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. As solar and volcanic forcings affect stratospheric ozone, this effect will be included in the hist-nat integration of DAMIP. To get a consistent ERF estimate modelling groups should include the same dataset in the RFMIP-ERF-HistNat experiment. David Plummer, in collaboration with Michaela Hegglin, has kindly agreed to develop prescribed ozone datasets for DAMIP hist-nat, which account for solar and volcanic effects and that data will be published on input4MIPs in due course. Therefore, we suggest that modelling centres using models without coupled chemistry wait until the prescribed ozone datasets are available before commencing these experiments.
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|