2000 years of Drought Variability in the Central United States

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(Lehmer 1970; Wendland 1978), and in northwestern Iowa after about A.D. 1100 and intensify- ing by A.D. 1200 (Bryson and Murray 1977). The next major drought is characterized prima- rily by an onset of eolian activ- ity in the western Great Plains. It is difficult to determine the ex- act date of onset, but activity be- gan sometime after —A.D. 950 (Forman et al. 1992; Forman et al. 1995; Madole 1994, 1995; Muhs et al. 1996). Other proxy data that help confirm this pe- riod of drought are those from North Dakota lake sedi- ments (Laird et al. 1996; Laird et al. 1998) and alluvial sediment records from western Nebraska and Kansas (Brice 1966; May 1989; Martin 1992). Although there is dendroclimatic and lake -level evidence of drought in the Sierra Nevada and White Mountains between —A.D. 900 and 1100, (LaMarche 1974; Stine 1994; Hughes and Graumlich 1996), there is no evidence of an onset of drought conditions occurring in the South- west at this time. The third major drought episode of the A.D. 1 -1200 period occurred roughly between A.D. 700 and 900. In archaeological evidence in the Four Corners area, A.D. 750 was the starting date for a drought that lasted several centuries (Euler et al. 1979; Dean et al. 1985; Peterson 1994), and a tree -ring reconstruction of drought for New Mexico also reflects this drought (Grissino -Mayer 1996). Drought is recorded in west- ern Minnesota lake varves at this time (Dean et al. 1994; Dean 1997) while North Dakota lake sediments indicate drought conditions typified the period A.D. 700 -850 (Fig. 11; Laird et al. 1996; Laird et al. 1998). In another more coarsely resolved record of lake sedi- ments in North Dakota, high salinity conditions are indicated to have begun about this time FIG. 11. North Dak and continued through the fif- 1996). Deviations fro m teenth century, a period contain- negative values indic ing the droughts of the tenth, high salinity and dry twelfth, and late thirteenth reflecting a change in to relatively wetter c o centuries (Fritz et al. 1991). In chronology is about o the central California drought nology of ±50-60 yr. record from giant sequoia, the this section of the cor FIG. 10. Paleoclimatic records of Great Plains and western U.S. century-scale drought, A.D. 1— present, as recorded in a variety of paleoclimatic data. The pale gray horizontal bars reflect the length of the series, and the dark gray indicate periods of drought. Orange verti- cal bars represent multidecadal droughts that appear to have been widespread. e. period A.D. 699 -823 had the highest drought frequency in the past 2000 years (Hughes and Brown 1992). Drought appears to have occurred in the White Moun- tains about this time as well (Hughes and Graumlich 1996). The onset of the earliest of these four droughts occurred about the middle of the third century and ap- pears to have lasted up to three centuries. A dendro- climatic reconstruction of precipitation for northern New Mexico (Grissino -Mayer 1996) shows this to be a period of consistently average to below - average pre- cipitation until about A.D. 500. Drought- sensitive gi- ant sequoia in central California suggest that the period A.D. 236 -377 was one of the three periods with the highest frequency of drought within the past two millennia (Hughes and Brown 1992). During the closely corresponding period, A.D. 200 -370, more fre- quent drought conditions were indicated by high lake salinity in North Dakota lake sediments (Laird et al. 1996; Laird et al. 1998). Archaeological remains in ota Moon Lake salinity record, here spanning A.D. 1 -1980 (Laird et al. the mean (based on past 2300 yr) log salinity values are shown with ating low salinity and wet conditions and positive values indicating conditions. Note the shift in salinity values around A.D. 1200, likely drought regime from more frequent, intense droughts prior to A.D. 1200 nditions in the last 750 yr. The average temporal resolution of the on sample per five years, with an estimated error in the absolute chro- The 92 -yr gap in the data from 1618 to 1710 is due to desiccation in Bulletin of the American Meteorological Society 2705