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curred about 1860 is notable in much of the historical <br />data [eolian activity, newspapers, and early meteoro- <br />logical records (Ludlum 1971; Bark 1978; Mock 1991; <br />ND <br />i y ' <br />Nt <br />f <br />±r S & 3'MS <br />39- <br />©K <br />Muhs and Holliday 1995)] and in drought reconstruc- <br />tions for the central and northern Great Plains (Fritts <br />1983; Stockton and Meko 1983), as well as in eastern <br />California (Hardman and Reil 1936) and throughout <br />the southwestern and western United States (Stockton <br />and Meko 1975; Meko et al. 1995). While the histori- <br />cal evidence of eolian activity suggests these two <br />nineteenth - century droughts were more severe than <br />twentieth - century droughts, it is not clear from the den - <br />drochronological records that nineteenth - century <br />droughts were indisputably more extreme. Rapid in- <br />creases in Native American and Euro - American popu- <br />lations as well as bison populations may have led to <br />severe land cover degradation and increased eolian <br />activity between 1820 and 1850 (West 1997). In any <br />case, it is clear that major multiyear Great Plains <br />drought has occurred naturally once or twice a century <br />over the last 400 years. <br />b. Thirteenth -to- sixteenth - century megadroughts <br />Prior to the seventeenth century, the availability of <br />high- resolution proxy data for Great Plains drought is <br />reduced, but useful information can still be gleaned <br />from a wide variety of proxy data, including data from <br />other areas of the western United States (Table 1). We <br />include these more distant records because they pro- <br />vide corroborative information for droughts docu- <br />mented in the few available Great Plains records and <br />allow an assessment of the extent of some of these <br />great droughts. Instrumental records indicate that the <br />major droughts impacting the Great Plains in the twen- <br />tieth century also affected areas of the western United <br />States (see Fig. 5); thus we feel that our use of proxy <br />data from the western United States to support evi- <br />dence of drought in the Great Plains is justified. There <br />are few tree -ring chronologies for the Great Plains that <br />extend prior to the 1600s, but there are long chronolo- <br />gies for other areas in the western United States that <br />reflect spatially extensive droughts. Other proxy data <br />with a coarser temporal resolution or less accurate tem- <br />poral control than tree -ring data include those from <br />lake sediment, alluvial, eolian, and archaeological <br />FIG. 4. (a) Locations of drought - sensitive tree -ring chronologies and reconstructions of precipitation or drought in the Great Plains. <br />Numbered dots are locations of Cook et al.'s (1996) gridded PDSI reconstructions. The key for lettered symbols is in Table 2. Statistical <br />relationships (explained variance) between observed and reconstructed series of tree -ring chronologies are listed by author and/or <br />grid number in Table 3. Note that while reconstructions are for regions within the Great Plains, the reconstructions are generated from <br />trees located in areas flanking the Great Plains_ reconstructions (the exception is Weakly's southwestern Nebraska chronology). Tree <br />growth reflects large -scale climate variations, and thus trees proximal to the Great Plains have been used successfully to reconstruct <br />climate variations in this region. (b) Locations of many of the paleoclimatic records documenting drought in the Great Plains and <br />western United States for the period A.D. 1 -1600. The key for lettered symbols is in Table 2. <br />2698 Vol. 79, No. 12, December 1998 <br />A � <br />Ro <br />- <br />s <br />y <br />s <br />r <br />M< <br />P <br />► - <br />3 <br />u_ <br />f <br />s <br />o <br />F <br />A <br />f yc <br />Muhs and Holliday 1995)] and in drought reconstruc- <br />tions for the central and northern Great Plains (Fritts <br />1983; Stockton and Meko 1983), as well as in eastern <br />California (Hardman and Reil 1936) and throughout <br />the southwestern and western United States (Stockton <br />and Meko 1975; Meko et al. 1995). While the histori- <br />cal evidence of eolian activity suggests these two <br />nineteenth - century droughts were more severe than <br />twentieth - century droughts, it is not clear from the den - <br />drochronological records that nineteenth - century <br />droughts were indisputably more extreme. Rapid in- <br />creases in Native American and Euro - American popu- <br />lations as well as bison populations may have led to <br />severe land cover degradation and increased eolian <br />activity between 1820 and 1850 (West 1997). In any <br />case, it is clear that major multiyear Great Plains <br />drought has occurred naturally once or twice a century <br />over the last 400 years. <br />b. Thirteenth -to- sixteenth - century megadroughts <br />Prior to the seventeenth century, the availability of <br />high- resolution proxy data for Great Plains drought is <br />reduced, but useful information can still be gleaned <br />from a wide variety of proxy data, including data from <br />other areas of the western United States (Table 1). We <br />include these more distant records because they pro- <br />vide corroborative information for droughts docu- <br />mented in the few available Great Plains records and <br />allow an assessment of the extent of some of these <br />great droughts. Instrumental records indicate that the <br />major droughts impacting the Great Plains in the twen- <br />tieth century also affected areas of the western United <br />States (see Fig. 5); thus we feel that our use of proxy <br />data from the western United States to support evi- <br />dence of drought in the Great Plains is justified. There <br />are few tree -ring chronologies for the Great Plains that <br />extend prior to the 1600s, but there are long chronolo- <br />gies for other areas in the western United States that <br />reflect spatially extensive droughts. Other proxy data <br />with a coarser temporal resolution or less accurate tem- <br />poral control than tree -ring data include those from <br />lake sediment, alluvial, eolian, and archaeological <br />FIG. 4. (a) Locations of drought - sensitive tree -ring chronologies and reconstructions of precipitation or drought in the Great Plains. <br />Numbered dots are locations of Cook et al.'s (1996) gridded PDSI reconstructions. The key for lettered symbols is in Table 2. Statistical <br />relationships (explained variance) between observed and reconstructed series of tree -ring chronologies are listed by author and/or <br />grid number in Table 3. Note that while reconstructions are for regions within the Great Plains, the reconstructions are generated from <br />trees located in areas flanking the Great Plains_ reconstructions (the exception is Weakly's southwestern Nebraska chronology). Tree <br />growth reflects large -scale climate variations, and thus trees proximal to the Great Plains have been used successfully to reconstruct <br />climate variations in this region. (b) Locations of many of the paleoclimatic records documenting drought in the Great Plains and <br />western United States for the period A.D. 1 -1600. The key for lettered symbols is in Table 2. <br />2698 Vol. 79, No. 12, December 1998 <br />