The first goal of RFMIP is to investigate the effective radiative forcing (ERF) across climate models and to understand the reasons for inter-model differences. The effective radiative forcing includes rapid adjustments in the troposphere such as the changes in clouds and vertical temperature structure, in addition to the well-known stratospheric adjustment that occurs in response to an abrupt change in greenhouse gas forcing. Rapid adjustments are distinct from climate feedbacks, which scale with changing near-surface air temperature.
As a minimum, a group of six 30-year fixed sea-surface temperature model runs will be requested from each modelling group (table 1). ERF will be diagnosed as the difference in net top-of-atmosphere radiative flux between each experiment and the model’s pre-industrial control run. Fixing sea-surface temperatures and sea-ice conditions suppresses climate feedbacks and allows ERF to be diagnosed as the difference in top-of-atmosphere (TOA) radiation fluxes between each experiment and the piClim-control run (Forster et al., 2016). Rapid adjustments, including those driven by cloud changes, can be determined using radiative kernels.
To understand the evolution of ERF over the 1850-2014 period, we also request transient simulations where forcing agents (greenhouse gases, aerosols, land-use changes, solar and volcanoes; table 2) vary to match the best-estimate historical forcing. Again, sea-surface temperatures and sea-ice are to be fixed at the pre-industrial level, to diagnose ERF as the difference of TOA fluxes.
In RFMIP-ERF-HistAer and RFMIP-ERF-Aer, ozone concentrations should be fixed at pre-industrial values. In concentration-based models, the concentrations of ozone should be fixed, and in emissions-based models, the ozone in the radiation code should be fixed.
Table 1: Present-day time-slice simulations
|Experiment title||CMIP6 label||Experiment description||Years||Major purposes|
|RFMIP-ERF-PI-Control||piClim-control||Pre-industrial conditions||30||Baseline for model-specific effective radiative forcing (ERF) calculations|
|RFMIP-ERF-Anthro||piClim-anthro||Present-day (2014) anthropogenic forcing (greenhouse gases, aerosols, ozone and land-use)||30||Quantify present day (2014) total anthropogenic ERF|
|RFMIP-ERF-GHG||piClim-ghg||Present-day (2014) non-ozone greenhouse gases||30||Quantify present-day (2014) ERF by greenhouse gases|
|RFMIP-ERF-Aer||piClim-aer||Present-day (2014) aerosols (ozone fixed at pre-industrial concentrations)||30||Quantify present-day (2014) ERF by aerosols|
|RFMIP-ERF-LU||piClim-lu||Present-day (2014) land-use||30||Quantify present-day (2014) ERF by land-use changes|
|RFMIP-ERF-4xCO2||piClim-4xCO2||CO2 concentrations set to 4 times pre-industrial||30||Quantify ERF of 4 × CO2|
Table 2: Historical transient simulations
|Experiment title||CMIP6 label||Experiment description||Start||End||Major purposes|
|RFMIP-ERF-HistAll||piClim-histall||Time-varying forcing from all agents||1850||2100||Diagnose transient ERF from all agents|
|RFMIP-ERF-HistNat||piClim-histnat||Time-varying ERF from volcanoes, solar (including spectral) variability, etc.||1850||2100||Diagnose transient natural ERF|
|RFMIP-ERF-HistGHG||piClim-histghg||Time-varying ERF from non-ozone greenhouse gases||1850||2100||Diagnose transient ERF from greenhouse gases|
|RFMIP-ERF-HistAer||piClim-histaer||Time-varying ERF from aerosols (ozone fixed at pre-industrial concentrations)||1850||2100||Diagnose transient ERF from aerosols|