THE PROBLEM OF DETECTING CLIMATIC-CHANGE IN THE PRESENCE OF CLIMATIC VARIABILITY

A global climatic model (GCM) with a simple Q-flux based ocean has bee n run out for a 500-year period in order to generate time series indic ative of natural climatic variability, especially global mean surface temperature. Multi-decadal warming and cooling episodes with a maximum range of 0.7-degrees-C were obtained. The geographical pattern of the warming episodes was similar to that obtained in simulations of the e nhanced greenhouse effect, with maximum response at high latitudes, in dicating the difficulty of distinguishing between these two phenomena. The observed warming trend for 1877-1987 was compared to the standard error of trends estimated from the power spectrum of the model result s. These comparisons indicate that the observed warming trend is unlik ely to be explained simply by natural variability as simulated here.