Severe and sustained drought in southern California and the West: Present conditions and insights from the past on causes and impacts

Southern California and much of the western United States face a chronic challenge of limited water supply due to high potential evaporation and low precipitation coupled with frequent droughts. Mitigation approaches include the use of ground water, reliance on water from river systems fed by mountainous regions that have relatively high precipitation, and the construction of extensive aqueduct and water storage systems. In southern California the present infrastructure is capable of insulating large water districts against the typical annual and multi-annual droughts experienced over the past 100 years. However, paleoclimatic records indicate that the region is also prone to much longer droughts, including a prolonged episode of generally arid conditions and severe droughts extending from the 9th through 14th centuries. This period is sometimes referred to as the medieval climate anomaly. Archaeological evidence suggests that prehistoric populations such as the Anasazi in the Southwest and the Chumash in southern California were impacted by mega-drought conditions during the medieval climate anomaly. These groups appear to have displayed a variety of responses—ranging from increased violence and the abandonment of some regions, to the development of greater cultural complexity and material infrastructure. Paleoclimatological and paleooceanographic data indicate that the arid conditions in western North America during the medieval climate anomaly were produced by the prolonged occurrence of cool surface waters in the eastern Pacific. Recent climate model experiments suggest relatively small increases in insolation and decreases in atmospheric volcanic emission concentrations can trigger such depressions of eastern Pacific temperatures. It is thus possible that a similar event could occur in the future due to natural or anthropogenic causes.