RFMIP

Radiative Forcing Model Intercomparison Project

Website: rfmip.github.io
Protocol paper: Kramer et al. (2026)

The Radiative Forcing Model Intercomparison Project (RFMIP) is a World Climate Research Programme endorsed community model intercomparison project (MIP) that seeks to evaluate why state-of-the-art climate models respond the way they do to different human and natural climate influences such as greenhouse gases, air pollutants, land use change, solar variability and volcanic eruptions.

In the last round of coordinated Earth System model experiments (CMIP6), RFMIP provided critical information on the energetic influence of different climate forcers over the historical period and their potential future effects (Smith et al., 2020). The diversity of model responses to the same aerosol emissions (figure below) can be used to train emulators such as fair to infer information about the Earth’s climate sensitivity (Smith et al., 2021).

RFMIP Phase 2 is now in progress, and will form a key evidence base for CMIP7 and the IPCC Seventh Assessment Report. I am co-chairing RFMIP with Ryan Kramer (NOAA GFDL) and Tim Andrews (UK Met Office).

Effective radiative forcing from aerosols in CMIP6 models (Smith et al., 2021)

References

2026

The Radiative Forcing Model Intercomparison Project (RFMIP2.0) for CMIP7

Ryan J. Kramer, Chris Smith, and Timothy Andrews

Geoscientific Model Development, May 2026

Abstract DOI URL

An external perturbation to the climate system from anthropogenic or natural activity first impacts the climate by inducing an alteration to Earth’s energy budget, known as a radiative forcing. The characteristics of the radiative forcing, such as its global-mean magnitude and spatial pattern, determine the subsequent climate response. Therefore, understanding projections of climate change first requires diagnosing radiative forcing and evaluating its persistent uncertainty in Global Climate Models. As part of the Coupled Model Intercomparison Project phase 7 (CMIP7), the second iteration of the Radiative Forcing Model Intercomparison Project (RFMIP2.0) will enable the systematic characterization of effective radiative forcing and its components across state-of-the-art climate models, through a set of fixed-sea-surface-temperature timeslice and transient experiments. The protocol for RFMIP2.0, introduced here, will in part serve as a continuity and an expansion of core RFMIP experiments first introduced in CMIP6, some of which have now been incorporated into the overarching CMIP7 DECK and Assessment Fast Track protocols given their broad utility. This will allow for a consistent estimate for radiative forcing across multiple model generations, which is valuable for model evaluation and future development. RFMIP2.0 also includes new experiments that will address open questions about the definition of radiative forcing, such as its sensitivity to evolving surface conditions, and will further enhance an ever-growing swath of science applications that rely on an understanding of Earth’s energy budget.

2021

Energy Budget Constraints on the Time History of Aerosol Forcing and Climate Sensitivity

C. J. Smith, G. R. Harris, M. D. Palmer, N. Bellouin, W. Collins, G. Myhre, M. Schulz, J. C. Golaz, M. Ringer, T. Storelvmo, and P. M. Forster

Journal of Geophysical Research: Atmospheres, May 2021

Abstract DOI URL

An observationally constrained time series of historical aerosol effective radiative forcing (ERF) from 1750 to 2019 is developed in this study. We find that the time history of aerosol ERFs diagnosed in CMIP6 models exhibits considerable variation and explore how the time history of aerosol forcing influences the probability distributions of present-day aerosol forcing and emergent metrics such as climate sensitivity. Using a simple energy balance model, trained on CMIP6 climate models and constrained by observed near-surface warming and ocean heat uptake, we derive estimates for the historical aerosol forcing. We find 2005–2014 mean aerosol ERF to be −1.1 (−1.8 to −0.5) W m−2 relative to 1750. Assuming recently published historical emissions from fossil fuel and industrial sectors and biomass burning emissions from SSP2-4.5, aerosol ERF in 2019 is −0.9 (−1.5 to −0.4) W m−2. There is a modest recovery in aerosol forcing (+0.025 W m−2 decade−1) between 1980 and 2014. This analysis also gives a 5%–95% range of equilibrium climate sensitivity of 1.8°C –5.1°C (best estimate 3.1°C) with a transient climate response of 1.2°C –2.6°C (best estimate 1.8°C).

2020

Effective radiative forcing and adjustments in CMIP6 models

Christopher J. Smith, Ryan J. Kramer, Gunnar Myhre, Kari Alterskjær, William Collins, Adriana Sima, Olivier Boucher, Jean-Louis Dufresne, Pierre Nabat, Martine Michou, Seiji Yukimoto, Jason Cole, David Paynter, Hideo Shiogama, Fiona M. O’Connor, Eddy Robertson, Andy Wiltshire, Timothy Andrews, Cécile Hannay, Ron Miller, Larissa Nazarenko, Alf Kirkevåg, Dirk Olivié, Stephanie Fiedler, Anna Lewinschal, Chloe Mackallah, Martin Dix, Robert Pincus, and Piers M. Forster

Atmospheric Chemistry and Physics, Aug 2020

Abstract DOI URL

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 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 m−2, comprised of 1.81 (±0.09) W m−2 from CO2, 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 m−2 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 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 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 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 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.