The Community Earth System Model Version 2 (CESM2)

TLDR

CESM2 is a next‑generation Earth system model that builds on CESM1, addressing development challenges to enhance climate simulation fidelity. The authors evaluate CESM2 using long preindustrial control and historical ensemble simulations at 1° resolution, comparing low‑top (40 km, limited chemistry) and high‑top (130 km, comprehensive chemistry) atmospheric configurations. CESM2 delivers substantial improvements over CESM1, including reduced low‑latitude precipitation and shortwave cloud forcing biases, better MJO and ENSO teleconnections, a land‑carbon trend matching observations, an equilibrium climate sensitivity of 5.1–5.3 °C, a transient climate response of 1.9–2.0 °C, and broadly similar performance between low‑ and high‑top setups, with outputs made publicly available.

Abstract

An overview of the Community Earth System Model Version 2 (CESM2) is provided, including a discussion of the challenges encountered during its development and how they were addressed. In addition, an evaluation of a pair of CESM2 long preindustrial control and historical ensemble simulations is presented. These simulations were performed using the nominal 1° horizontal resolution configuration of the coupled model with both the "low-top" (40 km, with limited chemistry) and "high-top" (130 km, with comprehensive chemistry) versions of the atmospheric component. CESM2 contains many substantial science and infrastructure improvements and new capabilities since its previous major release, CESM1, resulting in improved historical simulations in comparison to CESM1 and available observations. These include major reductions in low-latitude precipitation and shortwave cloud forcing biases; better representation of the Madden-Julian Oscillation; better El Niño-Southern Oscillation-related teleconnections; and a global land carbon accumulation trend that agrees well with observationally based estimates. Most tropospheric and surface features of the low- and high-top simulations are very similar to each other, so these improvements are present in both configurations. CESM2 has an equilibrium climate sensitivity of 5.1–5.3 °C, larger than in CESM1, primarily due to a combination of relatively small changes to cloud microphysics and boundary layer parameters. In contrast, CESM2's transient climate response of 1.9–2.0 °C is comparable to that of CESM1. The model outputs from these and many other simulations are available to the research community, and they represent CESM2's contributions to the Coupled Model Intercomparison Project Phase 6.