Variable winter moisture in the southwestern United States linked to rapid glacial climate shifts

During the last glacial period, the climate of the Northern Hemisphere was characterized by rapid, large-amplitude temperature fluctuations through cycles lasting a few thousand years(1-3). These fluctuations are apparent in Greenland temperature reconstructions(2,3), and corresponding temperature and hydrological variations have been documented throughout the Northern Hemisphere(4,5). Here we present a record of precipitation in the southwestern United States from 56,000 to 11,000 yr ago, on the basis of delta O-18 measurements of speleothem calcite from New Mexico. Our record shows that increased winter precipitation in the southwestern United States is associated with Northern Hemisphere cooling, which we attribute to a southward shift in the polar jet stream, which modulated the position of the winter storm track over North America. On the western side of the Pacific Ocean basin, decreases in summer monsoon precipitation are associated with Northern Hemisphere cooling, due to southward displacement of the intertropical convergence zone(4). We conclude that cooling and warming excursions in the Northern Hemisphere lead to concurrent latitudinal displacement of both the intertropical convergence zone and the polar jet stream over the Pacific Ocean. Our data are consistent with modern evidence for a northward shift of the polar jet stream in response to global warming(6-8), which could lead to increasingly arid conditions in southwestern North America in the future.