RELATIVE DETECTABILITY OF GREENHOUSE-GAS AND AEROSOL CLIMATE-CHANGE SIGNALS

The use of pattern correlations to compare observed temperature change s with predicted anthropogenic effects has greatly increased our confi dence in the reality of these effects. Here we use synthetic observed data to determine the expected behavior of the pattern correlation sta tistic, R(t), and hence clarify some results obtained in previous stud ies. We show that, for the specific case considered here (near-surface temperature changes), even with a perfectly-known signal, expected va lues of R(t) currently should be only of order 0.3-0.5, as observed; t hat R(t) may show markedly non-linear variations in time; that the CO2 -alone signal pattern should be difficult to detect today primarily be cause of data coverage deficiencies; and why the signal due to combine d CO2-aerosol forcing is easier to detect than either the CO2-alone or aerosol-alone signals. Finally, we show that little is to be gained a t present by searching for a time-dependent signal compared with a rep resentative constant signal pattern.