Quantify the contribution of IMO 2015/2020 aerosol reductions to clear-sky absorbed solar radiation changes

Determine the extent to which reductions in aerosol emissions associated with the International Maritime Organization fuel sulfur regulations implemented in 2015 and 2020 contributed to observed changes in clear-sky top-of-atmosphere absorbed solar radiation over the oceans, including the negative clear-sky absorbed solar radiation trends over the Atlantic and southern-hemisphere oceans during 2013–2022 and the positive clear-sky absorbed solar radiation anomalies in 2023 identified in CERES satellite data.

Background

The paper attributes much of the 2023 global temperature surge to a record-low planetary albedo, primarily driven by reductions in low-level cloud cover. Differences between CERES satellite observations and ERA5 reanalysis suggest complexities in partitioning the roles of clouds and aerosols in recent absorbed solar radiation (ASR) anomalies and trends.

Within the oceans, CERES clear-sky ASR patterns hint at possible direct aerosol effects, and recent IMO fuel sulfur regulations (2015 and 2020) are a major perturbation to shipping-related sulfate aerosols. However, while positive clear-sky ASR anomalies in 2023 are seen in some regions, the 2013–2022 clear-sky ASR trends were negative over much of the Atlantic and Southern Hemisphere oceans and cannot be explained by incident solar radiation trends alone, leaving the specific contribution of IMO-driven aerosol changes unresolved.