Natural climatic variability as an explanation for historical climatic fluctuations

Abstract

The question as to whether the climatic anomalies associated with the Medieval Warm Period and the Little Ice Age can be attributed to natural climatic variability is explored in this paper. The output from a 500-year run with a global climatic model is used for this purpose. The model exhibits multi-decadal variability in its climatic outputs, which appears to have many of the characteristics of observed climatic data over the last millennium. Global distributions of surface temperature associated with peak warming and cooling phases of the model run highlight the spatial variability which occurs, and the lack of synchroneity 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 model results suggest that such climatic phases should not be considered as lengthy periods of universal warming or cooling. Comparison of observed time series of land surface temperature for the northern hemisphere for the last 500 years with model output indicates that most of the observed features in this climatic record can be reproduced by processes associated with internal mechanisms of the climatic system as reproduced in the model. While the model results do not exclude the possible contribution of external forcing agents as a contributing factor to these climatic episodes, the perception is that such agents would enhance existing naturally-induced climatic features rather than initiate them, at least for this time frame. Given the omnipresent nature of natural climatic variability, it is assumed that such variability rather than external forcing agents has primacy in generating and maintaining 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 future climatic trends given the expected influence of the greenhouse effect.