Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science ProgramChapter 3irregular polygon delineates the boundariesof the area we refer to as Southwestern NorthAmerica and includes the Southwestern UnitedStates and Northern Mexico. It encompasses allgrid points on and within 27.5°–50°N. latitudeand 97.5°–125°W. longitude and was the areaused by Cook et al. (2004). The dashed linealong the 40th parallel separates the West intonorthwest and southwest sectors, which will becompared later.3.3 Medieval Megadroughts in theWestern United StatesCook et al. (2004) examined the NADA contentsback to A.D. 800 for the West to place thecurrent turn-of-the-century drought there (Seager,2007) in a long-term context. In so doing, aperiod of elevated aridity was found in the A.D.900–1300 period that included four particularlywidespread and prolonged multi-decadal megadroughts(Fig. 3.8). This epoch of large-scaleelevated aridity was corroborated by a numberof independent, widely scattered, proxy recordsof past drought in the West (Cook et al., 2004).In addition, the four identified megadroughtsagreed almost perfectly in timing with thoseidentified by Woodhouse and Overpeck (1998),which were based on far fewer data. These findingswere rather sobering for the West becausethey (1) verified the occurrence of several pastmultidecadal megadroughts prior to 1600,Figure 3.8. Percent area affected by drought (Palmer Drought Severity Index(PDSI)