A GCM Simulation of the Climate 6000 years ago

Two 10-yr integrations of the UGAMP GCM are presented. Each has a full seasonal cycle, T42 resolution, interactive land and sea ice, and prescribed sea surface temperatures. They differ in that one integration represents present day climate (PD) and the other has a perturbed orbit and reduced atmospheric concentrations of CO2 appropriate to the climate of 6000 years ago (6 kyr, hereafter 6k). The 6k integration produces enhanced continental warmth during summer and cold during winter. Changes in atmospheric temperature gradients brought about by the surface response lead to altered jet stream structures and transient eddy activity, which in turn affect precipitation patterns. Tropical “monsoon”-type circulation patterns are also affected, also leading to altered precipitation. Many of the changes in hydrology mimic the geological record remarkably well: the Sahel is much wetter, as are the midwestern United States and the Mediterranean regions; California and northern Europe are drier. Processes leading to the model’s surface responses in both temperature and hydrology are described in detail. Finally, the sensitivity of the results to an alternative, objective definition of the 6k calendar is investigated. This sensitivity is found to be smaller than the overall signal to the extent that the principal conclusions are not altered.