A global shipping detour just revealed a hidden climate twist

In a study published in Atmospheric Chemistry and Physics, Diamond and FSU Department of Earth, Ocean, and Atmospheric Science graduate student Lilli Boss found that new regulations cutting sulfur in marine fuel by roughly 80 percent also reduced cloud droplet formation by about 67 percent compared with older, more polluted fuels.

"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 results could ultimately improve global climate models and give scientists and policymakers better tools for evaluating future environmental regulations and public health protections.

How Reduced Sulfur Fuel Alters Cloud Formation

In January 2020, the International Maritime Organization (IMO) required a large cut in sulfur content in marine fuels to curb air pollution. Tiny particles in ship exhaust, especially sulfate aerosols, play a major role in how clouds form and how bright they become. These aerosols create clouds with many small droplets, which reflect more sunlight and increase cooling at the surface. This cooling effect has historically offset about one-third of the warming produced by greenhouse gases.

Despite this, the influence of aerosols remains highly uncertain. Greenhouse gases such as carbon dioxide, or CO₂, stay in the atmosphere for centuries, while aerosols last only days or weeks. Their short lifetime, combined with the natural variability of clouds, makes aerosol-cloud interactions the biggest source of uncertainty in climate forecasts.

Before the rerouting event, Diamond's previous work showed that clouds in heavily trafficked shipping corridors contained larger and fewer droplets after IMO 2020. Scientists continue to debate how the resulting increase in sunlight reaching the ocean may have influenced the 2023 and 2024 marine heatwaves in the Atlantic Ocean. There is also disagreement about how much overall cloudiness decreased after IMO 2020, with estimates ranging from a moderate 10% drop to an extreme 80% decline.

A Conflict-Driven Natural Experiment

Beginning in November 2023, attacks in the Bab al-Mandab Strait dramatically reduced traffic in the Red Sea and pushed ships toward the route around the Cape of Good Hope. The South Atlantic region -- known for persistent low-level clouds that respond strongly to ship pollution -- saw a rapid increase in shipping activity.

Because these changes were caused by conflict rather than weather patterns or new policies, scientists were able to observe how clouds reacted specifically to altered ship emissions. Clear cause-and-effect situations like this are nearly impossible to recreate in controlled experiments, giving researchers a rare natural experiment.

Satellite measurements showed a distinct rise in nitrogen dioxide, or NO2, across the southeastern Atlantic Ocean. NO2 is emitted by ship engines and was unaffected by the IMO 2020 sulfur rules, making it a reliable marker of increased ship traffic. This allowed researchers to directly compare cloud conditions before and after the fuel regulations under similar levels of shipping activity.

What the Data Revealed

Even with about twice as many ships passing through the region in 2024, cloud droplet formation was only slightly weaker than before IMO 2020. By comparing NO2, which remained unchanged by the regulations, with cloud droplet numbers, which respond to sulfur emissions, Diamond and Boss identified a 67% reduction in the ability of ships to influence cloud formation after the fuel rules were introduced. This finding strengthens the evidence that cleaner fuels have substantially reduced shipping's impact on cloud properties and provides an important constraint for improving future climate models.

Why These Results Matter

Understanding how clouds react to changes in aerosol levels is still one of the most difficult aspects of climate research.

The new findings help narrow the uncertainty surrounding Earth's energy balance. With clearer information, policymakers can make more informed decisions about environmental regulations while still considering long-term climate goals. The research also highlights the complex trade-offs that arise when improving air quality, since actions that protect human health can also affect cloud-induced cooling.

Although aerosols provide temporary cooling, they pose serious risks to people. Sulfur particles are harmful pollutants linked to respiratory and cardiovascular problems. According to estimates, the IMO regulation has already prevented tens of thousands of premature deaths.