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Well-preserved pine needles found in fossil packrat middens document the biogeographic responses of pinyon pines to changing climates over the last 50,000 years. During the full glacial Wisconsinan (MIS2), Pinus monophylla (single-needle pinyon), Pinus edulis (Colorado pinyon), and P. edulis var. fallax (Arizona singleleaf pinyon) all grew along the southern portions of their current ranges. P. monophylla extended from the southern Sierra Nevada across the Mojave Desert to northwestern Arizona. P. edulis grew from northwestern Arizona across central Arizona to New Mexico and south to westernmost Texas. P. edulis var. fallax grew throughout what is now the Sonoran Desert of southern Arizona. Application of the modern climate requirements for these species suggest that winter precipitation at this time was at least 150% of modern, but also that summer precipitation may have been somewhat greater than today, at least at the northernmost end of the Gulf of California. During the Bolling and Allerod intervals P. edulis and P. edulis var. fallax quickly expanded northward over the Mogollon Rim of central Arizona into the Little Colorado River basin and northwestern Arizona. This northerly expansion of the fallax variety during the Allerod interval suggests that temperatures were warmer than most of the latest Wisconsinan and that summer precipitation was at least 120% of modern. After the rapid warming at the start of the Holocene (11.7 ka), P. monophylla and P. edulis populations were reduced in extent as their retreating southerly stands were not immediately replaced by expansion into cooler regions. These populations slowly expanded 300-500 km northward at rates between 20 and 60 m y(-1), reaching some of their current northern limits only within the last millennium. Increases in temperature expected over the next several hundred years will result in a similar reduction in populations unless this warming is ameliorated by favorable increases in precipitation. The consistent needle anatomy and distributions of the three types relative to each other suggest that there has been little evolutionary change discernible from these needles over at least the last 25,000 years. (C) 2012 Elsevier Ltd and INQUA. All rights reserved.