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Effective radiative forcing and adjustments in CMIP6 modelsThe 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 17 contemporary climate models that are participating in the Coupled Model Intercomparison Project (CMIP6) and have contributed to the Radiative Forcing Model Intercomparison Project (RFMIP). Present-day (2014) global-mean anthropogenic forcing relative to pre-industrial (1850) levels from climate models stands at 2.00 (±0.23) W/sq. m, comprised of 1.81 (±0.09) W/sq. m from CO2, 1.08 (± 0.21) W/sq. m from other well-mixed greenhouse gases, −1.01 (± 0.23) W/sq. m from aerosols and −0.09 (±0.13) W/sq. m from land use change. Quoted uncertainties are 1 standard deviation across model best estimates, and 90 % confidence in the reported forcings, due to internal variability, is typically within 0.1 W/sq. m. The majority of the remaining 0.21 W/sq. m is likely to be from ozone. In most cases, the largest contributors to the spread in effective radiative forcing (ERF) is from the instantaneous radiative forcing (IRF) and from cloud responses, particularly aerosol–cloud interactions to aerosol forcing. As determined in previous studies, cancellation of tropospheric and surface adjustments means that the 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/sq. m, 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 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 and little evidence that modelling groups are systematically tuning climate sensitivity or aerosol forcing to recreate observed historical warming.
Document ID
20205006275
Document Type
Accepted Manuscript (Version with final changes)
Authors
Christopher J. Smith ORCID
(University of Leeds Leeds, United Kingdom)
Ryan J. Kramer ORCID
(Universities Space Research Association Columbia, Maryland, United States)
Gunnar Myhre ORCID
(Center for International Climate and Environmental Research Oslo, Norway)
Kari Alterskjær ORCID
(Center for International Climate and Environmental Research Oslo, Norway)
William Collins ORCID
(University of Reading Reading, United Kingdom)
Adriana Sima ORCID
(Sorbonne University Paris, France)
Olivier Boucher ORCID
(Institut Pierre-Simon Laplace Paris, France)
Jean-Louis Dufresne ORCID
(Laboratoire de Météorologie Dynamique Palaiseau, France)
Pierre Naba
(Centre National de Recherches Météorologiques Toulouse, France)
Martine Michou
(Centre National de Recherches Météorologiques Toulouse, France)
Seiji Yukimoto ORCID
(Meteorological Research Institute)
Jason Cole ORCID
(Environment Canada Gatineau, Quebec, Canada)
David Paynter ORCID
(Geophysical Fluid Dynamics Laboratory Princeton, New Jersey, United States)
Hideo Shiogama
(National Institute for Environmental Studies Tsukuba, Japan)
Fiona M. O’Connor
(Met Office Exeter, United Kingdom)
Eddy Robertson
(Met Office Exeter, United Kingdom)
Andy Wiltshire
(Met Office Exeter, United Kingdom)
Timothy Andrews
(Met Office Exeter, United Kingdom)
Cécile Hannay
(National Center for Atmospheric Research Boulder, Colorado, United States)
Ron Miller ORCID
(Goddard Institute for Space Studies New York, New York, United States)
Larissa Nazarenko
(Columbia University New York, New York, United States)
Alf Kirkevåg ORCID
(Norwegian Meteorological Institute Oslo, Norway)
Dirk Olivié
(Norwegian Meteorological Institute Oslo, Norway)
Stephanie Fiedler ORCID
(University of Cologne Cologne, Germany)
Anna Lewinschal
(Stockholm University Stockholm, Sweden)
Chloe Mackallah
(Commonwealth Scientific and Industrial Research Organisation Canberra, Australian Capital Territory, Australia)
Martin Dix
(Commonwealth Scientific and Industrial Research Organisation Canberra, Australian Capital Territory, Australia)
Robert Pincus ORCID
(University of Colorado Boulder Boulder, Colorado, United States)
Piers M. Forster
(University of Leeds Leeds, United Kingdom)
Date Acquired
August 12, 2020
Publication Date
August 17, 2020
Publication Information
Publication: Atmospheric Chemistry and Physics
Volume: 20
Issue: 16
ISSN: 1680-7316
Subject Category
Meteorology And Climatology
Funding Number(s)
CONTRACT_GRANT: NNH15CO48B
CONTRACT_GRANT: DOE-OBER-7457436
CONTRACT_GRANT: NSF-1852977
Distribution Limits
Public
Copyright
Portions of document may include copyright protected material.
Technical Review
External Peer Committee
Keywords
effective radiative forcing
rapid adjustments
oresent day

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