Understanding and Attributing Composition-Climate Feedbacks in the Earth System

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.

The main users of the research and strategies for dissemination to these groups are described below.

The UK Met Office

The outcomes of the project will contribute to the future development of the Met Office Unified Model. The UM has been adopted by more than 42 end-users around the globe, including the Centre for Australian Weather and Climate Research, the Korea Meteorological Administration and the National Institute of Water and Atmospheric Research (New Zealand), and many of these external partners will also benefit from the research.

The research will make a substantial contribution to the joint Met Office/NERC programme to develop the UK Earth System Model 1 as part of the Joint Weather and Climate Research Programme (JWCRP). Met Office scientists will be closely engaged with the research, both via existing collaborations established by development of UKCA and new links with the 'Understanding Climate Change' group (see letter of support). Regular visits to Exeter are planned.

Other users could benefit from the implementation of new diagnostic tools into UK-ESM1, and efforts will be made to get these lodged in the model code so that they are available to the wider community.

Other international modelling centres

Many international centres are currently developing Earth System Models. The research will directly impact on these centres and their activities through increases in fundamental understanding of Earth System processes. The research will be disseminated to these users through publications in peer-reviewed journals and through talks at international conferences and meetings. The extended visit to NOAA will also enable links to be made with the modelling group at the National Center for Atmospheric Research (NCAR) who are developing the CESM-WACCM Earth System Model.

Other researchers working on radiative forcing and climate feedbacks

There is a large international community working on topics related to radiative forcing and climate feedbacks who will be interested in the research. It will be disseminated to these users through my involvement with the Radiative Forcing Model Intercomparison Project (RFMIP), which is planned as part of CMIP6 (see attached letter of support from RFMIP co-lead Prof Piers Forster). This will strengthen links to international scientific assessments such as future IPCC reports.

Other researchers working on chemistry-climate processes and air quality

The research will make substantial steps forward in the role of understanding of chemical processes in the climate system and will therefore impact on the wider atmospheric chemistry community. Cambridge is closely involved in the Chemistry-Climate Model Initiative (CCMI), which will provide a channel for disseminating the research through meetings and workshops. There is a close symbiosis between air quality and climate for topics related to composition. Whilst the research will focus on climate, it is also anticipated that the air quality community will be interested in the work, and efforts will be made to engage with them where possible.

Policy makers and the general public

The research will be highly relevant to future international scientific assessments, such as Intergovernmental Panel on Climate Change reports and the World Meteorological Organisation (WMO) Ozone Assessments. I was invited to participate in the final review process for the 2014 WMO Ozone Assessment Report and therefore have contacts within this community that I will maintain during the research.

The research planned as part of this project is aimed at increasing our understanding of our environment and its sensitivity to mankind's actions. This will contribute to the large body of climate research that enables scientists to provide information to policy makers and the general public, and is therefore of indirect benefit to the wider society.