LTSM1 UKESM Extension

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, we develop Global Climate Models (GCMs) that describe the main physical processes in the coupled climate system. These models are integrated forwards in simulated time, from a pre-industrial period to present-day, forced by observed estimates of key greenhouse gases, aerosols and land-use. The models are then continued into the future forced by a range of greenhouse gas, aerosol and land-use scenarios. Each of the model future climates can then compared to the simulated present-day climates. This analysis results in an ensemble of climate change estimates that can be used to assess the socio-economic and ecological impacts of the simulated changes and aid in the development of mitigation and adaptation policies.

GCMs have been further developed into Earth system models (ESMs), as we did in the UKESM LTSM, where UKESM1 was developed from the physical model, HadGEM3-GC3.1. A key difference between ESMs and GCMs is the former include an interactive description of the global carbon cycle supporting analysis of both physical climate change and potential changes in the efficacy by which anthropogenic emitted CO2 is taken up by natural carbon reservoirs. A reduction in the uptake efficiency of Earth's natural carbon reservoirs may result in a larger fraction of emitted carbon dioxide remaining in the atmosphere to warm the planet. Accurate estimates of the future evolution of both the global climate system and the carbon cycle are therefore crucial for getting a clear picture of the future risks humanity faces, as well as for developing mitigation actions (that typically target modifying the efficacy of carbon uptake) to keep global warming to acceptable levels.

To address this need we developed the 1st UK Earth system model (UKESM1) and ran it for a large suite of experiments in the 6th Coupled Model Intercomparison Project (CMIP6). UKESM1 is the most advanced Earth system model in the world today and as well as a coupled physical climate model also includes interactive treatment of (i) the global carbon cycle and vegetation, (ii) atmospheric chemistry and aerosols and (iii) models for the Greenland and Antarctic ice sheets. We have run a large (19 member) ensemble of historical simulations with UKESM1 (1850 to 2015) and extended a number of these into the future (2015 to 2100) following 7 different future emission pathways from CMIP6 scenarioMIP. In this extension, we propose a detailed analysis of the UKESM1 historical ensemble and the suite of scenarioMIP projections. Our aims are (i) to better understand what drives observed historical Earth system change and ask how well UKESM1 represents these changes, (ii) with the knowledge from (i), analyse simulated Earth system change in the UKESM1 scenarioMIP ensemble, combining this with the CMIP6 multi-model ensemble, to document the range of simulated changes across the coupled Earth system over the coming century. Two primary emphases in this analysis will be; (a) to document and contrast regional changes at different levels of global mean warming (e.g. 2C or 3C) and (b) where possible, to constrain the various coupled feedbacks simulated by UKESM1 that drive the changes we see. In addition to these two science goals, we will also continue to provide support to the large UK UKESM user and model development community and plan to hold two consultation workshops with (i) UK climate policymakers and (ii) UK climate impacts researchers. In these workshops, we will present our findings on predicted future Earth system change and begin a two-way dialogue on how UK Earth system modelling can best serve the needs of these two groups, developing future collaborations based on mutual understanding of each groups needs and goals.