The UK Earth system modelling project.

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.

The main beneficiaries of the project will be:The IPCC and intergovernmental policy makers, including: UK government: particular DEFRA and DECC, DFID, DfT and MoD; UK governmental agencies and industries requiring climate services information.

Maximising the UK Contribution to the IPCC AR6
An important motivation for UKESM1 is the delivery of science to underpin the UK contribution to IPCC AR6, peer reviewed scientific outputs of UKESM1 are expected to contribute to: Working Group (WG) I assessment of the scientific aspects of climate change; WGII assessment of the vulnerability of socio-economic and natural systems to climate change, and adaptation options; WGIII, assessment options for mitigating climate change.

Both the UKESM models themselves and the resulting simulation-data will be made openly available to the NERC research community, supporting a wide spectrum of Earth system science over and above that performed directly in this project. In addition, the UKESM core group will provide support to a number of UK universities planning contributions to other Model Intercomparison Projects (MIPs) in CMIP6. The result of this national collaboration will be an unprecedented UK contribution to CMIP6 and, through this, also an unprecedented contribution to IPCC AR6. Importantly, this contribution will be a national effort, coordinated across the 8 NERC centres in this project, MOHC and UK universities.

Science into Policy:
The Met Office Hadley Centre (MOHC) will benefit from this project through an enhanced capacity to investigate future Earth system change and provide knowledge support to UK government . The ensemble of future projections made with UKESM1 will be made available to the UK research community, with an emphasis on supporting the UK climate impacts research. CMIP6 simulation-data will form the backbone of future data sets used by the Copernicus Climate Change Service (C3S), charged with providing underpinning climate services data to European governments, business and the public. UKESM1 simulations will therefore constitute the primary UK contribution to C3S. We will ensure a subset of the UKESM1 projections produce output required for dynamical downscaling over the UK using either the Met Office Hadley Centre (MOHC) Regional Climate Model of the new MOHC-NERC coupled UK Environmental Prediction model. This activity will be carried out external to the UKESM project by MOHC scientists. The resulting high-resolution (~1.5-4km grid box resolution over the UK) projections, covering the UK and adjacent coastal waters, will be a major resource for UK planners and stakeholders concerned with the risks and opportunities associated with global change. The knowledge developed from the combination of UKESM1 projections and accompanying downscaled data will be an important support for UK government departments involved in planning and policy negotiations with respect to future global change.

Wealth Creation:
There is a growing demand for information on potential future climate states, apparent in the emerging 'Climate Services' sector. The UKESM project will feed information into this sector with a wide range of potential beneficiaries, including agriculture/food security, transport and the insurance sector. This This link can be both directly from the project, through downstream research (e.g. in climate impacts) using UKESM1 data, through use of dynamically downscaled UKESM1 data covering various regions of the globe and through UKESM1 projections being part of the Copernicus Climate Change Service.

Media Relations and Public Engagement:
There is an increasing need for the public to be informed about the science of climate change, how climate projections are made, including explanation of where projection uncertainties arise and how they impact our the delivery of robust future climate estimates. We will endeavour to contribute to this requirement through public presentations and information sheets.