The UK Earth system modelling project.

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Global climate change is one of the leading environmental threats facing mankind. To develop appropriate mitigation and adaptation strategies requires accurate projections of the future state of the Earth's climate. To address this, the research community have developed Global Climate Models (GCMs) that describe the main physical processes in the coupled climate system. These mathematical-computer models are integrated forwards in simulated time, from a pre-industrial period (before ~1850) to present-day, forced by observed estimates of key greenhouse gases (e.g. carbon dioxide, methane, ozone), aerosols and land-use. The models are then continued into the simulated future forced by a range of greenhouse gas, aerosol and land-use scenarios representing plausible future socio-economic development pathways. Each of the time-evolving model future climates are then compared to the pre-industrial and present-day climates from the same model. This analysis results in an ensemble of climate change estimates, linked to each of the applied development pathways, that can be used to assess potential socio-economic and ecological impacts and aid in the development of climate change mitigation and adaptation policies.GCMs have recently been further developed into Earth system models (ESMs). A key difference between ESMs and GCMs is the former include an interactive description of the global carbon cycle. Climate change is primarily driven by human emissions of carbon dioxide which traps a fraction of the Earth's emitted radiation in the atmosphere, warming it and the Earth's surface. This direct warming from increasing carbon dioxide can be amplified or damped by various feedbacks in the climate system (e.g. involving water vapour, clouds or sea-ice). A key determinant of the climate change impact of human-emitted carbon dioxide is how much of the emitted gas actually stays in the atmosphere where it can interact with the Earth's emitted radiation. Presently, around 50% of the carbon dioxide emitted by humans stays in the atmosphere, the remaining 50% being taken up, in roughly equal measures, by the terrestrial biosphere and the world oceans. There is increasing evidence to suggest the efficiency of these natural carbon reservoirs in absorbing human-emitted carbon dioxide may change in the future, being sensitive to both the concentration of carbon dioxide in the Earth system and to the induced climate change. A reduction in the uptake efficiency of Earth's natural carbon reservoirs would result in a larger fraction of emitted carbon dioxide remaining in the atmosphere and thereby a larger climate change (warming) for a given cumulative emission of carbon dioxide.To address the need to simulate both the changing global climate and the carbon cycle response to a changing climate and changing atmospheric composition, we are developing the 1st UK Earth system model, based on the core physical GCM, HadGEM3, developed at the Met Office. This development is a major collaboration between NERC centres and the Met Office, integrating a large body of core research and development into a single, world-leading ESM. This proposal aims to secure the NERC funding to maintain this collaboration. The project will support the final development and community release of the 1st UKESM models, as well as application of these models to a range of collaborative science experiments carried out at the international level to support the IPCC AR6. The project has a major emphasis on evaluating the full range of climate and biogeochemical processes and interactions simulated by UKESM1 models with an aim to increase confidence in future projections made with the models. The project will also generate and analyse a suite of such projections and deliver a set of robust estimates of Earth system change to UK government, business and the public. Finally, the project will initiate long-term development of a 2nd version of the UKESM model, for release ~2023.