The effect of vegetation in a climate model simulation on the Younger Dryas

The effect of vegetation on the Younger Dryas (YD) climate is studied by co mparing the results of four experiments performed with the ECHAM-4 atmosphe ric general circulation model (AGCM): (1) modem control climate, (2) simula tion with YD boundary conditions, but with modern vegetation, (3 and 4) ide ntical to (2), but with paleo-vegetation. Prescribing paleo-vegetation inst ead of modem vegetation resulted in temperature anomalies (both positive an d negative) of up to 4 degreesC in the Northern Hemisphere mid-latitudes, m ainly as an effect of changes in forest cover (change in albedo). Moreover, changes in precipitation and evaporation were found, most notably during D ecember-January-February (DJF) in the tropics and were caused by the replac ement of forests by grasslands. These results are consistent with other mod el studies on the role of vegetation changes on climate and they suggest th at it is important in paleoclimate simulation studies to prescribe realisti c vegetation types, belonging to the period of interest. However, in our ca se the addition of YD vegetation did not improve the agreement with proxy d ata in Europe, as the temperatures were increasing during winter compared t o the YD simulation with modem vegetation. It must be noted that this incre ase was not statistically significant. The model-data mismatch suggests tha t other factors probably played an important role, such as permafrost and a tmospheric dust. We infer that during the last glacial-interglacial transit ion, the time lag between the first temperature increase and the northward migration of trees, estimated at 500-1000 years, could have delayed the war ming of the Eurasian continent. The relatively open vegetation that existed during the early stages of the last glacial-interglacial transition had a relatively high albedo, thus tempering warming up of the Eurasian land surf aces. (C) 2000 Elsevier Science B.V. All rights reserved.