Attacks in the Red Sea forced ships to detour around the Cape of Good Hope, creating a natural experiment that revealed how cleaner marine fuels reduce cloud formation. Researchers found that low-sulfur fuels cut the cloud-boosting effect from ship emissions by about two-thirds. This discovery helps refine climate models and highlights trade-offs in pollution regulations.
In November 2023, militia attacks in the Bab al-Mandab Strait disrupted shipping through the Red Sea, prompting vessels to reroute around Africa's Cape of Good Hope. This conflict-driven change increased traffic in the South Atlantic, a region with persistent low-level clouds sensitive to pollution, allowing scientists to study the impacts of 2020 marine fuel regulations.
The International Maritime Organization's January 2020 rule slashed sulfur content in marine fuels by about 80% to reduce air pollution. Sulfate aerosols from ship exhaust previously seeded clouds with many small droplets, enhancing reflectivity and offsetting roughly one-third of greenhouse gas warming. However, the new fuels diminished this effect, reducing cloud droplet formation by approximately 67% compared to pre-2020 levels.
Florida State University atmospheric scientist Michael Diamond and graduate student Lilli Boss analyzed satellite data in a study published in Atmospheric Chemistry and Physics. They observed a rise in nitrogen dioxide (NO2) emissions—a marker unaffected by sulfur rules—indicating doubled ship traffic in 2024. Despite this, cloud responses were weaker post-regulation. "The unexpected rerouting of global shipping gave us a unique opportunity to quantify aerosol-cloud interactions, reducing the largest source of uncertainty in global climate projections," Diamond said. "When your 'laboratory' is the atmosphere, it's not every day you can run experiments like this one."
These findings narrow uncertainties in climate forecasts, as aerosols' short lifespan contrasts with long-lasting greenhouse gases. While the regulations have prevented tens of thousands of premature deaths from harmful sulfur particles, they also lessen temporary cloud cooling. The results inform policies balancing air quality improvements with climate goals.