When attacks by militias disrupted shipping in the Red Sea, cargo ships were forced to reroute around the Cape of Good Hope. This shift in global trade routes provided researchers at Florida State University (FSU) with a unique opportunity to study how changes in ship emissions affect cloud formation and, ultimately, climate projections.
Michael Diamond, an assistant professor in FSU’s Department of Earth, Ocean and Atmospheric Science, along with graduate student Lilli Boss, analyzed the effects of new international regulations that significantly reduced sulfur content in marine fuels. Their research, published in Atmospheric Chemistry and Physics, found that these cleaner fuels led to about an 80 percent reduction in sulfur emissions from ships and a corresponding 67 percent decrease in cloud droplet formation compared to previous fuel standards.
“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,” said Diamond. “When your ‘laboratory’ is the atmosphere, it’s not every day you can run experiments like this one. It was an invaluable opportunity to get a more accurate picture of what’s happening on Earth.”
The study’s findings could help improve global climate models by providing better data on how clouds respond to pollution changes. This information may assist policymakers as they consider environmental regulations aimed at both protecting human health and addressing climate change.
In January 2020, the International Maritime Organization (IMO) implemented new rules requiring lower sulfur levels in marine fuels. Aerosols from ship emissions—especially sulfate particles—affect cloud brightness and reflectivity by causing clouds to form with smaller droplets that reflect more sunlight. This process has historically offset some warming caused by greenhouse gases but remains a major source of uncertainty for climate scientists because aerosols are short-lived compared to long-lasting greenhouse gases like carbon dioxide.
Diamond’s earlier work indicated that after IMO 2020 took effect, clouds over major shipping lanes began forming with larger and fewer droplets. There is ongoing debate among scientists about how much this has affected ocean heatwaves and overall cloudiness since the regulation was introduced.
A sharp increase in shipping traffic around southern Africa following Red Sea disruptions created what researchers describe as a “natural experiment.” Because this rerouting resulted from conflict rather than weather or policy decisions, it allowed scientists to isolate the impact of increased ship emissions on local cloud patterns without other confounding factors.
Satellite data showed higher concentrations of nitrogen dioxide (NO2)—a pollutant released by ship engines unaffected by sulfur regulations—over the southeastern Atlantic Ocean during this period. By comparing NO2 levels with measurements of cloud droplet numbers sensitive to sulfur emissions, Diamond and Boss determined that ships’ ability to alter clouds dropped by 67 percent after implementation of IMO regulations.
Their results provide strong evidence that cleaner fuels have lessened shipping’s influence on cloud formation. This helps clarify how air pollution controls can affect both human health and broader climate processes.
Quantifying these effects remains critical for improving estimates related to Earth’s energy balance—a key factor for setting future environmental policies. While aerosols from ships have temporarily cooled parts of the planet by making clouds brighter, they also pose significant health risks; exposure to sulfur particles is linked with respiratory and cardiovascular diseases. The IMO regulation is estimated to have already prevented tens of thousands of premature deaths worldwide.
