations (e.g., the fall of the Maya civilization [53]). Several natural and documentary sources record strong drought events over the past millennium in different regions. For example, ecclesiastical documents have recorded frequent and severe droughts in different regions of Europe [41]. Dendrochronological records also record the same behavior in other regions of the world, among others, China [58] and North America [33]. Instrumental records available from the eighteenth century in some regions have allowed determining drought frequency and severity with more spatial and temporal details than climate reconstructions, and they have allowed to characterize strong drought events affecting different regions of the world in the past three centuries (e.g., the 1930s Dust Bowl [108]). Therefore, drought is a natural phenomenon that occurs independently of anthropogenic effects and recent climate change processes. Nevertheless, it is hypothesized on the effects of anthropogenic climatic change on drought severity recorded in the last decades and also under future projections [38]. The main characteristic of the climate change processes is the strong temperature rise [45,62]. In fact, In some areas, such as Europe, the rising of temperatures over the last 500 years seems to be unprecedented [78], consistent with global studies [141]. Moreover, the rising of temperatures has been mainly observed during summer [92], when their effect on water availability and drought is stronger. The effect of temperature rise on drought severity is mainly driven by its control on the atmospheric evaporative demand (AED) since according to the Clausius–Clapeyron relationship, the water-holding capacity of air increases by about 7% per 1°C warming [122]. It is argued that increased vapor pressure deficit driven by higher temperatures would change the AED and would affect drought severity, mainly in arid and semiarid regions in which the AED cannot be compensated by a higher water supply to the atmosphere [144]. Moreover, climate change is not only characterized by strong temperature rise, but it also affects the evolution of other climate elements like cloudiness and sunshine duration [145], atmospheric water content [113], and wind speed [85]. The evolution of all these elements should be considered together to understand current climate change effects on drought severity [86]. Precipitation deserves independent mention given its primary importance to define drought severity. Increased AED may produce higher water vapor in the atmosphere and cause higher precipitation at the global scale, accompanied by higher frequency and severity of intense precipitation events [122], which would suggest decreased drought severity under warming conditions. Nevertheless, although this pattern may be valid at the global scale, anthropogenic atmospheric forcing may also affect atmospheric circulation patterns and alterations in the magnitude and direction of flows [127,155], which may cause noticeable drying at the regional scale. Thus, negative precipitation trends have been recorded in different regions of the world like the Mediterranean [57] and Australia [88], associated with the anthropogenic forcing. This chapter discusses the influence of climate change on drought severity, and it emphasizes how challenging it is to find an answer to the following question: Is climate change increasing drought severity worldwide? This chapter focuses on the climate forcing that may cause modifications in drought severity and revise recent ideas on the evolution of drought severity worldwide. Finally, it provides objective evidences of possible increased drought severity based on drought impacts in drought-prone natural systems and economic sectors.
16.2 Climate Impacts on Drought Severity Drought cannot be directly measured by any instrument. It affects a number of natural systems and economic sectors, and it is usually detected only when an impact is recorded (streamflow reduction, crop failure, urban supply restrictions, forest mortality, etc.). Therefore, drought stress is defined as a function
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of the impact it exerts on any system. Determining the effect of climate change on drought severity is a difficult task due to the lack of long time series and accurate measurements to assess objective drought conditions at the global scale (e.g., streamflows, soil moisture, and lake levels). This situation worsens if the effects of water management and land transformation on drought severity are considered, making a separation of climatic and artificial signals difficult. For these reasons, the assessments of climate warming impacts on drought trends at the global scale have been based on climatic information [17,37,110,125]. Sometimes, there is no agreement between climate anomalies and water shortages in a given natural or economic system since there are other relevant components, such as the drought vulnerability and possible adaptation measures. Nevertheless, although human management may also determine drought severity, the origin of most drought events is related to climate variability. For these reasons, and also given the available climate information with a wide spatial and temporal perspective, drought trend analyses have been usually based on climatic drought indices, which try to be synthetic measures of drought severity that are calculated using time series of different climate variables, mainly precipitation data [84] or precipitation and the AED [17,91,128]. In general, these indices are good proxies to determine drought conditions in a variety of environmental, hydrological, and agricultural systems [132]. To determine the impact of climate change on drought, it is necessary to determine the effect of different climate elements on drought severity. The impact of precipitation on drought severity is evident. A long period characterized by a precipitation decrease would produce a cascade effect on different usable water sources: soil moisture, streamflow, reservoir storages, groundwater, etc. [27,84]. The severity of drought would be proportional to the magnitude of the precipitation decrease. Although global precipitation shows a dominant positive trend [115], there are strong spatial differences. For instance, in Western Africa, the Sahel, eastern Australia, and the Mediterranean, the precipitation has noticeably decreased in the last five decades [115]. It is evident that drought severity and climate aridity have increased as a consequence of the precipitation evolution in these regions [11,57]. Whereas the effect of precipitation on drought is evident, much less is known on the possible effects of the other major climate components of drought severity, that is, the AED. AED is very difficult to measure, and although specific instruments are available (e.g., pans, evaporimeters, lysimeters, and eddy covariance towers), measurements usually have several uncertainties [1] and they are commonly only available at very sparse punctual measurements. AED depends on the combination of two components [95]: (1) radiative, which is related to the available energy necessary to transform sensible to latent heat, and (2) aerodynamic, which is related to the air capacity to store water vapor. The radiative component depends on the available solar radiation, which is determined by the variability in the cloud coverage [106] and also by the contents of particles in the atmosphere, as aerosols [81]. The aerodynamic component depends on the air temperature, which determines the vapor pressure and the relative humidity, which represents the difference between the quantity of water that the atmosphere may hold (as a function of the temperature) and the total vapor content, and the wind speed, which determines the rate of replacement of saturated air by unsaturated air to continue evaporating. Changes in any of these variables, and not only changes in temperature, affect the AED [86]. The AED is also commonly enunciated as potential (PET) or reference evapotranspiration (ETo) [143]. Nevertheless, the use of PET is strongly discouraged due to ambiguities in its definition. Here, it is referred to ETo as the best estimation of the AED. Moreover, the concepts of actual evapotranspiration (ETa) and reference evapotranspiration (ETo) must be clearly defined. ETa is the water lost under real conditions (e.g., water availability, vegetation type, physiological mechanisms, climate), whereas ETo represents the AED of a reference surface (generally a grass crop having specific characteristics), and it is assumed that water supply from the land is unlimited [4]. Thus, ETa will be less than or equal to ETo, but never greater. Some authors suggested that using ETa is better than ETo to determine drought severity under global warming conditions since ETa and not ETo would determine the surface water balance and the drought conditions [36,61]. The proponents of this idea (i.e., the use of the difference between precipitation and ETa) explain that, compared to ETo, ETa would always be a better estimation of the amount of water actually transferred to the atmosphere. Thus, ETa would allow for obtaining a better estimation of the soil water balance than ETo. Nevertheless, ETa does not
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consider the highest possible AED with the current water availability. ETa would be a poor estimator of this demand, since it depends in turn on the current water availability. On the other hand, the very definition of ETo indicates that it refers to the maximum amount of water that would be transferred to the atmosphere by the soils and vegetation if there were no water supply deficit. Using the ETo as an estimator of the true evaporative demand seems, thus, a more convenient choice. In fact, using the ETa as a component of the drought severity would make sense as a replacement of precipitation. Indeed, the ETa would be a better estimator than the amount of rain water actually used by the vegetation; hence, the balance ETa–ETo would provide a better indicator of the stress (or lack of stress) that the system is undergoing in any given moment. Nevertheless, precipitation is commonly used as an estimator of water available to the system because of the difficulties involved in estimating ETa (see Beguería et al. [17] for further analysis and discussion of ETa and ETo use in drought indices). It is commonly assumed that global warming conditions increase AED given the higher water- holding capacity of the air. Indeed, global warming is expected to increase the intensity of the global hydrological cycle, with increases predicted in both ETa and precipitation [22,59]. Observations show that AED may show contrasting trends among regions and periods as a consequence of the evolution of the different climatic factors that control this variable. Some studies suggest that the vapor pressure deficit, that is, the difference between the amount of moisture in the air and how much moisture the air can hold when it is saturated and determined by warming processes and available absolute humidity, is driving the evaporative demand of the atmosphere, mainly in semiarid regions [144]. Other studies have argued that the effect of climate warming on AED is minimal, and other meteorological variables (including solar radiation and wind speed) are more important [87,104,105]. Brutsaert [22] indicated that current AED processes may not be linked to any individual process, and their changes are partially attributed to modifications in solar irradiance, relative humidity, and wind speed changes. Therefore, there are strong uncertainties in current AED trends and their driving factors worldwide. Regional and local studies based on observational datasets have shown a variety of results in different regions of the world. In some cases, the trends in AED have been negative, including for the Yangtze River [154], the Yellow River [79], and the Tibetan Plateau [159] in China. These trends have mainly been linked to changes in agricultural practices (increased irrigated lands), decreases in wind speed and solar radiation, and an increase in relative humidity [51]. Other studies have shown positive trends in the AED, including areas in Central India [39], Iran [66,119], and Florida [1]. Moreover, in some areas (e.g., Australia), there has been a large spatial variability in the evolution of ETo over the recent decades [42]. The regional evolution of aerodynamic and radiative components would explain the divergent AED trends. For example, Lawrimore and Peterson [67] and Golubev et al. [48] suggest that soil water content, ETa rates, and their influence on the vapor pressure deficit would play a major role to explain ETo variability and trends in water-limited environments. Thus, the importance of the aerodynamic component has also been recently stressed by Wang et al. [144], who indicated that their trends accounted for 86% of the long-term trends of the evaporation between 1973 and 2008 at the global scale. Nevertheless, other studies have provided an alternative theory, in which a decline in incoming solar energy at the global scale would drive ETo changes [71,82,104,117]. Some regional studies support this hypothesis. In Greece, Papaioannou et al. [93] and Kitsara et al. [65] have recently shown close agreement between ETo evolution and sunshine duration variability. Ambas and Baltas [8] provided similar results in Macedonia, and Fan and Thomas [46] in Southwest China. The Iberian Peninsula, as a representative example in the Mediterranean region, is an area that has experienced a strong temperature increase in the last five decades [20] and general precipitation decreases [136], which coincided with a general increase of the AED [21,28,137]. For example, in the Iberian Peninsula, the AED has increased by 122.5 mm per year between 1961 and 2011 [137] (Figure 16.1). The results found in the Iberian Peninsula, and particularly in Spain, highlight the complexity of knowing what is the evolution and drivers of the AED since although wind speed in Spain decreased between 1961 and 2011 [13], surface solar radiation and sunshine duration (cloudiness) have markedly increased (decreased), particularly since the 1980s (1960s) [106]. Moreover,
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relative humidity in Spain decreased by 1% per decade between 1961 and 2011, as a consequence of the reduced water supply from the land and oceans [136]. These changes are clearly driving a large increase in ETo, mainly as a consequence of the evolution of the aerodynamic component [138]; nevertheless, it is completely impossible to separate the effects of relative humidity, temperature, and precipitation in explaining changes of ETo, because the observed changes of relative humidity are strongly linked to the strong warming process with decreased precipitation [135] and reduced soil moisture leading to reduced water potentially available to the atmosphere and dramatically lower relative humidity.
16.3 Drought Frequency and Severity under a Changing Climate Given the strong uncertainty in determining changes in the AED, it is not surprising to find no consensus on the evolution of drought severity in the last decades under global warming conditions. Thus, a recent Intergovernmental Panel on Climate Change report on climate extremes indicates only low-tomedium confidence of drought trends at the global scale [109]. This statement is not only related to AED issues but also to strong problems for drought definition and quantification [73,149,150] and also to how the different drought indices account for precipitation and AED effects on drought severity [140]. Dai [37] suggested that global warming is noticeably increasing drought severity at the global scale. He used a popular drought index, the Palmer Drought Severity Index (PDSI), and global coarse resolution gridded datasets to affirm that the PDSI shows a dominant negative trend in the last decades as a consequence of increased AED. He used two different algorithms to estimate AED: the Thornthwaite equation [121], which is based on air temperature data, and the FAO-56 Penman–Monteith equation [4] based on air temperature, relative humidity, solar radiation, and wind speed. He showed small differences among the two methods used to estimate the AED. Sheffield et al. [110] also used both methods to estimate AED at the global scale but showed that drought severity has not changed noticeably in the last decades. The latter authors stressed the limitations of using temperature-based methods to estimate the AED and the need for considering the whole aerodynamic and radiative components. Donohue et al. [42] analyzed recent changes in ETo in Australia using five different formulae. They reported very diverse spatial and temporal changes based on the various methods and indicated that those methods based only on temperature variables (e.g., Thornthwaite) tended to underestimate ETo-positive and ETo-negative changes. Chen et al. [30] also compared the FAO-56 Penman–Monteith and Thornthwaite methods using 580 meteorological stations in China and showed that the Thornthwaite method overestimated or underestimated ETo, depending on the region being analyzed. Moreover, van der Schrier et al. [125] showed that uncertainties in drought severity trends assessed by means of the PDSI are not only related to the AED methodology and the use of different calibration periods in the PDSI may have noticeable impacts on drought trends; this introduces more uncertainties on current changes in drought severity. The PDSI has noticeable limitations, which have been widely reviewed by several authors [3,7,63,116,146]. One of the main problems is that the parameters necessary to calculate
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the PDSI are determined empirically, and most of the data used in testing the index were derived in the United States and are not applicable to other regions [3], significantly limiting geographical comparisons [50,55]. This problem was partially solved by the development of the self-calibrated (sc) PDSI [147], but the problems in spatial comparability in drought severity have not been completely solved. Wells et al. [147] stated that “It is important to note that, while the (sc) PDSI is more spatially comparable than the PDSI…, it is not as comparable as an index computed using nonlinear methods (e.g., the Standardized Precipitation Index).” The problems of spatial comparability for the PDSI and (sc) PDSI were clearly illustrated by Vicente-Serrano et al. [129], who showed that the PDSI represents water deficits at different time scales, depending on the region under consideration. This problem was initially investigated by Guttman [49], who showed that the spectral characteristics of the PDSI varied from site to site. In other words, the time scales of the PDSI and the (sc) PDSI are not fixed because they depend on the characteristics of the sites and vary spatially. This makes it difficult to assess what kind of deficit the index is representing and makes spatial comparisons problematic. The non-normality of the (sc) PDSI series, which show more frequent dry than humid periods in different regions of the world, and uncertainties in estimates of the water field capacity also limit its use. Thus, Karl [63] analyzed the sensitivity of the PDSI to the water field capacity parameter and, consistent with the findings of Weber and Nkemdirim [146], reported that areas of greater water field capacity are more likely to be affected by drought. Moreover, the PDSI lacks flexibility to adapt to the intrinsic multiscalar nature of drought, which is necessary in determining the varied impacts of drought for hydrological, ecological, and agricultural systems [131]. For these reasons, the use of a robust nonlinear drought index, which can be calculated on different time scales and can account for the effect of both precipitation and ETo on drought severity, appears preferable to the use of the PDSI (or scPDSI). The Standardized Precipitation Evapotranspiration Index (SPEI) [128] combines the sensitivity of the PDSI to changes in evaporation demand (caused by temperature fluctuations and trends) and the simplicity of calculation and the multitemporal nature of the Standardized Precipitation Index (SPI) formulated by McKee et al. [84]. The SPI is based on the conversion of precipitation data to probabilities based on long-term precipitation records computed for different time scales, and it has been accepted by the World Meteorological Organization as the reference drought index [54]. Nevertheless, the SPEI resolves the main criticism of the SPI, namely, that it is based on precipitation data alone. As the SPI, the SPEI is perfectly comparable in time and space and across different time scales, as it corresponds to a standard normal variable. Thus, the same SPEI values occur with the same frequency in all regions of the world, independent of the climate characteristics of the region. This index provides objective information on climatic drought conditions, as it relies only on climate data and is not influenced by external variables. The SPEI works as a perfect supply and demand system equally sensitive to changes in precipitation and ETo [140]. Using data of precipitation and ETo from the Climate Research Unit (CRU) [52] at the spatial resolution of 0.5°, 12-month SPI and SPEI at the global scale were obtained. The CRU ETo is calculated using the FAO-56 Penman–Monteith equation assuming a constant 2 m s−1 wind speed at the global scale. Ordinary least-squares regression was performed for determining the magnitude and significance of the deviations between the time series of SPI and SPEI at a cell-by-cell basis. Time series of the monthly difference SPEI–SPI were regressed upon time at each grid cell. The decadal change in SPEI with respect to SPI was mapped for those cells that had significant trends at a significance level of α < 0.05. Comparison of the time series of global SPI and SPEI datasets revealed a distinct behavior of the two indices, most notably toward the end of the study period (Figure 16.2). Before ca. 1995, all the indices showed a clear decreasing trend, indicating a progressive shift toward drier conditions. This was a direct consequence of the evolution of global precipitation. After ca. 1995, however, the departure between the SPI and the SPEI was very pronounced, with a clear positive trend in the case of the SPI, which changed its main behavior of the previous decades due to increasing global precipitation, while the SPEI continued decreasing, that is, toward drier conditions. The most remarkable result is that, even though the most humid years with respect to precipitation occurred in the last 10 years of the study period, the global SPEI remained in very low (negative) values due to increased potential evapotranspiration. Cell-by-cell
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evolution of the SPI and SPEI shows different patterns in large regions of the world with more negative SPEI evolution in comparison to the SPI (Figure 16.3). Cell-by-cell regression of the SPI–SPEI difference showed that this behavior was not spatially homogeneous but predominated over certain regions of the world (Figure 16.4). Significant negative trends predominated overall (46.1% of nondesert lands), while significant positive trends were marginal (7.2%). The difference between both indices increased from the equator, where there was almost no change, toward higher latitudes where the average difference was −0.04 standard deviations per decade (up to −0.3 standard deviations at the end of the period). Some areas specially affected by the shift in the SPEI were the northern and southern extremes of America, most part of Africa, Europe, and Australia, and some regions in central and East Asia. On the contrary, no significant or even positive trends were found in the conterminous United States, Central and South America, and in some regions in central Asia. This analysis based on robust drought indices suggests that drought severity may be increasing in the last decades associated with higher AED at the global scale. Nevertheless, strong uncertainties remain, which are related with the available global data sources that avoid for definitive conclusions on the global evolution of drought severity. Trenberth et al. [123] have suggested that uncertainties in global gridded datasets are very important. This is observed for variables characterized by sparse net of observatories (e.g., relative humidity, solar radiation, and wind speed), but also precipitation nets are showing strong deficiencies related to the number of available observatories and the homogeneity of the datasets. These statements stress the need for working with high-quality data to increase the goodness of drought severity trends, being this only possible at the regional level. An example of the evolution of drought severity based on high-quality and homogenized data of the different variables necessary to obtain accurate evolution of the AED is shown for the Iberian Peninsula. In this region, drought severity and the surface affected by drought have increased in the last two decades associated with increased ETo [139]. Thus, comparing the SPI and SPEI, the percentage of the surface affected by drought has increased by 40% in recent drought episodes given the influence of the AED (Figure 16.5). The current availability of regional studies with high-quality data is not enough to allow drawing definitive conclusions on recent drought trends at the global scale. Moreover, since global studies based on gridded datasets show strong uncertainty given data deficiencies, the best approach to deepen the knowledge of the recent drought severity evolution is to focus on impact studies in natural systems and economic sectors highly vulnerable to the drought severity, such as water resources, forests, and crops.
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16.4 Drought Impacts in a Changing Climate The hydrological processes are complex in nature, and there exists a high degree of uncertainty at different spatial scales. Therefore, isolating the effects of recent drought severity on water resources is not an easy task because other factors, such as socioeconomic variables, can modify the processes mainly by altering land use and plant cover changes (e.g., creation of new irrigation areas, farming processes, and
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changes in crops). Drought effects on systems also depend on the building of new infrastructures (dams and irrigation channels) or shifting socioeconomic changes in water demands for human, touristic, or industrial consumption. These complex relationships explain the lack of studies trying to quantify the possible consequences of warming processes on water availability and objective drought conditions. Notwithstanding, some invaluable findings are revealed by experimental research, confirming the negative impacts of global warming on water availability. In the United States, Walter et al. [142] quantified the warming effect by catchment balance in areas with low perturbation and reported an increase of water losses by ETo from 1950 onward. Cai and Cowan [23] employed the same approach in Australia, suggesting a 15% decrease of water losses for each +1°C of warming between 1950 and 2007. Similar results have also been presented, such as those by Lespinas et al. [68] in south of France and Yulianti and Burn [157] in Canada. Recently, Liang et al. [70] used spectral analyses to indicate that in the last decades the response of discharge of the Yellow River in China is more strongly related to warming than precipitation, suggesting the intensification of ETo effects on discharge. García–Ruiz et al. [47] provided a complete review of the evolution of water resources in the Mediterranean region, showing a dominant decrease of streamflows related to land transformations and human management but also to recent climate variability. Teuling et al. [120] analyzed in four European monitored basins the influence of the AED on streamflows, showing that a significant increase in evapotranspiration during drought episodes acts amplifying the storage anomalies. This pattern has also been observed in other world regions; in south Quebec, Assani et al. [12] showed a significant decrease of winter streamflow in spite of a significant increase in fall precipitation and streamflow. These authors suggested that the increase in evaporation (and decreased infiltration) could account for this. In the Murray–Darling basin (Australia), recent droughts have seen a larger reduction in annual runoff in many places compared to historical droughts with similar annual rainfall reductions, suggesting that the AED may be playing a noticeable role to increase streamflow drought severity [24,31,96,101,156]. Silverstein et al. [112] showed the same pattern in 13 other major fresh water and brackish river basins in southwestern Australia, in which clearly streamflow has decreased much stronger than observed precipitation decrease. In the Iberian Peninsula, Vicente-Serrano et al. [139] have demonstrated that increased AED has contributed to the decrease in the surface water resources. Figure 16.6 shows the correlation between the average monthly streamflow droughts, characterized by means of the Standardized Streamflow Index (SSI) [133] and the monthly SPI and SPEI at time scales from 1 to 24 months in natural basins of the Iberian Peninsula (not affected by regulation and/or water extraction upstream the gauging stations). The SPEI, which accounts for both precipitation and AED, correlated with SSI much better than SPI over the basins. Thus, in summer months, in which dryness is more important, and the availability of water resources is much lower, the SSI interannual variability shows stronger correlations with the SPEI than with SPI, which also suggests that the AED increase is largely contributing to increasing drought severity. All these studies seem to agree on the negative impact of temperature and AED on water resources, mainly in water-limited areas such as arid and semiarid regions. The evolution of streamflow records in temperate basins of the northern hemisphere was analyzed from the monthly streamflow dataset provided by Dai et al. [35], which contains global monthly river discharges from 1950 to 2004. Basins located in cold regions were not included since the effects of increased AED and temperature may be complex given interactions with snow melt processes and permafrost depletion. One hundred and thirty-seven basins from the cited dataset were selected (Figure 16.7a) and annual streamflow anomalies from 1950 to 2004 were calculated (Figure 16.7b). There is a strong decrease in the average annual streamflow in these basins between 1950 and 2004, mainly from 1980 onward, which means an acute decrease in the availability of water resources. There is a significant relationship of streamflow anomalies with the SPI (α < 0.001, R2 = 0.274). Nevertheless, the relationship is much stronger considering the SPEI (α < 0.001, R2 = 0.523). The dashed line in Figure 16.7 shows the evolution of the annual average SPEI in these basins (calculated from the CRU dataset), which shows high agreement with the annual streamflow anomalies. This suggests that water yield in basins mostly driven by precipitation–runoff processes depends on recent AED evolution that is increasing the severity of streamflow droughts in comparison to the expected decrease by the precipitation variability.
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Evolution of vegetation in water-limited natural systems may also provide information on how drought severity may be changing under climate change conditions. Arid and semiarid regions are among the most vulnerable to climate variability and drought given unstable land conditions and the marked abiotic constraints driving the typically unstable equilibrium of these ecosystems, which are a product of both water limitations and millennial human pressures [43,69]. Drought has commonly been considered a major factor triggering desertification processes, and several studies have highlighted the importance of drought episodes in explaining the occurrence of serious vegetation degradation [89,97]. Drought itself cannot be considered a factor explaining degradation trends, given the common resilience of semiarid vegetation communities and the general recovery of the vegetation following an increase in precipitation [56,90]. However, a higher drought frequency associated with global warming could trigger land degradation and desertification. This pattern has been observed in highly vulnerable semiarid shrublands on a gypsum substrate of northeast Spain. Vicente-Serrano et al. [134] found there a dominant trend toward decreased vegetation cover, mainly in summer and in areas affected by the most stressing drought conditions (low precipitation, higher evapotranspiration rates, and sun-exposed slopes). In this region, no significant trend was found for annual precipitation (Figure 16.8). In contrast, ETo has dramatically increased from approximately 1150 mm in the 1970s to approximately 1250 mm in the 2000s, which suggest that the ETo
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increase related to global warming is driven by higher drought severity in comparison with precipitationrelated droughts and it is in the root of the observed shrubland degradation. Nevertheless, although water resources in drought-prone basins and land degradation in semiarid regions are providing evidences of possible increased drought severity associated with higher AED, the strongest evidences are recorded in forest ecosystems. In recent decades, there are recorded unprecedented impacts of drought in forest ecosystems across the world, including high frequency of extraordinary mortality and forest dieback episodes [5,9,98,152]. Allen et al. [6] reviewed the impacts of warming and drought on tree mortality worldwide. Although they stated that episodic mortality occurs in the absence of climate change, the high number of existing evidences suggests that at least some of the world’s forested ecosystems already may be responding to climate change. This indicates that forests may become increasingly vulnerable to higher background tree mortality rates and die-off in response to warming and drought, even in environments (e.g., temperate areas) that are not normally considered water limited. The most relevant characteristic of this pattern is that it has been recorded both in humid and arid sites and in regions in which precipitation has not decreased in the last decades [94,102,153]. These studies have stressed that increased drought impacts on forests are mainly related to temperature rise and/or the increase of AED. The strong role of temperatures on drought severity was evident in the devastating 2003 central European heat wave, in which extreme high temperatures dramatically increased evapotranspiration and exacerbated summer drought stress [103], drastically reducing aboveground net primary production (ANPP) [32]. Similar patterns were observed in the summer of 2010 with a strong heat wave that increased drought stress in forests and produced large forest fires in eastern Europe and Russia [16]. Thus, empirical studies have demonstrated that higher temperatures amplify drought stress and enhance forest mortality under precipitation shortages [2]. To illustrate how warming processes are reinforcing drought stress and related ecological impacts worldwide, Breshears et al. [19] enunciated the term global-changetype drought to refer to drought under global warming conditions. In the presence of plant cover, the atmospheric demand of water would be supplied by plant transpiration. Under such circumstances, warming would imply an increase in transpiration to control plant temperature, and then responses to air warming would include such increased transpiration [22,42]. Water flows through the vascular system of tracheophytes (plants with lignified conducting cells) due to the existence of a water potential gradient between the roots in the soil and the transpiring leaves in order to satisfy the evaporative demand set by the atmospheric conditions. Together with water, part of the nutrients and other chemical substances (e.g., sugars) are transported within the plant. Under drought conditions (deficient water availability to meet the atmospheric demand), the hydraulic tension within the xylem may increase excessively, depending on stomatal conductance, provoking xylem cavitation and drought-induced embolism that in turn causes a severe loss in hydraulic conductivity [29,83]. Plants may cope with this excessive water loss by closing their stomata and thus reducing ETa. Nevertheless, independently of soil water availability and ETa, if water demand (ETo) increases over a certain threshold, the physiological mechanisms may collapse producing cellular and tissue damage and even plant death. That is, plants may die due to high water demand by the atmosphere (ETo) and even when precipitation or ETa shows little change. Therefore, the hypothesis of global warming strengthening drought severity has strong consistency from an ecological point of view. There are a number of recent studies that have related higher tree mortality, including dieback events and growth decline in response to warming-related increase of drought stress [14,15,18,26,64,72,80,130]. Zhao and Running [160] showed at a global scale that between 2000 and 2009 the annual ANPP decreased because of the combined effects of severe drought stress and high temperatures that induced high autotrophic respiration levels, indicating that NPP decreases because of warming-associated drying trends. Peng et al. [94] estimated tree mortality in natural stands throughout Canada’s boreal forests and found that tree mortality rates increased by an overall average of 4.7% per year from 1963 to 2008 associated with a global-change-type drought. Van Mantgem and Stephenson [126] tracked the fates of 21,338 trees in a network of old-growth forest plots in the Sierra Nevada of California and found that the mortality rate increased significantly over the 22 years of measurement (1983–2004) in different taxonomic groups
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FIGURE 16.9 Radial growth patterns (mean basal area increment ± SE) of Pinus halepensis trees near Zaragoza (NE Spain) related to the June SPEI calculated at scales of 10 months (SPEI10). The vertical gray boxes indicate the severe 1995, 2005, and 2012 droughts.
and elevational positions. They attributed this pattern to a temperature-driven increase in drought severity and suggested that these forests may be highly sensitive to temperature-driven drought stress. An example of the forest decline associated with increased drought severity driven by enhanced AED has recently been shown by Camarero et al. [25] in semiarid forests of northeast Spain. In this area, Aleppo pine (Pinus halepensis) are dominant, being acclimated to frequent drought episodes [158]. Thus, P. halepensis trees at least 100 years old are found in that region despite living under highly limiting conditions. Forest growth in the region shows a strong year-to-year variability associated with changes in precipitation [40]. Nevertheless, in the last decades, these forests show unprecedented growth decline and dieback episodes characterized by rapid defoliation and increasing mortality rates (Figure 16.9), which may not be solely explained by the precipitation evolution, but by drought reinforcement by enhanced AED. In northeastern Spain forest, a strong AED increase in the last decades has been associated with rising temperatures and increasing vapor pressure deficit [138], which is one of the main factors of the aerodynamic component in ETo. Williams et al. [153] also showed in Southwestern United States that the aerodynamic components are a potent driver of regional forest drought stress and tree mortality. Indeed, they derived a forest drought-stress index using a comprehensive tree-ring dataset and showed that forest drought stress is mainly driven by the warm-season vapor pressure deficit. Thus, they showed that in the last two decades forest activity has noticeably decreased and mortality increased without recording noticeable changes in precipitation. Other recent studies have also stressed the importance of changes in the aerodynamic component of the AED to explain current trends in forest mortality [10,44,151,153]. The vapor pressure deficit has been suggested by Vicente-Serrano et al. [138] as the main driver of increased ETo in the western Mediterranean region under conditions of reduced precipitation and relative humidity. Although there are a few regional studies that have determined possible evolution of the vapor pressure deficit with confident data [60], Simmons et al. [113] have suggested a recent general decline of relative humidity at the global scale, and Wang et al. [144] have shown that vapor pressure deficit has been the main driving factor of ETo trends in the last two decades worldwide. These results seem to reinforce that the aerodynamic component of the equation developed by Penman in 1948 may play an important role in the increase of drought severity under global warming conditions, suggesting that assumptions on constant relative humidity in a warming world [34] should be revisited. Thus, global warming impacts on drought’s severity seem also to be recorded in other highly relevant human sectors such as agricultural production [100]. Warming processes are also probably the triggering factor of the decline in world agricultural productions observed in the last few years since short-term
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enhanced temperatures can cause largely reduced yields of major crops [74]. These effects were reviewed for wheat by Porter and Gawith [99] and for annual crops by Wheeler et al. [148]. Trnka et al. [124] analyzed the agroclimatic conditions in Europe under climate change and showed that for many environmental zones, there were clear signs of deteriorating agroclimatic conditions in terms of increased drought stress associated with warmer summer conditions. There are also empirical studies, using crop yields records, demonstrating that global warming is noticeably increasing drought-related impacts. Lobell et al. [75] used a dataset of more than 20,000 historical maize trials in Africa and showed that each degree day spent above 30°C reduced the final yield by 1% under optimal rain-fed conditions and by 1.7% under drought conditions. Also Lobell et al. [76] used 9 years of satellite measurements of wheat growth in northern India to monitor the rates of wheat senescence following exposure to temperatures greater than 34°C and detected a statistically significant acceleration of senescence from extreme heat above and beyond the effects of increased average temperatures. More recently, Lobell et al. [77] have also analyzed the sensitivity of maize yields to drought in the United States from 1995 to 2012 showing that although yields have increased in absolute value under all levels of stress for both crops, the sensitivity of maize yields to drought stress associated with high vapor pressure deficits has increased and that greater sensitivity has occurred despite cultivar improvements and increasing carbon dioxide concentrations in the atmosphere. The evolution of global wheat crop yields between 1960 and 2010 was analyzed. They were obtained from the U.N. Food and Agriculture Organization (http://faostat.fao.org). A detrended global wheat crop yield time series was obtained by subtracting the linear trend that is attributable to technological advances in cropping systems [74]. SPI and SPEI time series calculated from the CRU dataset were then constructed for wheat yields as a weighted average over the grid cells. Weights for each cell were calculated based on the map of the percent land occupied by wheat crops (http://harvestchoice.org; Figure 16.10). Ordinary least-squares regression was conducted between the wheat yields and the SPI and SPEI series. ANOVA test was used for checking the significance of the models. There was a good correspondence between the time variation of global wheat crop yields and the SPEI (Figure 16.10). Regression of wheat crop yields against the SPI was not significant (α = 0.202), while the one calculated with the SPEI was (α < 0.001). This demonstrates that wheat crops are influenced not only by precipitation but also by changes in ETo. A multivariate regression model was capable of explaining up to 54.5% of the variance of global crop yields. According to the model, the decrease in wheat crop yield due to drought was −0.23 Mg ha−1 (or −7.25% with respect to the expected yield in 2006), and such reduction in yield occurred when the divergence between the SPI and the SPEI was the largest.
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16.5 Summary and Conclusions This book chapter has revised a highly relevant topic related to drought: how this phenomenon may be changing under current climate change scenarios. It discusses the main drivers of drought severity, and it stresses uncertainties in the estimation of one of the main components of drought severity: the AED. Under climate warming conditions, the severity of drought episodes is not only driven by precipitation, but also AED is gaining importance. Nevertheless, the complexity of determining AED makes it difficult to assess the role of its recent evolution on drought severity. AED does not only depend on air temperature but on the combination of radiative and aerodynamic components. In addition to air temperature, other variables are necessary to estimate the AED: relative humidity (or vapor pressure deficit), solar radiation, and wind speed. To determine how AED is changing, we need to consider the observed evolution in all these variables together; on the contrary, any AED estimation will be biased [86]. Global studies on the evolution of drought severity considering the current warming scenarios show strong uncertainties related to the availability and quality of the climate observations necessary to have a reliable assessment of the AED evolution. Precipitation gridded datasets also show some uncertainty. Nevertheless, robust drought indices like the SPEI, which consider both the contribution of precipitation and the AED, suggest an enhancement of drought severity associated with precipitation decreases at the regional scale and the generalized temperature increase at the global scale. Thus, it is suggested that global warming may be causing the increase in vapor pressure deficit, which contributes to increased land aridity and triggers extreme drought events. The available water resources in large areas of the world, in which precipitation does not show changes (or even if it has decreased), cannot supply
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the atmospheric water demand to maintain relative humidity constant [111]. Lower temperature rise in oceans in comparison to lands may also contribute to this pattern [136]. Global and regional studies in water-limited systems (streamflows, arid ecosystems, forest landscapes, and agricultural areas) suggest lower water availability in the last decades related to increased AED. The recent evolution of water resources in water-limited basins seems more related to the evolution of both precipitation and AED than only to the precipitation variability. This chapter suggests that AED is essential to know the drought severity evolution since the evolution of water resources, observed desertification processes, and the enhanced frequency of forest dieback episodes cannot be only explained by the evolution of precipitation. In summary, there are still large uncertainties in the evolution of drought severity given the complexity of the drought phenomenon, the difficulties for drought quantification, and the large existing data gaps for both objective drought quantifications based on impacts and the assessment based on climate variables, mainly for the AED assessment. In any case, the results of several regional studies from different areas of knowledge and the evidences provided in this chapter suggest that the severity of drought may be increasing in recent decades as a consequence of global warming. The state-of-the-art climate change projections for the twenty-first century predict enhanced warming at the global scale with low uncertainty among models [115]. It is expected that under these projections, drought will be a more complex and severe phenomenon. Associated agricultural, environmental, and economic impacts will probably increase, being important subjects for future global policies [114].
Authors Sergio M. Vicente-Serrano is currently a researcher in the Department of Geoenvironmental Processes and Global Change at the Pyrenean Institute of Ecology (Spanish National Research Council). Dr. Vicente-Serrano’s main interest deals with different environmental topics related to global change such as changes in land cover, the influence of general atmospheric circulation on water resources availability, and mainly drought studies from different perspectives: drought indices, variability, trends, environmental consequences, and mapping. He has worked in national and European projects devoted to remote sensing, water resources management, and droughts and published more than 150 papers in international journals of the fields of meteorology and atmospheric sciences, water resources, geosciences, remote sensing, etc. Dr. Vicente-Serrano received a bachelor’s degree in geography from the University of Zaragoza, a master’s degree in remote sensing from the Instituto de Estudios Espaciales de Cataluña (IEEC), and a doctor’s degree in physical geography from the University of Zaragoza. Santiago Beguería is a tenured scientist working in the Department of Soil and Water at Estación Experimental de Aula Dei (EEAD-CSIC) in Zaragoza, Spain. Prior to this, he held research positions at Utrecht University (the Netherlands) and at CSIC (Spain). He holds a PhD in physical geography at the University of Zaragoza (Spain). His research interests range from hydrology and water resources management to climate science and ecology. A specialist on spatial statistics, process-based modeling, and numerical methods in general, but at the same not indifferent to other, softer, approaches to complex environmental problems involving natural and socioeconomic aspects. He has authored or coauthored more than 100 scientific publications, including more than 90 articles in international peer-reviewed scientific journals. Jesús Julio Camarero is a forest ecologist and dendroecologist working in the Department of Biodiversity and Conservation, Pyrenean Institute of Ecology (Spanish National Research Council, CSIC). His main interests include understanding how major global-change drivers (climate warming, land use changes, increasing atmospheric CO2, and nitrogen deposition) and climatic stressors such as drought determine patterns and processes in forests. He is mainly interested in long-term growth responses to those drivers and related dynamics such as regeneration, mortality, and ecotone shifts. He has published several
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articles on these subjects in international journals of the fields of ecology, plant sciences, global change, forest sciences and climatology. Dr. J. Julio Camarero received a bachelor’s degree in biology from the University of Salamanca and a PhD in ecology from the University of Barcelona.
Acknowledgments This work was supported by the research project CGL2014-52135-C3-1-R and Red de variabilidad y cambio climático RECLIM (CGL2014-517221-REDT) financed by the Spanish Ministry of Economy and FEDER and “LIFE12 ENV/ES/000536-Demonstration and validation of innovative methodology for regional climate change adaptation in the Mediterranean area (LIFE MEDACC)” financed by the LIFE program.
References
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