2000 years of Drought Variability in the Central United States

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e land use practices, and 2) enhanced likelihood of drought due to global warming. As the limits of productive agricultural lands have been reached, more marginally arable lands have been put into agricultural production in times of favorable climatic conditions and through the use of irrigation. This practice has resulted in an increasing vulnerabil- ity to drought in many areas of the Great Plains (Lockeretz 1978; Barr 1981; Hecht 1983). Although the total acreage of irrigated land is not great, irriga- tion has been an important factor in the increase in cultivated acreage. The High Plains (Ogallala) Aqui- fer supplies 30% of the ground water used for irriga- tion in the United States (United States Geological Survey 1997) and is the primary source of water for irrigation in the Great Plains. Since the time of devel- opment, pumping of this ground water resource has resulted in water -level drops of more than 15 m in parts of the central and southern plains, with drops that ex- ceed 30 m in several locations, and is already depleted in some areas (Glantz 1989; White and Kromm 1987; United States Geological Survey 1997). The impacts of drought in these marginal areas have been tempered through social support, but these mitigation measures have been costly. Federal aid costs (disaster assistance, crop insurance, and emer- gency feed assistance) for the 1988 drought amounted to $7 billion with additional aid supplied by individual states (Riebsame et al. 1991). Total costs associated with this most recent severe drought amounted to over $39 billion (Riebsame et al. 1991). The duration of this drought was about 3 years and the percent of the con- tiguous United States in severe or extreme drought (Palmer Drought Hydrologic Index <— —3.0) peaked at 37% in 1988 (Riebsame et al. 1991). In contrast, the 1930s drought lasted about 7 years, and at its peak al- most 70% of the contiguous United States experienced severe or extreme drought (Riebsame et al. 1991). It is difficult to calculate and compare the costs and losses associated with drought, but the costs of miti- gating impacts of a 1930s- magnitude drought today would surely be considerable. General circulation models (GCMs) have been used to estimate the climate change that will accom- pany increases in tropospheric greenhouse gases lead- ing to a doubling of atmospheric CO,, calculated to occur in the mid— to late twenty -first century. Most state -of -the -art simulations suggest drier summers will prevail in the central United States under a 2 x CO. climate scenario ( Manabe and Wetherald 1987; Rind et al. 1990; Wetherald and Manabe 1995; Gregory et al. 1997). Model simulations show an earlier dry- ing of soils in spring due to the coincidence of less winter precipitation in the form of snow and warmer temperatures, conditions leading to greater evapotrans= piration relative to precipitation in late spring and sum= . mer (United States Global Chance Research Program 1995; Gregory et al. 1997). Dry conditions may be further enhanced by a decrease in summer precipita- tion and relative humidity (Wetherald and Manabe 1995; Gregory et al. 1997). In addition, some GCM studies have suggested an increase in the occurrence of extreme events with global warming (Overpeck et al. 1990; Rind et al. 1990), and although recent mod- eling results report modest decreases in mean values of summer precipitation and soil moisture in the cen- tral United States, a marked increase in the frequency and duration of extreme droughts under 2 x CO,_ con- ditions is also reported (Gregory et al. 1997). Paleoclimatic data strongly support evidence for Great Plains droughts of a magnitude greater than those of the twentieth century, while current land use practices and GCM predictions point to an increased vulnerability to Great Plains droughts in the next cen- tury. Given the likelihood that we are not able to pre- dict the exact extent and duration of the next major drought, it would be wise to adopt a probabilistic ap- proach to drought forecasting and planning that incor- porates the range of variability suggested by the proxy data. The paleoclimatic data suggest a 1930s- magnitude Dust Bowl drought occurred once or twice a century over the past 300 -400 years, and a decadal- length drought once every 500 years. In addition, paleoclimatic data suggest a drought regime change about 800 years ago, which was likely due to some change in the base state of the climate. An increase in global temperatures is one mechanism that could pos- sibly induce such a base -state change in climate and thus confront society with some costly surprises in the form of multidecadal drought. The prospect of great drought in the future highlights the need to place higher priority on narrowing the uncertainty about future drought by improving our understanding of the causes of drought and our ability to predict great droughts in the future. Assessments of future drought variability must tap paleoclimatic data, in combination with climate mod- els, to understand the full range of natural interannual to interdecadal drought variability, and to estimate the human- induced climate changes that might occur, superimposed on natural variability. Our current un- derstanding of drought and drought prediction is based 2710 Vol. 79, No. 12, December 1998