The CESM2 Single Forcing Large Ensemble and Comparison to CESM1: 1 Implications for Experimental Design

: Single Forcing Large Ensembles (SFLEs) are a relatively new tool for quantifying the contributions of different anthropogenic and natural forcings to the historical and future projected evolution of the climate system. This study introduces a new SFLE with the Community Earth system Model version 2 (CESM2) which can be used to separate the influences of: (1) greenhouse gases; (2) anthropogenic aerosols; (3) biomass burning aerosols; and (4) all remaining forcings, on the evolution of the Earth System from 1850 to 2050. Here, the forced responses of global near surface temperature and associated drivers is examined in CESM2 and compared with that in an SFLE with CESM2’s predecessor, CESM1. The experimental design, the imposed forcing and the model physics all differ between the CESM1 and CESM2 SFLEs. In CESM1 an “all-but-one” approach was used where everything except the forcing of interest is time evolving, while in CESM2 an “only” approach is used, where only the forcing of interest is time evolving. This experimental design choice is shown to matter considerably for anthropogenic aerosol forced change in CESM2, due to state dependence of cryospheric albedo feedbacks and non-linearity in the Atlantic Meridional Overturning Circulation (AMOC) response to forcing. This impact of experimental design is, however, strongly dependent on the model physics and/or the imposed forcing as the same sensitivity to experimental design is not found in CESM1, which appears to be an inherently less non-linear model in both its AMOC behavior and cryospheric feedbacks.

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