INFLUENCES OF ANTHROPOGENIC AND OCEANIC FORCING ON RECENT CLIMATE-CHANGE

We report a new approach to climate change detection and attribution u sing an atmospheric general circulation model (AGCM), complementary to the traditional approach using coupled ocean-atmosphere models (CGCM) . Ensembles of simulations were run with an AGCM forced with the obser ved history of sea-surface temperature (SST) and sea-ice extent and re peated with a variety of forcing factors added incrementally. SST chan ges alone give a warming of only about 0.15 degrees C in annual global land surface air temperature between 1950 and 1994. Addition of chang ing greenhouse gases, including off-line calculations of tropospheric ozone, give a further warming of 0.15 degrees C, still 0.2 degrees C l ess than observed. This deficit in warming derives from the Northern H emisphere (NH) winter half-year as the summer half-year NH temperature is well-simulated. In the lower stratosphere,little cooling is simula ted using the observed changes of SST alone but increasing the concent ration of greenhouse gases and decreasing the concentration of stratos pheric ozone leads to a cooling close to that observed. Inclusion of c hanges to tropospheric ozone with other forcing factors, the first tim e this has been attempted, gives good simulations of tropospheric and stratospheric temperature changes; these are significantly more simila r to observations than using SST variations alone. Despite the uncerta inties, these simulations strongly indicate a discernible anthropogeni c effect on the annual mean thermal structure of the atmosphere, the f irst time this has been shown in the presence of the observed variatio ns of SST and sea-ice extent.