A 15,000 yr paleoenvironmental record at the elevation of lower timberline in central Colorado suggests two major paleoclimatic changes during that interval: (1) at 10,000 yr B.P., from cool-moist conditions to warm-moist conditions and (2) at 4,000 yr B.P., to warm-dry conditions. This sequence can be interpreted as a shift from precipitation predominantly originating from Pacific winter storms (before 10,000 yr B.P.) to a predominantly monsoonal-type regime (lasting until 4,000 yr B.P.) to the present pattern. Contemporaneity of higher upper timberline and lower lower timberline during the 10,000 to 4,000 yr B.P. interval is not contradictory but a result of changes in those circulation patterns.