Aerosol Effective Radiative Forcing Accelerates 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 and continues to accelerate in recent years \cite{hansen_earths_2005,trenberth_perspective_2022,loeb_observational_2024}. 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\textsubscript{ACI}), remains highly uncertain and closely intertwined with cloud feedback \cite{bellouin_bounding_2020,raghuraman_forcing_2023, kramer_observational_2021}. Here we estimate ERF\textsubscript{ACI} using satellite observations and show it has been an important contributor to the EEI increase over the past two decades. The ERF\textsubscript{ACI} exhibits a significant warming trend of ${0.33 \pm 0.03 Wm^{-2}/decade}$ averaged over oceans between 60\textsuperscript{o}S and 60\textsuperscript{o}N. The warming trend of ERF\textsubscript{ACI} stems from a global decline in cloud droplet number concentration driven by decreasing anthropogenic aerosol emissions. It is similar to the combined instantaneous forcing from greenhouse gases and aerosol-radiation interactions estimated by radiative kernel calculations \cite{kramer_observational_2021}. Our results can close the gap between simulated and observed EEI trends while implying a weak total cloud feedback. Our findings have important implications for studying decadal changes, cloud feedback, and mitigation.