EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Effective Radiative Forcing and Adjustments in CMIP6

Christopher Smith1, Ryan Kramer2,3, Gunnar Myhre4, Kari Alterskjær4, Bill Collins5, Robert Pincus6, Piers Forster1, and the RFMIP modelling groups*
Christopher Smith et al.
  • 1University of Leeds, Institute for Climate and Atmospheric Science, Earth & Environment, Leeds, United Kingdom of Great Britain and Northern Ireland (
  • 2Climate and Radiation Laboratory, NASA Goddard Space Flight Center, Geenbelt, MD 20771 USA
  • 3Universities Space Research Association, 7178 Columbia Gateway Drive, Columbia, MD 21046, USA
  • 4CICERO, Oslo, Norway
  • 5Department of Meteorology, University of Reading, UK
  • 6Cooperative Institute for Reseearch in Environmental Sciences, University of Colorado Boulder, CO, USA
  • *A full list of authors appears at the end of the abstract

The effective radiative forcing, which includes the instantaneous forcing plus adjustments from the atmsophere and surface, as emerged as the key metric of evaluating human and natural influence on the climate. We evaluate effective radiative forcing and atmospheric 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 ranges 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 forcing. The spread of present-day aerosol forcing has narrowed compared to CMIP5 models to the range of -0.63 to -1.37 W m-2, with a less negative mean. The spread in CO2 forcing has also narrowed in CMIP6 compared to CMIP5, which may be a consequence of improving radiative transfer parameterisations. We also find that present-day aerosol forcing is uncorrelated with equilibrium climate sensitivity. Therefore, there is no evidence to suggest that the higher climate sensitivity in many CMIP6 models is a consequence of stronger negative present-day aerosol forcing.

RFMIP modelling groups:

Adriana Sima, Olivier Boucher, Jean-Louis Dufresne, Pierre Nabat, Martine Michou, Seiji Yukimoto, Jason Cole, David Paynter, Hideo Shiogama, Fiona O'Connor, Eddy Robertson, Andy Wiltshire, Timothy Andrews, Cecile Hannay, Ron Miller, Larissa Nazarenko, Alf Kirkevag, Dirk Olivie, Stephanie Fiedler

How to cite: Smith, C., Kramer, R., Myhre, G., Alterskjær, K., Collins, B., Pincus, R., and Forster, P. and the RFMIP modelling groups: Effective Radiative Forcing and Adjustments in CMIP6, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10294,, 2020

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Presentation version 1 – uploaded on 05 May 2020
  • CC1: Comment on EGU2020-10294, Tuuli Miinalainen, 06 May 2020

    Comprehensive analysis! I didn't fully understand the phrase in the slide number 7, "The negative forcing from aerosol-cloud interactions tends to outweigh the positive adjustment from GHGs in the anthropogenic sum". Which processes or mechanisms are included for this, only shortwave component or?

    • AC1: Reply to CC1, Christopher Smith, 06 May 2020

      Hello Tuuli, thanks for the feedback! The SW effect is greatest in both the GHG positive adjustment (top row) and the aerosol-cloud interaction (middle row; note we don't call this an adjustment, because the Twomey effect is defined as a component of the instantaneous forcing following the definition of AR5 WG1 Chapter 7). The anthropogenic in the bottom row is roughly the sum of the two (there are some effects from ozone which aren't included in well-mixed GHG, and land-use change effects are very small). It appears that the cloud changes response to the aerosol has a larger effect on cloud responses to total anthropogenic than the GHGs do. There are offsetting LW effects, but comparing the SW, LW  and the net: 2nd, 3rd and 4th columns - you will see the patterns of the SW and the net are very similar. 

      • CC2: Reply to AC1, Tuuli Miinalainen, 07 May 2020

        Thanks for the clarification! Should check your ACP preprint as well. :-)