Aerosol effective radiative forcing increases Earth's energy imbalance in recent decades

Earth's energy imbalance (EEI), a key driver of climate change, has risen markedly over the last two decades. Greenhouse gas forcing, aerosol forcing, and cloud feedback all contribute to this increase. However, the role of aerosol forcing, particularly effective radiative forcing through aerosol-cloud interactions (ERF_ACI), remains highly uncertain and closely intertwined with cloud feedback. Here, we estimate shortwave ERF_ACI using satellite observations and show its importance as a contributor to the EEI increase between 2003 and 2023. ERF_ACI exhibits a significant increasing trend of 0.33 Wm^-2/decade averaged over oceans between 60^∘S and 60^∘N. The increasing trend of ERF_ACI stems from a global decline in cloud droplet number concentration driven by decreasing anthropogenic aerosol emissions. It is of similar magnitude as the combined instantaneous forcing trend from greenhouse gases and aerosol-radiation interactions. Our results may reduce the gap between the simulated and observed shortwave contribution to EEI trends while implying a weak shortwave cloud feedback. They also suggest a stronger aerosol forcing than the multi-model mean.