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Preprints
https://doi.org/10.5194/acp-2019-1152
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-1152
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  03 Feb 2020

03 Feb 2020

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This preprint is currently under review for the journal ACP.

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

Fiona M. O'Connor1, N. Luke Abraham2,3, Mohit Dalvi1, Gerd Folberth1, Paul Griffiths2,3, Catherine Hardacre1, Ben T. Johnson1, Ron Kahana1, James Keeble2,3, Byeonghyeon Kim4, Olaf Morgenstern5, Jane P. Mulcahy1, Mark G. Richardson6, Eddy Robertson1, Jeongbyn Seo4, Sungbo Shim4, Joao C. Teixeira1, Steven Turnock1, Jonny Williams5, Andy Wiltshire1, and Guang Zeng5 Fiona M. O'Connor et al.
  • 1Met Office, Exeter, UK
  • 2National Centre for Atmospheric Science, University of Cambridge, UK
  • 3Department of Chemistry, University of Cambridge, UK
  • 4National Institute of Meteorological Sciences, Seogwipo-si, Jeju-do, Korea
  • 5National Institute, for Water and Atmospheric Research, Wellington, New Zealand
  • 6Centre for Environmental Modelling And Computation, University of Leeds, UK

Abstract. Quantifying forcings from anthropogenic perturbations to the Earth System (ES) is important for understanding changes in climate since the pre-industrial period. In this paper, we quantify and analyse a wide range of present-day (PD) anthropogenic climate forcings 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, by quantifying effective radiative forcings (ERFs) that include rapid adjustments within a full ESM, it enables the role of various climate-chemistry-aerosol-cloud feedbacks to be quantified.

Global mean ERFs are 1.83, 0.13, −0.33, and 0.93 W m−2 at the PD (Year 2014) relative to the pre-industrial (PI; Year 1850) for carbon dioxide, nitrous oxide, ozone-depleting substances, and methane, respectively. The PD total greenhouse gas ERF is 2.89 W m−2, larger than the sum of the individual GHG ERFs.

UKESM1 has an aerosol forcing of −1.13 W m−2. A relatively strong negative forcing from aerosol-cloud interactions and a small negative instantaneous forcing from aerosol-radiation interactions are partially offset by a substantial forcing from black carbon absorption. Internal mixing and chemical interactions mean that neither the forcing from aerosol-radiation interactions nor aerosol-cloud interactions are linear, making the total aerosol ERF less than the sum of the individual speciated aerosol ERFs.

Tropospheric ozone precursors, in addition to exerting a positive forcing due to ozone, lead to oxidant changes which in turn cause an indirect aerosol ERF, altering the sign of the net ERF from nitrogen oxide emissions. Together, aerosol and tropospheric ozone precursors (near-term climate forcers, NTCFs) exert a global mean ERF of −1.12 W m−2, mainly due to changes in the cloud radiative effect. There is also a negative PD ERF from land use (−0.32 W m−2). It is outside the range of previous estimates, and is most likely due to too strong an albedo response. In combination, the net anthropogenic ERF is potentially biased low (1.61 W m−2) relative to other estimates, due to the inclusion of non-linear feedbacks and ES interactions.

By including feedbacks between greenhouse gases, stratospheric and tropospheric ozone, aerosols, and clouds, some of which act non-linearly, this work demonstrates the importance of ES interactions when quantifying climate forcing. It also suggests that rapid adjustments need to include chemical as well as physical adjustments to fully account for complex ES interactions.

Fiona M. O'Connor et al.

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Fiona M. O'Connor et al.

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Latest update: 07 Aug 2020
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Short summary
This paper calculates how changes in emissions and/or concentrations of different atmospheric constituents since the pre-industrial era have altered the Earth's energy budget at the present day, using a metric called effective radiative forcing. The impact of land use change has also been assessed. We find that individual contributions do not add linearly and different Earth System interactions can affect the magnitude of the calculated effective radiative forcing.
This paper calculates how changes in emissions and/or concentrations of different atmospheric...
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