Assessing Earth's energy imbalance trend in the early 21st century in two high-resolution coupled models

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Satellite observations since 2001 show an increasing Earth’s energy imbalance (EEI) at the top of the atmosphere of \SI{0.45}{W.m^{-2}.dec^{-1}}, a trend understated in coupled climate models. We assess the 2001–2024 EEI trend in historical simulations of two high-resolution coupled models, CM4X, a revised version of CM4.0, and CESM-HR, a high-resolution version of CESM1, each with 10 realizations. The ensemble-mean EEI trends in CM4X and CESM-HR capture only 25\% and 55\% of the observed trend, respectively, due to underestimated shortwave trends originating from the tropics and Southern Hemisphere extratropics. CESM-HR exhibits twice the intramodel variability in EEI and surface warming trends than CM4X, and biases in EEI trends are not linked to warming-pattern differences. One CESM-HR realization matches the observed EEI trend but via strong Antarctic offshore polynyas absent from observations. These results highlight the potentially critical influence of sea-ice processes on near-term climate projections in high-resolution models.

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