| Summary: | Aerosols are short-lived in the atmosphere, and so their distribution and climate forcing is very inhomogeneous. To understand the behaviour of the climate system in response to inhomogeneous forcing, and to inform emission policy choices, we must investigate how emissions from individual geographic regions affect the climate regionally and globally. I present here the results of two interwoven modelling studies. First, I analyse the simulated temperature response to perturbing sulfur dioxide emissions over a specific region – China – in three current generation climate models. Second, I systematically investigate with a single model the temperature and precipitation responses to black carbon and sulfur dioxide emissions from the United States, Europe, East Asia, and India. These simulations reveal in the first instance that there is very large uncertainty around aerosol-climate interactions in present climate models. Removing SO2 emissions from China results in a six-fold difference in the optical depth and short-wave flux changes over China between different models, and the resulting surface temperature response is poorly constrained. However, the subsequent systematic perturbations indicate that in the event the regional forcing is large, then there are striking features of the climate response that are consistent across different perturbation locations. Emission changes always result not only in a strong local response around the emission region, but also a strong remote response, the pattern of which is insensitive to the original location of emission changes. There is, however, variation in the efficacy with which emissions from different regions force the climate, with US and European sulphur dioxide emission changes having a larger effect than East Asian emission changes. The results presented here are relevant for understanding the effect of potential future emission controls, and also for understanding how the climate responds to different localised forcings, which has not been tested in complex coupled climate models previously.
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