Assessment of pre-industrial to present-day anthropogenic climate forcing in UKESM1

Abstract

Abstract. Quantifying forcings from anthropogenic perturbations to\nthe Earth system (ES) is important for understanding changes in climate\nsince the pre-industrial (PI) period. Here, we quantify and analyse a wide\nrange of present-day (PD) anthropogenic effective radiative forcings (ERFs)\nwith the UK s Earth System Model (ESM), UKESM1, following the protocols\ndefined by the Radiative Forcing Model Intercomparison Project (RFMIP) and\nthe Aerosol and Chemistry Model Intercomparison Project (AerChemMIP). In\nparticular, quantifying ERFs that include rapid adjustments within a full\nESM enables the role of various chemistry–aerosol–cloud interactions\nto be investigated. Global mean ERFs for the PD (year\xa02014) relative to the PI (year\xa01850)\nperiod for carbon dioxide (CO2), nitrous oxide (N2O),\nozone-depleting substances (ODSs), and methane (CH4) are 1.89\u2009±\u20090.04, 0.25\u2009±\u20090.04, −0.18\u2009±\u20090.04, and 0.97\u2009±\u2009 0.04\u2009W\u2009m−2, respectively. The total greenhouse gas (GHG) ERF is 2.92\u2009±\u20090.04\u2009W\u2009m−2. UKESM1 has an aerosol ERF of −1.09\u2009±\u20090.04\u2009W\u2009m−2. A relatively\nstrong negative forcing from aerosol–cloud interactions (ACI) and a small\nnegative instantaneous forcing from aerosol–radiation interactions (ARI)\nfrom sulfate and organic carbon (OC) are partially offset by a substantial\nforcing from black carbon (BC) absorption. Internal mixing and chemical\ninteractions imply that neither the forcing from ARI nor ACI is linear,\nmaking the aerosol ERF less than the sum of the individual speciated aerosol\nERFs. Ozone (O3) precursor gases consisting of volatile organic compounds\n(VOCs), carbon monoxide (CO), and nitrogen oxides (NOx), but excluding\nCH4, exert a positive radiative forcing due to increases in O3. However, they also lead to oxidant changes, which in turn cause an\nindirect aerosol ERF. The net effect is that the ERF from PD–PI changes in\nNOx emissions is negligible at 0.03\u2009±\u20090.04\u2009W\u2009m−2, while the ERF\nfrom changes in VOC and CO emissions is 0.33\u2009±\u20090.04\u2009W\u2009m−2.\nTogether, aerosol and O3 precursors (called near-term climate forcers\n(NTCFs) in the context of AerChemMIP) exert an ERF of −1.03\u2009±\u20090.04\u2009W\u2009m−2, mainly due to changes in the cloud radiative effect (CRE). There\nis also a negative ERF from land use change (−0.17\u2009±\u20090.04\u2009W\u2009m−2).\nWhen adjusted from year 1850 to 1700, it is more negative than the range of\nprevious estimates, and is most likely due to too strong an albedo response.\nIn combination, the net anthropogenic ERF (1.76\u2009±\u20090.04\u2009W\u2009m−2) is\nconsistent with other estimates. By including interactions between GHGs, stratospheric and tropospheric\nO3, aerosols, and clouds, this work demonstrates the importance of ES\ninteractions when quantifying ERFs. It also suggests that rapid adjustments\nneed to include chemical as well as physical adjustments to fully account\nfor complex ES interactions.\n