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.

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Most glaciers and ice caps (GIC) are out of balance with the current climate. To return to equilibrium, GIC must thin and retreat, losing additional mass and raising sea level. Because glacier observations are sparse and geographically biased, there is an undersampling problem common to all global assessments. Here, we further develop an assessment approach based on accumulation-area ratios (AAR) to estimate committed mass losses and analyze the undersampling problem. We compiled all available AAR observations for 144 GIC from 1971–2010 and found that most glaciers and ice caps are farther from balance than previously believed. Accounting for regional and global undersampling errors, our model suggests that GIC are committed to additional losses of 30 ± 11% of their area and 38 ± 17% of their volume if the future climate resembles the climate of the past decade. These losses imply global mean sea-level rise of 163 ± 73 mm, assuming total glacier volume of 430 mm sea-level equivalent. To reduce the large uncertainties in these projections, more long-term glacier measurements are needed in poorly sampled regions

The Greenland Ice Sheet (GrIS) mass balance is examined with an Earth system/ice sheet model that interactively couples the GrIS to the broader Earth system. The simulation runs from 1850 to 2100, with historical and SSP5-8.5 forcing. By the mid-21st century, the cumulative GrIS contribution to global mean sea level rise (SLR) is 23 mm. During the second half of the 21st century, the surface mass balance becomes negative in all drainage basins, with an additional SLR contribution of 86 mm. The annual mean GrIS mass loss in the last two decades is 2.7-mm sea level equivalent (SLE) year −1 . The increased SLR contribution from the surface mass balance (3.1 mm SLE year −1 ) is partly offset by reduced ice discharge from thinning and retreat of outlet glaciers. The southern GrIS drainage basins contribute 73% of the mass loss in mid-century but 55% by 2100, as surface runoff increases in the northern basins. Plain Language SummaryThe Greenland Ice Sheet (GrIS) gains mass at the surface from snowfall, and it loses mass from melting and runoff and from glacier calving at the ocean front. When these processes are in balance, the ice sheet does not contribute to global sea level change. Recent observations have shown that the ice sheet is losing mass and raising global mean sea level.This study uses a global Earth system model that calculates ice flow of the GrIS, as well as processes in the atmosphere, ocean, land, and sea ice. For a modern reference, the model is forced with atmospheric greenhouse gas concentrations for the period 1850-2014. Next, the model is forced for the rest of the 21st century following the SSP5-8.5 scenario to study how the GrIS and the Earth system respond to a worst-case scenario.By 2050, the GrIS loses mass that is equal to 23 mm of global mean sea level rise. During the second half of the 21st century, all regions of the GrIS lose mass because of increased surface melting and runoff, with the dry north playing a greater role. By 2100, the projected GrIS contribution to sea level rise is 109-mm sea level equivalent.