The Radiative Forcing Model Intercomparison Project for CMIP7

Reducing uncertainties in future climate projections requires improved understanding of both the effective radiative forcing (ERF) and the climate response. The Radiative Forcing Model Intercomparison Project (RFMIP) for CMIP7 proposes a set of diagnostic experiments in global climate models to evaluate ERF in these models.

Experiments in RFMIP are run with a pre-industrial climatology of sea surface temperatures (labelled piClim) to minimize the influence of surface temperature change on the top-of-atmosphere energy budget. With an atmosphere-only configuration, as there is no slow ocean response, models equilibrate quickly to a change in forcing and do not require long run times. We use 30-year “time slice” experiments to diagnose ERF to steady-state changes to different combinations of forcing agents. We also propose a set of 251-year (1850-2100) “transient” experiments where forcings are prescribed following historical and future trajectories. 

Diagnosing radiative forcing is a core model property, and therefore three previous RFMIP experiments have been selected to be included in the CMIP7 DECK:

• piClim-control (timeslice; baseline comparison for other experiments)
• piClim-4xCO2 (timeslice; quadrupling of pre-industrial CO2 concentrations)
• piClim-anthro (timeslice; present-day anthropogenic forcers)

Furthermore a set of RFMIP experiments are identified as highly societally relevant and have been included in the CMIP7 Assessment Fast Track (AFT): 

• piClim-aer (timeslice; present-day aerosols)
• piClim-histaer (transient; historical and future aerosols)
• piClim-histall (transient; historical and future all forcings)

Outside of DECK and AFT, we organise RFMIP experiments into three tiers by priority and the likelihood of modelling centers’ ability to run them. All CMIP6 RFMIP experiments are present in Tiers 1 and 2. We propose novel extensions in Tier 3, including fixing land as well as sea surface temperatures (piClim-FixedTL) in order to more accurately estimate the ERF; evaluating CO2 ERF at different concentrations other than a quadrupling to assess deviation from logarithmic behaviour; changing the climatic baseline to investigate surface temperature effects; and including biogeochemically and radiatively decoupled analogues of piClim-4xCO2.