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Discussion papers
https://doi.org/10.5194/acp-2019-1212
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-1212
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 20 Jan 2020

Submitted as: research article | 20 Jan 2020

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This preprint is currently under review for the journal ACP.

Effective radiative forcing and adjustments in CMIP6 models

Christopher J. Smith1, Ryan J. Kramer2,3, Gunnar Myhre4, Kari Alterskjær4, William Collins5, Adriana Sima6, Olivier Boucher6, Jean-Louis Dufresne6, Pierre Nabat7, Martine Michou7, Seiji Yukimoto8, Jason Cole9, David Paynter10, Hideo Shiogama11,12, Fiona M. O'Connor13, Eddy Robertson13, Andy Wiltshire13, Timothy Andrews13, Cécile Hannay14, Ron Miller15, Larissa Nazarenko15, Alf Kirkevåg16, Dirk Olivié16, Stephanie Fiedler17, Robert Pincus18,19, and Piers M. Forster1 Christopher J. Smith et al.
  • 1School of Earth & Environment, University of Leeds, LS2 9JT, UK
  • 2Climate and Radiation Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
  • 3Universities Space Research Association, 7178 Columbia Gateway Drive, Columbia, MD 21046, USA
  • 4CICERO, Oslo, Norway
  • 5Department of Meteorology, University of Reading, Reading, UK
  • 6Laboratoire de Météorologie Dynamique, Institut Pierre-Simon Laplace, Sorbonne Université/CNRS, 4 Place Jussieu, 75252 Paris Cedex 05, France
  • 7CNRM, Université de Toulouse, Météo-France, CNRS, Toulouse, France
  • 8Meteorological Research Institute, Tsukuba, Japan
  • 9Canadian Centre for Climate Modelling and Analysis, Environment Canada, Victoria, British Columbia, Canada
  • 10Geophysical Fluid Dynamics Laboratory, Princeton University Forrestal Campus, 201 Forrestal Road, Princeton, NJ 08540-6649, USA
  • 11Center for Global Environmental Research, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan
  • 12Atmosphere and Ocean Research Institute, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan
  • 13Met Office Hadley Centre, FitzRoy Road, Exeter, EX1 3PB, UK
  • 14NCAR/UCAR, Boulder, Colorado, USA
  • 15NASA Goddard Institute for Space Studies, New York, NY 10025, USA
  • 16Norwegian Meteorological Institute, Oslo, Norway
  • 17Max-Planck-Institut für Meteorologie, Hamburg, Germany
  • 18Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA
  • 19NOAA/ESRL Physical Sciences Division, Boulder, CO, USA

Abstract. The effective radiative forcing, which includes the instantaneous forcing plus adjustments from the atmosphere and surface, has emerged as the key metric of evaluating human and natural influence on the climate. We evaluate effective radiative forcing and adjustments in 13 contemporary climate models that are participating in CMIP6 and have contributed to the Radiative Forcing Model Intercomparison Project (RFMIP). Present-day (2014) global mean anthropogenic forcing relative to pre-industrial (1850) from climate models stands at 1.97 (± 0.26) W m−2, comprised of 1.80 (± 0.11) W m−2 from CO2, 1.07 (± 0.21) W m−2 from other well-mixed greenhouse gases, −1.04 (± 0.23) W m−2 from aerosols and −0.08 (± 0.14) W m−2 from land use change. Quoted uncertainties are one standard deviation across model best estimates, and 90 % confidence in the reported forcings, due to internal variability, is typically within 0.1 W m−2. The majority of the remaining 0.17 W m−2 is likely to be from ozone. As determined in previous studies, cancellation of tropospheric and surface adjustments means that the traditional stratospherically adjusted radiative forcing is approximately equal to ERF for greenhouse gas forcing, but not for aerosols, and consequentially, not for the anthropogenic total. The spread of aerosol forcing ranges from −0.63 to −1.37 W m−2, exhibiting a less negative mean and narrower range compared to 10 CMIP5 models. The spread in 4 × CO2 forcing has also narrowed in CMIP6 compared to 13 CMIP5 models. Aerosol forcing is uncorrelated with equilibrium climate sensitivity. Therefore, there is no evidence to suggest that the increasing spread in climate sensitivity in CMIP6 models, particularly related to high-sensitivity models, is a consequence of a stronger negative present-day aerosol forcing.

Christopher J. Smith et al.

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Status: open (until 31 Mar 2020)
Status: open (until 31 Mar 2020)
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Latest update: 25 Feb 2020
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Short summary
The spread in effective radiative forcing for both CO2 and aerosols is narrower in the latest CMIP6 generation of climate models than it is in CMIP5. For the case of CO2 it is likely that model radiation parameterisations have improved. Tropospheric and stratospheric radiative adjustments to the forcing behave differently for different forcing agents, and there is still significant diversity in how clouds respond to forcings, particularly for total anthropogenic forcing.
The spread in effective radiative forcing for both CO2 and aerosols is narrower in the latest...
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