Distinguishing Aerosol Impacts on Climate over the Past Century

Aerosol direct ( DE), indirect (IE), and black carbon-snow albedo (BAE) effects on climate between 1890 and 1995 are compared using equilibrium aerosol-climate simulations in the Goddard Institute for Space Studies General Circulation Model coupled to a mixed layer ocean. Pairs of control (1890)-perturbation ( 1995) with successive aerosol effects allow isolation of each effect. The experiments are conducted both with and without concurrent changes in greenhouse gases (GHG). Anew scheme allowing dependence of snow albedo on black carbon snow concentration is introduced. The fixed GHG experiments global surface air temperature ( SAT) changed by -0.2 degrees, -1.0 degrees, and +0.2 degrees C from the DE, IE, and BAE. Ice and snow cover increased 1% from the IE and decreased 0.3% from the BAE. These changes were a factor of 4 larger in the Arctic. Global cloud cover increased by 0.5% from the IE. Net aerosol cooling effects are about half as large as the GHG warming, and their combined climate effects are smaller than the sum of their individual effects. Increasing GHG did not affect the IE impact on cloud cover, however they decreased aerosol effects on SAT by 20%, and on snow/ice cover by 50%; they also obscure the BAE on snow/ ice cover. Arctic snow, ice, cloud, and shortwave forcing changes occur mostly during summer-fall, but SAT, sea level pressure, and longwave forcing changes occur during winter. An explanation is that aerosols impact the cryosphere during the warm season but the associated SAT effect is delayed until winter.