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Abstract

The enhancement of the stratospheric sulphate aerosol layer has been proposed as a method to abate the global warming caused by anthropogenic greenhouse gas emissions. In this thesis we present a newly developed global atmosphere-aerosol model, evaluate its performance against observations, and apply it to study the effectiveness and impacts of this possible form of climate engineering. We find that the achievable cooling effect per injected sulphur mass unit may decrease more drastically for larger injections than previously estimated and that injections at higher altitude or over larger areas do not result in a stronger cooling. The effectiveness of the method may therefore be rather limited, at least when using tropical injections of sulphur dioxide as in our model experiments. In addition, there are several potentially harmful side effects, such as stratospheric heating due to absorption of radiation by the aerosol causing strong perturbations in atmospheric dynamics, composition, and high-level clouds. Furthermore, we find that the radiative effects of stratospheric aerosol injection and marine cloud brightening, another proposed geoengineering technique, would be largely additive and complementary when applying them together. This might allow the design of portfolio approaches to achieve specific climate goals and reduce unintended side effects of climate engineering.