GREENHOUSE WARMING AND THE EURASIAN BIOTA - ARE THERE ANY LESSONS FROM THE PAST

Climate models predict a rise in global mean temperature of around 2-4 -degrees-C by the end of the next century, with far greater rises in t he high latitudes. Mean annual temperature rises of 6-8-degrees-C are predicted for 65-degrees-N, and as much as 10-12-degrees-C for above 7 0-degrees-N. There can be little doubt that such changes will have pro found effects on boreal and arctic ecosystems, both through the temper ature effects themselves and through associated changes in water balan ce. There is abundant evidence of climatic change in the high latitude s from the last 2.4 million years of the Quaternary. In a succession o f glacial-interglacial cycles, high latitude temperatures seem to have fluctuated overall by about the same amount as is projected for the n ext century. Perhaps it is possible to use our knowledge of such past changes to understand what might happen to the high latitude ecosystem s once the future greenhouse warming gets under way?There are many pot ential pitfalls in using data from the past to attempt to predict the future. In addition to the limitations in the data, there are also man y important differences in the rate and setting of changes that should be borne in mind. With regard to climatic time-scale, the biogeograph ical patterns which we observe for the past are far more likely to rep resent equilibrium situations than those which we will observe in the future. Equilibrium data can itself be useful in that it provides indi cations of the distribution of climate conditions towards which the Ea rth will move. For example, it provides support for the notion that th e climatic models are indeed correct in predicting that the strongest warming will occur in the high latitudes. Even following the relativel y slow climate changes of the Quaternary high latitudes, there is abun dant evidence of disequilibrium in tree species migrations, lasting fo r millennia in some cases. The survival of nearly all the high-latitud e forms of plants and animals known from the Pleistocene fossil record -despite the repeated climatic fluctuations-may provide reassuring evi dence of their future resilience. However, the extinctions of many lar ge arctic mammals at around the time of the most recent warming phase may provide warning of what will occur in the future to certain specie s whose populations are already depleted by human activity. The except ions to this pattern of gradual change are the sudden climatic shifts which have occurred in the North Atlantic region on several occasions during the late Quaternary. These may offer the closest analogues that we have to the effects of a future greenhouse warming on high-latitud e plant and animal communities. It seems that some groups of organisms , such as insects, molluscs and water plants were able to respond rapi dly to the climate warming, perhaps on the timescale of decades. Howev er, tree populations were left far, behind and took centuries or milen nia to catch up with the changed climate, resulting in unfamiliar ecol ogical scenarios in the mid and high latitudes.