Understanding and Attributing Composition-Climate Feedbacks in the Earth System

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The Earth's surface has warmed by ~0.8 degrees Celsius over the past 130 years. A large body of scientific evidence indicates that the majority of this warming has been driven by mankind's activities: principally the emission of greenhouse gases, like carbon dioxide, into the atmosphere. If greenhouse gas emissions continue unabated, it is considered likely that dangerous climate change will occur in many parts of the globe by the end of the century or earlier. It is therefore crucial that the scientific community is able to provide quantitative, reliable information about climate in the coming decades to enable governments to implement appropriate adaptation and mitigation strategies in good time. Whilst the fundamental physics that links increases in greenhouse gases to global warming is very robust and well understood, the exact magnitude of surface temperature change depends upon a complex array of feedback loops that amplify or damp the response, much like in an electronic circuit. For example, the melting of Arctic sea ice and glaciers is a positive feedback that enhances surface warming because the Earth's surface can absorb more incoming energy from the Sun leading to further warming. A comprehensive understanding of these feedbacks is required if we are to be able to provide quantitative information about future climate.Climate projections are strongly reliant upon complex climate 'simulators' that capture a wide range of physical processes. These large computer programs are run of the world's most powerful supercomputers and have developed in leaps and bounds over the past few decades. They now include sophisticated representations of the atmosphere, ocean, sea ice, vegetation, land surface, ocean biogeochemistry and atmospheric chemical processes: so-called Earth System Models. This wide array of processes means that new interactions and feedback loops will be captured that could be important for our understanding of climate; many of these loops may have been previously ignored or not well represented in less comprehensive climate models and therefore scientific research is required to understand them in detail.An important candidate for 'new' interactions in the Earth system is atmospheric chemical processes. Both chemical and transport processes are sensitive to climatic conditions. This means that the distribution of other gases in the atmosphere, such as ozone, will also change as levels of carbon dioxide increase. Ozone is also a greenhouse gas, so changes in its abundance will have further effects on our climate. This coupling between carbon dioxide, ozone levels and climate is one example of a chemistry-climate feedback loop. These feedbacks are complex to understand because they involve sequences of processes that are intimately coupled. The main goal of this research is to investigate the two-way interactions between climate and the composition of Earth's atmosphere, and to determine which feedback loops are important for our understanding of climate change.Such studies are only now possible because of the recent rapid progress in our capabilities to simulate the Earth system. The project will track the cutting edge developments in this area by using a state-of-the-art Earth System Model currently being built by the UK scientific community. This research will improve our fundamental understanding of how chemical processes affect Earth's climate and will shed light on their role in determining how climate may evolve in the coming decades.