Climatic changes of the last 18,000 years: observations and model simulations

Changes in solar radiation arising from changes in the orientation of the earth's axis had pronounced effects on tropical monsoons and mid-latitude climates as well as on ice-sheet configuration during the last 18,000 years. COHMAP (Cooperative Holocene Mapping Project) has assembled a global array of well-dated paleoclimatic data and used general-circulation models to identify and evaluate causes and mechanisms of climatic change. For the northern tropics, particularly in Africa and Asia, data and model results show that the orbitally induced increase in solar radiation in summer 12,000 to 6,000 years ago enhanced the thermal contrast between land and sea and thus produced strong summer monsoons, which raised lake levels in regions that are arid today. In middle to high latitudes the climatic response to both the insolation changes and to the retreating ice sheets led to readjustments in the vegetation in both the Northern and Southern hemispheres. Model results show that the large North American ice sheet split the westerly jet stream into northern and southern branches over North America. An increase in storms associated with the southern branch helps explain high lake levels and increased woodlands in the southwestern United States during full-glacial conditions. Comparisons of paleoclimatic data with the model simulations are important because models provide a theoretical framework for evaluating mechanisms of climatic change, and such comparisons help to evaluate the potential of general circulation models for predicting future climates.