Effects of anthropogenic GHGs and aerosols on the 20th century summer temperature variations in East Asia from a GCM experiment

The effects of the emissions of greenhouse gases (GHGs) and aerosols on the land-mean summer surface air temperature (T2) are examined in a model sensitivity experiment in which the historical emissions of GHGs and aerosols are implemented in four combinations. The CONT run shows that the natural variability is not related to any long-term T2 trend, i.e., the multi-decadal T2 variations over EA in the 20^th century are primarily related to the anthropogenic emissions of GHGs and aerosols. The HIST run well produces the observed land-mean summer T2 variations for East Asia (EA) which are characterized by warming trends in early and late 20^th century with cooling trend between the two periods. According to the observed T2 trends, three 30-year periods, 1911-1940 (P1), 1941-1970 (P2), and 1971-2000 (P3), are defined for analyzing the multi-decadal T2 trends in the 20^th century. Analysis of the model results shows that the T2 trend in each period is determined by unique effects of GHGs and aerosols. The positive T2 trend in P1 is determined by the warming effects of aerosols by decreases in the shortwave (SW)-reflecting biomass burning aerosols and the GHG-aerosol interaction; the effects of GHGs on T2 are small in P1. The negative T2 trend in P2 results from that the cooling effects of aerosols and the interaction term exceed the warming effects of GHGs. In P3, the warming effects of GHGs and the interaction term exceeds the cooling effects of aerosols. The regional variations in the T2 trends are positively related to the variations in the SW trends and negatively related to the cloudiness trend. Thus, the effects of GHGs and aerosols on the EA T2 are substantially shaped through feedback within the cloud system that involves clouds.