EFFECT OF VEGETATION ON AN ICE-AGE CLIMATE MODEL SIMULATION

A growing number of studies suggest that vegetation changes can signif icantly influence regional climate variations. Herein we utilize a cli mate model (GENESIS) with a land surface vegetation package to evaluat e the potential role of the very large vegetation changes that occurre d during the last glacial maximum (LGM), In particular, we focus on th e potential response to a significant reduction in the area of tropica l rainforest. Simulations employed a global vegetation reconstruction for the LGM and Climate/Long-Range Investigation, Mapping and Predicti on (CLIMAP) sea surface temperature (SST) estimates. Results indicate that expansion of dryland vegetation causes a 15-30% additional LGM co oling for Australia (0.4 degrees C) and Africa (0.9 degrees C), respec tively, Turnover from conifer to tundra also causes cooling of 2 degre es-4 degrees C or mon in western Europe and Siberia. However, for the largest rainforest area (Amazon Basin), inclusion of realistic vegetat ion increased modeled temperatures 2 degrees-4 degrees C and decreased precipitation by 10-35%. These latter results are similar to those ob tained with sensitivity experiments of the effects of future Amazon de forestation, Initial assessment of the potential effect of decreased s tomatal resistance due to lower ice age CO2 levels indicates little si gnificant response to this effect. Comparison of model-predicted low-e levation LGM temperature Changes with estimates from proxy data indica te that inclusion of realistic vegetation estimates for the LGM result s in slightly more than 50% agreement between models and data for low- elevation sites in low-mid latitudes. Data at variance with model pred ictions would appear to be explainable by considering additional chang es in vegetation, ice age dust, or a 1 degrees-2 degrees C cooling bel ow CLIMAP values. This conclusion is at Variance with a 3 degrees-4 de grees C tropical cooling suggested by some studies for explaining esti mated land temperature changes during the LGM. In some western Europea n sites model temperatures are colder than proxy data by 2 degrees-8 d egrees C. This model-data discrepancy may be explained by less sea ice in the subpolar North Atlantic than stipulated by CLIMAP, a conclusio n consistent with new marine data from that region.