CESM2 Single Forcing Large Ensemble Project

The CESM2 “Single Forcing” Large Ensemble Project is a publicly available set of climate model simulations useful for addressing the roles of individual forcings in historical and future climate change. This simulations use the same model and forcings as the CESM2 Large Ensemble Project and, therefore, can be used to parse the relative roles of different forcings in responses found in that ensemble where all forcings are applied together. The ensemble members are initialized from 1850 from the same initial conditions that were used to initialize the "macro" members of the CESM2 Large Ensemble and they extend to 2050, following CMIP6 historical forcings prior to 2015 and SSP3-7.0 forcings, thereafter. Note that the smoothed biomass burning emissions that were used in the second 50 members of the CESM2 large ensemble are used, so these simulations should be compared with the second 50 members of the CESM2 large ensemble. Four primary ensembles are available in which different forcings are time evolving while all other forcings are held fixed at 1850's values i.e, the "only" method is used. Note that this differs from the CESM1 Large Ensemble which used the "all-but-one" method where all forcings were evolving except the one of interest. In the CESM2 ensembles, only the forcing of interest is evolving. Four ensembles are available using the following time-evolving forcings:

GHG = only greenhouse gases evolving (15 members) AAER = Only anthropogenic aerosols evolving (20 members) BMB = Only biomass burning aersols evolving (15 members) EE = everything else evolving i.e., all forcings other than greenhouse gases (15 members)

Note that by "anthropogenic aerosols" here we refer to all industrial, agricultural, domestic and transport related emissions and acknowledge that this does not include any anthropogenic influences on biomass burning, which is instead present in the BMB ensemble. Each forcing that is time evolving in the CESM2 Large Ensemble is time evolving in one of these four sub-ensembles to allow additivity to be tested. As discussed in the release paper cited below, substantial non-linearities are present in the CESM2 Single Forcing Large Ensemble. As part of the investigation into this non-linearity, an additional ensemble was performed which was initialized in 1920 of the CESM2 Large Ensemble members and run with all forcings except anthropogenic aerosols i.e., following the same "all-but-one" approach that was used in CESM1. This demonstrated that there is considerable sensitivity of the anthropogenic aerosol forced response to the method used i.e., "all-but-one" versus "only". We have, therefore, expanded this ensemble to a 10 member ensemble, referred to as xAER, so that any sensitivity of the anthropogenic aerosol influence to the method used can be tested.

xAER = Everything time evolving except anthropogenic aerosols (10 members)