Assessing Earth's Energy Imbalance Trend in the Early 21st Century in Two High-Resolution Coupled Models

Satellite observations since 2001 show an increasing Earth's energy imbalance (EEI) at the top of the atmosphere of 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 in 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.