Effective radiative forcing and adjustments in CMIP6 models

Christopher J Smith , Ryan J Kramer , Gunnar Myhre (1) , Kari Alterskjaer (2) , William Collins , Adriana Sima (3) , Olivier Boucher (4) , Jean-Louis Dufresne (5) , Pierre Nabat (6) , Martine Michou (6) , Seiji Yukimoto (7) , Jason M. Cole (8) , David Paynter , Hideo Shiogama (9) , Fiona M. O'Connor (10) , Eddy Robertson (10) , Andy Wiltshire (11) , Timothy J. Andrews (12) , Cecile Hannay (13) , Ron Miller , Larissa Nazarenko , Alf Kirkevåg , Dirk Olivié , Stephanie Fiedler (14) , Anna Lewinschal , Chloe Mackallah , Martin Dix , Robert Pincus , Piers M Forster (15)

1 CICERO - Center for International Climate and Environmental Research [Oslo]
2 Department of Geosciences [Oslo]
3 LMD - Laboratoire de Météorologie Dynamique (UMR 8539)
4 IPSL (FR_636) - Institut Pierre-Simon-Laplace
5 LMD - Laboratoire de Météorologie Dynamique (UMR 8539)
6 CNRM - Centre national de recherches météorologiques
7 MRI - Meteorological Research Institute [Tsukuba]
8 Blackett Laboratory
9 NIES - National Institute for Environmental Studies
10 MOHC - Met Office Hadley Centre
11 Fitzroy Road
12 Department of Psychology [York, UK]
13 ESRL - NOAA Earth System Research Laboratory
14 KIT - Karlsruhe Institute of Technology
15 University of Leeds

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.

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 17 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 2.00 (± 0.23) W m −2 , comprised of 1.81 (± 0.09) W m −2 from CO 2 , 5 1.08 (± 0.21) W m −2 from other well-mixed greenhouse gases, −1.01 (± 0.23) W m −2 from aerosols and −0.09 (± 0.13) W 1 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 0.21 W m −2 is likely to be from ozone. In most cases, the largest contributors to the spread in ERF is from the instantaneous radiative forcing (IRF) and from cloud responses, particularly aerosol-cloud interactions to aerosol forcing. As determined in previous 10 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 m −2 , exhibiting a less negative mean and narrower range compared to 10 CMIP5 models. The spread in 4×CO 2 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 15 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.

Domaines: Océan, Atmosphère; Climatologie; Météorologie

Christopher J Smith, Ryan J Kramer, Gunnar Myhre, Kari Alterskjaer, William Collins, et al.. Effective radiative forcing and adjustments in CMIP6 models. Atmospheric Chemistry and Physics, 2020, ⟨10.5194/acp-20-9591-2020⟩. ⟨hal-02913575⟩