NATURAL CLIMATIC VARIABILITY AS AN EXPLANATION FOR HISTORICAL CLIMATIC FLUCTUATIONS

The question as to whether the climatic anomalies associated with the Medieval Warm Period and the Little Ice Age can be attributed to natur al climatic variability is explored in this paper. The output from a 5 00-year run with a global climatic model is used for this purpose. The model exhibits multi-decadal variability in its climatic outputs, whi ch appears to have many of the characteristics of observed climatic da ta over the last millennium. Global distributions of surface temperatu re associated with peak warming and cooling phases of the model run hi ghlight the spatial variability which occurs, and the lack of synchron eity in the response from region to region. Considerable year-to-year variability occurs in temperature anomaly patterns during the warming and cooling phases, indicating the complexity of the responses. The mo del results suggest that such climatic phases should not be considered as lengthy periods of universal warming or cooling. Comparison of obs erved time series of land surface temperature for the northern hemisph ere for the last 500 years with model output indicates that most of th e observed features in this climatic record can be reproduced by proce sses associated with internal mechanisms of the climatic system as rep roduced in the model. While the model results do not exclude the possi ble contribution of external forcing agents as a contributing factor t o these climatic episodes, the perception is that such agents would en hance existing naturally-induced climatic features rather than initiat e them, at least for this time frame. Given the omnipresent nature of natural climatic variability, it is assumed that such variability rath er than external forcing agents has primacy in generating and maintain ing the underlying observed climatic variability. An understanding of the mechanisms and behaviour of such climatic features is becoming of increasing importance, in view of their possible role in modulating fu ture climatic trends given the expected influence of the greenhouse ef fect.