Climate response to indirect anthropogenic sulfate forcing

Abstract

A general circulation model (GCM) has been used to conduct sensitivity tests of the climatic influence imparted by a cloud albedo change hypothesized to result from anthropogenic increases in atmospheric sulfur. The global distribution of anthropogenic sulfate aerosols is computed with a simplified 3‐D transport model. The NCAR CCM1 has been run with a cloud albedo perturbation that is a function of the distribution of anthropogenic sulfur particles. We report climate statistics from the last 20 years of 30 year GCM control and experiment runs. The climate response is strongest in the northern hemisphere winter, with cooling over the North Atlantic and North Pacific oceans on the order of 2–6°C. The 500 mb geopotential height field shows a significant deepening over the Canadian provinces, enhancing the northerly flow over the North American and North Atlantic regions during boreal winter. The equilibrium climate does not, however, cool over central Europe in northern hemisphere winter, despite this region being one of the most heavily impacted areas in the world by sulfate aerosol. The anthropogenic sulfate ‘indirect’ forcing elicits a highly non‐linear climate response that can be explained through changes in the hemispheric wave train. These results may assist in explaining the long‐standing climate change issue of what causes the cooling over the North Atlantic and North Pacific over the last decades, a feature that is not explained by increases in greenhouse gases alone.