A 1,200-year perspective of 21st century drought in southwestern North America

Abstract

A key feature of anticipated 21st century droughts in Southwest North America is the concurrence of elevated temperatures and increased aridity. Instrumental records and paleoclimatic evidence for past prolonged drought in the Southwest that coincide with elevated temperatures can be assessed to provide insights on temperature-drought relations and to develop worst-case scenarios for the future. In particular, during the medieval period, ∼AD 900-1300, the Northern Hemisphere experienced temperatures warmer than all but the most recent decades. Paleoclimatic and model data indicate increased temperatures in western North America of approximately 1 °C over the long-term mean. This was a period of extensive and persistent aridity over western North America. Paleoclimatic evidence suggests drought in the mid-12th century far exceeded the severity, duration, and extent of subsequent droughts. The driest decade of this drought was anomalously warm, though not as warm as the late 20th and early 21st centuries. The convergence of prolonged warming and arid conditions suggests the mid-12th century may serve as a conservative analogue for severe droughts that might occur in the future. The severity, extent, and persistence of the 12th century drought that occurred under natural climate variability, have important implications for water resource management. The causes of past and future drought will not be identical but warm droughts, inferred from paleoclimatic records, demonstrate the plausibility of extensive, severe droughts, provide a long-term perspective on the ongoing drought conditions in the Southwest, and suggest the need for regional sustainability planning for the future.

Fig. 1.

Total seasonal precipitation and mean seasonal temperature averaged over Colorado, Utah, New Mexico, and Arizona (17); five-year running means, 1900–2008. Precipitation in Blue Line (Horizontal Line is the average), temperature in Brown. Cool season (November–March), Top. Warm season (May–October), Bottom. Shading indicates periods of below average precipitation.

Fig. 2.

Global, hemispheric, and regional proxy and model data documenting medieval period conditions. A Solar irradiance (69), B two estimates of Northern Hemisphere land temperatures, departures from 1850–1995 (32), C ECHO-G (60) modeled average annual temperature for 34°–40° N, 104°–124° W, and departures from 1890–1990, D reconstructed Colorado Plateau mean maximum temperatures (13), E reconstructed water year streamflow, Colorado River at Lees Ferry (41) and Sacramento Four Rivers index flow (40), percent of average based on AD 901–1977, and F reconstructed Southwest Drought Area Index (5). All series except (A) were smoothed with a 20-year spline. Light Shading indicates medieval period, Dark Shading indicates mid-1100s period.

Fig. 3.

Composite maps of summer PDSI. A Gridded PDSI, reconstructed, 1146–1145, Left; instrumental, 1951–1960, Right (5). B. Divisional PDSI, 1999–2008, Left; 1951–1960, Right for comparison with gridded PDSI (, Image provided by the NOAA/ESRL Physical Sciences Division, Boulder Colorado from their Web site at http://www.psd.noaa.gov/). Note slightly different scales; Red indicates dry conditions and Blue indicates wet.

Fig. 4.

Observed annual flow of the Colorado River at Lees Ferry. Horizontal Line is the median of the 1906–2006 observed flows (18.32 billion cubic meters). Light Gray line is the 10-year running average. Observed flows are version 6.18.08 of the natural flows from the U.S. Bureau of Reclamation (http://www.usbr.gov/lc/riverops.html), appended with provisional flows for water years 2007–09 that were estimated by the Bureau of Reclamation with data currently available.