TOWARDS THE DETECTION AND ATTRIBUTION OF AN ANTHROPOGENIC EFFECT ON CLIMATE

It has been hypothesized recently that regional-scale cooling caused b y anthropogenic sulfate aerosols may be partially obscuring a warming signal associated with changes in greenhouse gas concentrations. Here we use results from model experiments in which sulfate and carbon diox ide have been varied individually and in combination in order to test this hypothesis. We use centered [R(t)] and uncentered [C(t)] pattern similarity statistics to compare observed time-evolving surface temper ature change patterns with the model-predicted equilibrium signal patt erns. We show that in most cases, the C(t) statistic reduces to a meas ure of observed global-mean temperature changes, and is of limited use in attributing observed climate changes to a specific causal mechanis m. We therefore focus on R(t), which is a more useful statistic for di scriminating between forcing mechanisms with different pattern signatu res but similar rates of global mean change. Our results indicate that over the last 50 years, the summer (JJA) and fall (SON) observed patt erns of near-surface temperature change show increasing similarity to the model-simulated response to combined sulfate aerosol/CO2 forcing. At least some of this increasing spatial congruence occurs in areas wh ere the real world has cooled. To assess the significance of the most recent trends in R(t) and C(t), we use data from multi-century control integrations performed with two different coupled atmosphere-ocean mo dels, which provide information on the statistical behavior of 'unforc ed' trends in the pattern correlation statistics. For the combined sul fate aerosol/CO2 experiment, the 50-year R (t) trends for the JJA and SON signals are highly significant. Results are robust in that they do not depend on the choice of control run used to estimate natural vari ability noise properties. The R(t) trends for the CO2-only signal are not significant in any season. C(r) trends for signals from both the C O2-only and combined forcing experiments are highly significant in all seasons and for all trend lengths (except for trends over the last 10 years), indicating large global-mean changes relative to the two natu ral variability estimates used here. The caveats regarding the signals and natural variability noise which form the basis of this study are numerous. Nevertheless, we have provided first evidence that both the largest-scale (global-mean) and smaller-scale (spatial anomalies about the global mean) components of a combined CO2/anthropogenic sulfate a erosol signal are identifiable in the observed near-surface air temper ature data. If the coupled-model noise estimates used here are realist ic, we can be highly confident that the anthropogenic signal that we h ave identified is distinctly different from internally generated natur al variability noise. The fact that we have been able to detect the de tailed spatial signature in response to combined CO2 and sulfate aeros ol forcing, but not in response to CO2 forcing alone, suggests that so me of the regional-scale background noise (against which we were tryin g to detect a CO2-only signal) is in fact part of the signal of a sulf ate aerosol effect on climate. The large effect of sulfate aerosols fo und in this study demonstrates the importance of their inclusion in ex periments designed to simulate past and future climate change.