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 pre scribed sea surface temperatures. They differ in that one integration represents present day climate (PD) and the other has a perturbed orbi t and reduced atmospheric concentrations of CO2 appropriate to the cli mate of 6000 years ago (6 kyr, hereafter 6k). The 6k integration produ ces enhanced continental warmth during summer and cold during winter. Changes in atmospheric temperature gradients brought about by the surf ace response lead to altered jet stream structures and transient eddy activity, which in turn affect precipitation patterns. Tropical ''mons oon''-type circulation patterns are also affected, also leading to alt ered precipitation. Many of the changes in hydrology mimic the geologi cal record remarkably well: the Sahel is much wetter, as are the midwe stern United States and the Mediterranean regions; California and nort hern Europe are drier. Processes leading to the model's surface respon ses in both temperature and hydrology are described in detail. Finally , the sensitivity of the results to an alternative, objective definiti on of the 6k calendar is investigated. This sensitivity is found to be smaller than the overall signal to the extent that the principal conc lusions are not altered.