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Plant macrofossils, percentage abundance of grass taxa, fecal-pellet delta(13)C, and plant-cuticle contents from 49 fossil rodent middens dated by C-14 record changes in local vegetation and precipitation since 45 ka (calibrated or measured to thousands of calendar years before present) in the central Atacama Desert (fat 22degrees24degreesS) of northern Chile. The midden sites are along the hyperarid upper margin (2400-3100 m) of the "absolute desert," in an extreme environment sparsely vegetated by annual herbs and halophytic shrubs. Conditions between 40 and 22 ka may have been at least intermittently dry, and possibly cooler, as implied by four middens with low species richness. We infer a large increase in summer rainfall between 16.2 and 10.5 ka on the basis of the lowering of steppe grasses by as much as 1000 m, prominence of C, grasses and summer annuals, high species richness, and displacement of northern species at least 50 kin south of their modern ranges. The precipitation increase was greatest for a cluster of middens between 11.8 and 10.5 ka. Abrupt drying, evident in a dramatic decrease in grass abundance, occurred after 10.5 ka at all four midden localities. Increased percentages of grass, higher species richness, and extralocal taxa record slightly wetter conditions between 7.1 and 3.5 ka. The present hyperarid conditions were established after 3 ka.

Present-day variability of summer precipitation in the central Atacama Desert and adjacent Altiplano is related to the intensity and position of upper air circulation anomalies, which in turn respond to Pacific sea-surface temperature anomalies. Summer insolation over the central Andes (lat 20degreesS) was at its minimum during the latest glacial to early Holocene transition, so regional insolation forcing cannot account for intensified pluvial conditions in the central Atacama. Summer precipitation collapsed abruptly between 10.5 and 10 ka, indicating either nonlinear relationships with seasonal insolation or a change in intensity of upper air circulation over the Altiplano, effectively blocking moisture transport to the Atacama Desert. Here, we suggest that precipitation variations on millennial time scales in the central Atacama are the result of extraregional forcing of the South American Summer Monsoon through intensified Walker Circulation (stronger easterlies) and La Nina-like conditions operating through insolation anomalies (i.e., departures in insolation values) directly over central Asia and the equatorial Pacific.