Coupled Climate Models Systematically Underestimate Radiation Response to Surface Warming

A realistic representation of top-of-the-atmosphere (TOA) radiation response to surface warming is key for trusting climate model projections. We show that coupled models with freely evolving ocean-atmosphere interactions systematically underestimate the observed global TOA radiation trend during 2001-2022 in 552 simulations. Locally, even if a simulation spontaneously reproduces observed surface temperature trends, TOA radiation trends are more likely under- than overestimated. This response bias stems from the models' inability to reproduce the observed large-scale surface warming pattern and from errors in the atmospheric physics affecting short- and longwave radiation. Models with a better representation of the TOA radiation response to local surface warming have a relatively low equilibrium climate sensitivity. Our bias metric is a novel process-based approach which links a model's current response to climate change to its behavior in the future. A realistic representation of the radiation balance at the top of the Earth's atmosphere (TOA) by coupled climate models is essential for trust in future climate projections. Despite that relevance, it is still not clear whether the models correctly simulate the coupling between surface warming and TOA radiation because of the short observational record. We show that climate models systematically underestimate the observed increase in global TOA radiation during 2001-2022 in 552 simulations. Locally, even if a simulation reproduces observed changes in surface temperature, changes in TOA radiation are more likely under- than overestimated. This response bias stems from the models' inability to reproduce the observed large-scale patterns of surface warming and from errors in the atmospheric physics which suppress the communication of the surface information to the TOA. Models that better represent the TOA radiation response to local surface warming have a relatively low equilibrium climate sensitivity, that is, a weak global-mean surface warming in response to a doubling of the atmospheric CO2 concentration above pre-industrial levels. Our new bias metric links a model's current response to climate change to its behavior in the future. Climate models underestimate the observed global TOA radiation trend during 2001-2022 Surface warming patterns and atmospheric physics matter for the observation-model discrepancy Models with a small bias in the coupling between surface warming and TOA radiation trends have a relatively low climate sensitivity