Assessment of pre-industrial to present-day anthropogenic climate forcing in UKESM1
Authors: O'Connor Fiona M. , Abraham N. Luke , Dalvi Mohit , Folberth Gerd A. , Griffiths Paul T. , Hardacre Catherine , Johnson Ben T. , Kahana Ron , Keeble James , Kim Byeonghyeon , Morgenstern Olaf , Mulcahy Jane P. , Richardson Mark , Robertson Eddy , Seo Jeongbyn , Shim Sungbo , Teixeira João C. , Turnock Steven T. , Williams Jonny , Wiltshire Andrew J. , Woodward Stephanie , Zeng Guang 
Published: January 2021

Abstract. Quantifying forcings from anthropogenic perturbations to
the Earth system (ES) is important for understanding changes in climate
since the pre-industrial (PI) period. Here, we quantify and analyse a wide
range of present-day (PD) anthropogenic effective radiative forcings (ERFs)
with the UK's Earth System Model (ESM), UKESM1, following the protocols
defined by the Radiative Forcing Model Intercomparison Project (RFMIP) and
the Aerosol and Chemistry Model Intercomparison Project (AerChemMIP). In
particular, quantifying ERFs that include rapid adjustments within a full
ESM enables the role of various chemistry–aerosol–cloud interactions
to be investigated. Global mean ERFs for the PD (year 2014) relative to the PI (year 1850)
period for carbon dioxide (CO2), nitrous oxide (N2O),
ozone-depleting substances (ODSs), and methane (CH4) are 1.89 ± 0.04, 0.25 ± 0.04, −0.18 ± 0.04, and 0.97 ±  0.04 W m−2, respectively. The total greenhouse gas (GHG) ERF is 2.92 ± 0.04 W m−2. UKESM1 has an aerosol ERF of −1.09 ± 0.04 W m−2. A relatively
strong negative forcing from aerosol–cloud interactions (ACI) and a small
negative instantaneous forcing from aerosol–radiation interactions (ARI)
from sulfate and organic carbon (OC) are partially offset by a substantial
forcing from black carbon (BC) absorption. Internal mixing and chemical
interactions imply that neither the forcing from ARI nor ACI is linear,
making the aerosol ERF less than the sum of the individual speciated aerosol
ERFs. Ozone (O3) precursor gases consisting of volatile organic compounds
(VOCs), carbon monoxide (CO), and nitrogen oxides (NOx), but excluding
CH4, exert a positive radiative forcing due to increases in O3. However, they also lead to oxidant changes, which in turn cause an
indirect aerosol ERF. The net effect is that the ERF from PD–PI changes in
NOx emissions is negligible at 0.03 ± 0.04 W m−2, while the ERF
from changes in VOC and CO emissions is 0.33 ± 0.04 W m−2.
Together, aerosol and O3 precursors (called near-term climate forcers
(NTCFs) in the context of AerChemMIP) exert an ERF of −1.03 ± 0.04 W m−2, mainly due to changes in the cloud radiative effect (CRE). There
is also a negative ERF from land use change (−0.17 ± 0.04 W m−2).
When adjusted from year 1850 to 1700, it is more negative than the range of
previous estimates, and is most likely due to too strong an albedo response.
In combination, the net anthropogenic ERF (1.76 ± 0.04 W m−2) is
consistent with other estimates. By including interactions between GHGs, stratospheric and tropospheric
O3, aerosols, and clouds, this work demonstrates the importance of ES
interactions when quantifying ERFs. It also suggests that rapid adjustments
need to include chemical as well as physical adjustments to fully account
for complex ES interactions.