2000 years of Drought Variability in the Central United States

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the Four Corners (Euler et al. 1979; Dean et al. 1985) indicate drought conditions from A.D. 250 to 400. 3. The paleoclimatic perspective: A summary Paleoclimatic data provide evidence that twentieth - century droughts are nbt representative of the full range of drought variability that has occurred over the last 2000 years. The collection of dendroclimatic re- constructions for the Great Plains region suggests that the severe droughts of the twentieth century, although certainly major in terms of their societal and economic impacts, are by no means unprecedented in the past four centuries. Moreover, when all proxy data, includ- ing historical accounts of eolian activity, are consid- ered, it is likely that droughts of a magnitude at least equal to those of the 1930s and 1950s have occurred with some regularity over the past 400 years. A look farther back in time reveals evidence that the multidecadal drought events of the late thirteenth and/or sixteenth centuries were of a much greater du- ration and severity than twentieth - century droughts. Interestingly, in the interval between these two big droughts, there is little evidence of severe and/or wide- spread drought. Laird et al. (1996) and Laird et al. (199 8) suggest that their North Dakota lake sediment data reflect a drought regime shift occurring around A.D. 1200, with droughts prior to this time characterized by much greater intensity and frequency (Fig. 11). Although the type of proxy data that extend back several thousand years tend to have a decadal- to century-scale tempo- ral resolution and dating accuracy that confounds close comparisons, the few annually resolved paleoclimatic records that ao exist for this period provide some evi- dence for longer periods of drought or periods of more frequent drought prior to the thirteenth century (LaMarche 1974; Dean 1994; Grissino -Mayer 1996; Hughes and Graumlich 1996). Several tree -ring records and reconstructions for the Southwest and the White Mountains /Great Basin region support the idea of a major drought regime shift after the late thirteenth - century drought. The timing is somewhat later than suggested by Laird et al. (1996) and Laird et al. (1998), but the difference may be due to the greater precision in dendrochronological dating compared to radiocar- bon dating. For the most part, these longer records have been analyzed in terms of low- frequency varia- tions, but twentieth - century variations can still be evaluated in the context of the previous 2000 years. Dendroclimatic evidence suggests that many droughts prior to the late thirteenth - century drought were at least decades in duration. In contrast, the droughts since the thirteenth- century event apparently have tended to be a decade or less in duration, with the exception of the late sixteenth - century multidecadal drought in the Southwest. The North Dakota lake sediment record along with these tree -ring records from the Southwest; the Great Basin, and the White Mountains suggest that a drought regime shift may have occurred not only in the Great Plains, but over much of the western United States as well. The evidence for a drought regime shift around 700 years ago is intriguing, but more investi- gations incorporating millennium- length records of highly resolved, precisely dated paleoclimatic data are needed to confirm and understand the full nature and extent of this event. An assessment of the available proxy data suggests that droughts of the twentieth century have been char- acterized by moderate severity and comparatively short duration, relative to the full range ofpast drought variability. This indicates the possibility that future droughts may be of a much greater severity and dura- tion than what we have yet experienced. It is impera- tive to understand what caused the great droughts of the past 2000 years and if similar droughts are likely to occur in the future. 4. The causes of Great Plains drought An inquiry into the mechanisms behind Great Plains drought begins with an examination ofprecipi- tation climatology and the atmospheric conditions as- sociated with twentieth - century drought. The semiarid to subhumid climate of the Great Plains is influenced by several different air masses, each with spatially and seasonally varying impacts on the region: dry west- erly flow of air from the Pacific; the cold, dry arctic air masses from the north; and the warm, moist tropi- cal air masses from the south (Bryson 1966; Bryson - and Hare 1974). The polar jet stream brings Pacific moisture to the area in the cool season, but the region is generally quite dry in winter (Doesken and Stanton 1992). In summer, although the central Great Plains is under the drying influence of a strong subtropical ridge, moisture is drawn into the area from the Gulf of Mexico by the Great Plains nocturnal low -level jet (LLJ). The LLJ is a synoptic -scale feature associated with convective storm activity and represents the in- 2706 Vol. 79, No. 12, December 1998