A global reduction in sulfur pollution from shipping that has inadvertently contributed to recent warming of the Earth is providing insights into the challenge of evaluating one of two major proposed solar geoengineering approaches, marine cloud brightening.

In 2020, the International Maritime Organization imposed a sharp reduction in the level of sulfur in ship fuel to reduce emissions of sulfate aerosols and sulfur dioxide — pollutants associated with premature deaths from asthma, lung cancer, cardiovascular, and pulmonary diseases, to improve air quality in coastal and large port communities. 

By decreasing sulfur pollution, the cleaner fuels also reduced the reflectivity of low-level marine clouds that aerosol particles in the exhaust had previously brightened — reducing the cooling effect of clouds that shaded the ocean from the sun’s rays. 

Soon, observers noticed a decrease in “ship tracks,” those distinct, bright white, linear clouds that form in the trail of large ships, allowing more sunlight to heat the ocean surface. But quantifying the warming effect of increased sunshine on the ocean surface using conventional climate models has proven challenging because they simply can’t represent clouds well enough.

“Clouds are one of the most ephemeral natural phenomena, changing all the time as they drift through the sky,” said Graham Feingold, a program leader at the NOAA's Chemical Sciences Laboratory, CIRES fellow, and co-author of the study. “How do you design an experiment to understand their impact on the temperature of Earth’s surface below?”