Aerosol Effective Radiative Forcing Accelerates Earth’s Energy Imbalance In Recent Decades
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This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
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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. 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 <sub>ACI<sub>), remains highly uncertain and closely intertwined with cloud feedback. Here we estimate ERF <sub>ACI<sub> using satellite observations and show it has been an important contributor to the EEI increase over the past two decades. The ERF <sub>ACI<sub> exhibits a significant warming trend of 0.33 ± 0.03 Wm⁻² averaged over oceans between 60°S and 60°N. The warming trend of (ERF <sub>ACI<sub> 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. 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.