Assessment of the pre-industrial to present-day anthropogenic forcing, including apportionment of the methane and halocarbon forcings: First results from UKESM1

The role of different forcing agents in historical and future climate change remains a key motivation and scientific question for the 6th Coupled Model Intercomparison Project (CMIP6). Fundamental to this question is a quantitative understanding of their impacts on the Earth's radiative balance, linked to the physical and chemical perturbations that arise from anthropogenic activities and natural processes. In this work, effective radiative forcings (ERFs) are quantified for a wide range of anthropogenic and natural forcing agents using the UK's Earth System Model, UKESM1. By adopting the protocol from the Radiative Forcing Model Intercomparison Project (RFMIP), pre-industrial (PI) to present-day (PD) ERFs are calculated consistently for all climate forcers, including long-lived well-mixed greenhouse gases, ozone, aerosols, land use, solar activity, and volcanic eruptions. Additional UKESM1 simulations apportion the methane ERF to forcing by methane itself, and to resulting changes in ozone, aerosols, and stratospheric water vapour. The PI-to-PD methane ERF is at least 50% larger than the forcing by methane alone. We find that the PI-to-PD change in methane also gives rise to a positive aerosol forcing; the mechanisms for this aerosol forcing are discussed. A similar apportionment approach is used to elucidate the PD forcing by halocarbons. Apportionment of the ERFs in this way helps to explain some of the multi-model spread in PD estimates of climate forcing.