Climate Sensitivity and Carbon Feedbacks in UK Earth System Models: HadGEM3-GC3.1 and UKESM1

Climate and carbon cycle feedbacks are critical determinants of future warming under any emissions scenario, but their magnitudes are highly uncertain and remain scientific priorities for the 6th Coupled Model Intercomparison Project (CMIP6). Earth system models are one of the few tools available to understand the processes which control these feedbacks. We report the climate sensitivity of two CMIP6-generation models (a physical climate model HadGEM3-GC3.1 and an Earth system model UKESM1) and present analysis of the key feedbacks controlling the sensitivity of these models. In particular we examine increases in climate sensitivity that arise from recent improvements in physical processes. We further report on carbon cycle feedbacks in UKESM1 and analyse the key processes determining carbon-climate and carbon-concentration feedbacks. The inclusion of nitrogen cycling in UKESM1 significantly alters both of these feedbacks, primarily through changes in the response of soil carbon to system perturbations. We consider the implications of these changes in physical and carbon cycle feedbacks for CO ₂ emissions pathways consistent with given warming targets.