Twenty-nine radiocarbon-dated pack rat middens document woodland communities in the deserts of the southwestern United States less than 10,000 years ago. A synchronous change from woodland to desert or grassland occurred about 8000 years ago in the Chihuahuan, Sonoran, and Mohave deserts. A shift of the Aleutian low and the winter storm track to the north, which resulted in drastically reduced winter precipitation in these areas, is inferred. The shift to nonpluvial climates in the Southwest lagged behind the beginning of nonglacial climates in the North because the melting continental glaciers contined to affect general circulation patterns.