Book 2.indb - US Climate Change Science Program

Abrupt Climate Changethat even the past 400 years were insufficientto capture the frequency and occurrence ofmegadroughts that clearly exceeded anythingin the historical records in many regions.3.2 The North American Drought AtlasSince that time, great progress has been madein expanding the spatial coverage of tree-ringPDSI reconstructions to cover most of NorthAmerica (Cook and Krusic, 2004a,b; Cooket al., 2004). The grid used for that purposeis shown in Figure 3.7. It is a 286-point 2.5°by 2.5° regular grid that includes all of theregions described in Woodhouse and Overpeck(1998), Cook et al. (1999), and Stahle et al.(2000). In addition, the reconstructions wereextended back 1,000 or more years at manylocations. This was accomplished by expandingthe tree-ring network from the 425 tree-ringchronologies used by Cook et al. (1999) to 835series used by Cook et al. (2004). Several ofthe new series also exceeded 1,000 years inlength, which facilitated the creation of newPDSI reconstructions extending back into themegadrought period in the Western UnitedStates prior to 1600. Extending the reconstructionsback at least 1,000 years was an especiallyimportant goal. Woodhouse and Overpeck(1998) summarized evidence for at least fourwidespread multi-decadal megadroughts in theGreat Plains and the Western United States duringthe A.D. 750–1300 interval. These includedtwo megadroughts lasting more than a centuryeach during “Medieval” times in California’sSierra Nevada (Stine, 1994). Therefore, beingable to characterize the spatial and temporalproperties of these megadroughts in the WesternUnited States was extremely important.Using the same basic methods as those inCook et al. (1999) to reconstruct droughtover the conterminous United States, newPDSI reconstructions were developed on the286-point North American grid (Fig. 3.7) andincorporated into a North American DroughtAtlas (NADA; Cook and Krusic, 2004a,b; Cooket al., 2007). The complete contents of NADAcan be accessed and downloaded at http://iridl.ldeo.columbia.edu/SOURCES/.LDEO/.TRL/.NADA2004/.pdsi-atlas.html. In Figure 3.7, theFigure 3.7. Map showing the distribution of 286 grid points of drought reconstructed for much ofNorth America from long-term tree-ring records. The large, irregular polygon over the West is thearea analyzed by Cook et al. (2004) in their study of long-term aridity changes. The dashed line at 40°N.divides that area into Northwest and Southwest zones. The dashed-line rectangle defines the GreatPlains region that is also examined for long-term changes in aridity here.87