Only in the last few years have high-resolution paleoclimatic data become available from coastal southern California. Recent research in the California Channel Islands, drawing on some of these data, attributes settlement disruptions, disease, and violence to maritime subsistence distress attendant to elevated sea temperatures in the period from A.D. 1150 to 1300. A broad range of paleoenvironmental, archaeological, and human osteological data suggest that these stress indicators are more convincingly correlated with severe late Holocene drought episodes during a portion of the medieval climatic anomaly (ca. A.D. 800 to 1400). Based on these data, cultural changes in coastal southern California, including violence, declining health, and emergent social complexity, are similar to events documented in the American Southwest. Cultural adaptations in both regions appear to have been responding to persistent drought conditions during the late Holocene.

Ground-penetrating radar and other geophysical techniques are known to produce useful data when deposits are crisply structured, as in the case of sub-surface masonry walls or large ditches. New studies of Californian coastal sites find the methods are effective in tracing the less sharp distinctions that define clay and sand house floors within these large and dense hunter-gatherer middens.

Two recent developments in southwestern archaeology are brought together in this paper. First, theoreticians have begun to argue that the archaeological record should be viewed as the product of selection-driven evolution. Second, tree-ring research has produced a highly detailed history of climate for a large area of the northern Southwest. We view the record of climatic oscillations and extreme events as a record of the strength of selection favoring stabilization of specialized agricultural strategies in the arid northern Southwest. Published data from Black Mesa provide a cultural record of sufficient precision to permit comparison with the climatic record, while new data from Vermillion Cliffs, southern Utah, document one local end-product of an evolutionary sequence shaped to an important degree by the long-term variability of climate. Anasazi occupation of many regions failed to persist through the “Great Drought” of the 1270s. From a local perspective, this extreme climatic event caused adaptations shaped by selection prior to the 1270s to fail; from a broader temporal-spatial perspective, however, the drought must be seen as part of the selective regime that shaped subsequent human adaptation to the northern Southwest.

Two major drought episodes, A.D. 1000 to 1015 and A.D. 1120 to 1150, contributed to significant change in adaptive strategies of the Virgin Branch Anasazi, a prehistoric population that occupied the southwestern Great Basin between A.D. 100 and A.D. 1150. The first extreme climatic event promoted the adoption of several alternative buffering strategies including intensive agricultural practices, increased reliance on storage, and the organization of large residential labor groups. The second drought, which followed 150 years of favorable climatic conditions and high levels of population growth, had a devastating impact upon the Virgin Branch Anasazi resulting in the complete abandonment of the southwestern Great Basin by that group. These two climatic events required entirely different responses, which suggest that shifts in climate are best viewed as triggering culture change. The preconditions of population growth set the various levels of sensitivity to extreme climatic events and determine the precise nature of the culture changes.