An overview of the E3SM version 2 large ensemble and comparison to other E3SM and CESM large ensembles

Abstract

Abstract. This work assesses a recently produced 21-member climate model large ensemble (LE) based on the U.S. Department of Energy's Energy Exascale Earth System Model (E3SM) version 2 (E3SM2). The ensemble spans the historical era (1850 to 2014) and 21st century (2015 to 2100), using the SSP370 pathway, allowing for an evaluation of the model's forced response. A companion 500-year preindustrial control simulation is used to initialize the ensemble and estimate drift. Characteristics of the LE are documented and compared against other recently produced ensembles using the E3SM version 1 (E3SM1) and Community Earth System Model (CESM) versions 1 and 2. Simulation drift is found to be smaller, and model agreement with observations is higher in versions 2 of E3SM and CESM versus their version 1 counterparts. Shortcomings in E3SM2 include a lack of warming from the mid to late 20th century, likely due to excessive cooling influence of anthropogenic sulfate aerosols, an issue also evident in E3SM1. Associated impacts on the water cycle and energy budgets are also identified. Considerable model dependence in the response to both aerosols and greenhouse gases is documented and E3SM2's sensitivity to variable prescribed biomass burning emissions is demonstrated. Various E3SM2 and CESM2 model benchmarks are found to be on par with the highest-performing recent generation of climate models, establishing the E3SM2 LE as an important resource for estimating climate variability and responses, though with various caveats as discussed herein. As an illustration of the usefulness of LEs in estimating the potential influence of internal variability, the observed CERES-era trend in net top-of-atmosphere flux is compared to simulated trends and found to be much larger than the forced response in all LEs, with only a few members exhibiting trends as large as observed, thus motivating further study.

Unique Temperature Trend Pattern Associated With Internally Driven Global Cooling and Arctic Warming During 1980–2022

“…The model simulations come from large ensembles included in CMIP6 using 10 different models that contain 10 to 50 ensemble members. Aside from the CMIP6 models, we also include the CESM2 large ensemble with updated biomass burning aerosol emissions (Fasullo et al, 2022;Rodgers et al, 2021), as well as the E3SM2 large ensemble (Fasullo et al, 2024). SAT trend maps are calculated by finding the ordinary least-squares regression at each grid point using 43-year periods (the duration of the satellite era from 1980 to 2022) separated by five years spanning 1850-2047 (i.e., 1850-1892, 1855-1897, …, 1980-2022, …, 2005-2047).…”