A conflict-driven reroute of international ships exposed how dramatically cleaner fuel reduces cloud formation. The discovery strengthens climate models and highlights the complex trade-offs between cleaner air and planetary warming. Credit: Shutterstock

A sudden shift in global shipping routes created an accidental large-scale experiment that allowed scientists to observe how cleaner ship fuel alters cloud formation.

Their analysis showed a dramatic drop in cloud-forming particles, confirming that cleaner fuel makes ships far less capable of brightening clouds and temporarily cooling the planet.

Shipping Disruptions Spark an Unexpected Climate Opportunity

When militia attacks interfered with shipping routes in the Red Sea, few expected the consequences to reach the skies above the South Atlantic. For Florida State University atmospheric scientist Michael Diamond, however, the forced shift in cargo ship routes opened an unusual chance to explore a key climate question — How much do cleaner fuels change how clouds form?

In a study published in Atmospheric Chemistry and Physics, Diamond and FSU Department of Earth, Ocean, and Atmospheric Science graduate student Lilli Boss reported that recent fuel rules lowering sulfur content by about 80 percent also reduced cloud droplet formation by roughly 67 percent when compared with earlier high-sulfur 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.”

Their findings could help sharpen climate models, offering scientists and policymakers clearer predictions and a better understanding of how environmental regulations support public health.

Michael Diamond is an assistant professor in Florida State University’s Department of Earth, Ocean and Atmospheric Science. Credit: Devin Bittner/FSU College of Arts and Sciences

How Fuel Regulations Alter Cloud Behavior

In January 2020, the International Maritime Organization (IMO) required a major drop in sulfur levels in marine fuel in order to improve air quality. Ship exhaust releases aerosols, especially sulfate particles, that influence how clouds form and how bright they appear. These tiny particles encourage the formation of many small droplets, which produce brighter clouds that reflect more sunlight. This creates a cooling effect that has historically offset about one-third of the warming caused by greenhouse gases.

Despite this cooling influence, the impact of aerosols remains difficult to pin down. Greenhouse gases such as carbon dioxide, or CO2, stay in the atmosphere for centuries, while aerosols remain for only days or weeks. Their short lifetime, combined with cloud variability, makes aerosol-cloud interactions the biggest source of uncertainty in climate projections.

Diamond’s previous research showed that after IMO 2020, clouds in major shipping corridors began forming with fewer and larger droplets. Researchers are still debating how the resulting increase in sunlight absorbed by the ocean may have contributed to the 2023 and 2024 marine heatwaves in the Atlantic Ocean. There is also ongoing disagreement about how much cloud cover decreased after the fuel regulations, with estimates ranging from about 10 percent to as high as 80 percent.

A Large-Scale Natural Test Emerges

Beginning in November 2023, attacks in the Bab al-Mandab Strait sharply reduced Red Sea traffic and shifted vessels toward the route around the Cape of Good Hope. This sudden change meant the South Atlantic region — an area highly sensitive to ship emissions because of its regular low-lying clouds — saw a dramatic surge in ship traffic.

Because this shift was caused by conflict rather than weather patterns or policy changes, scientists were able to observe cloud behavior driven solely by changes in ship emissions. Such direct cause-and-effect conditions are extremely rare outside laboratory settings, making this situation a valuable natural experiment.

Satellite measurements revealed a clear increase in nitrogen dioxide, or NO2, across the southeastern Atlantic Ocean. NO2 is released by ship engines and was not affected by the IMO 2020 sulfur rules, so it offered a dependable indicator of intensified ship traffic. This allowed researchers to compare cloud conditions before and after the fuel regulations under similar levels of shipping activity.

Key Results From the Analysis

Even though there were nearly twice as many ships operating in 2024, the overall reduction in cloud droplet formation was only slightly less pronounced than before IMO 2020. When Diamond and Boss compared NO2 levels, which were unchanged by sulfur regulations, with cloud droplet numbers, which respond to sulfur emissions, they found a 67 percent decline in ships’ ability to modify cloud properties after the regulations took effect.

This provides strong additional evidence that cleaner fuels have weakened shipping’s influence on cloud formation and offers a clearer measure of how pollution and cloud behavior are linked, an important factor for improving climate simulations.

Why These Findings Matter

Understanding how clouds respond to aerosols remains one of climate science’s most challenging problems.

The new research helps reduce the uncertainty surrounding Earth’s energy balance, enabling policymakers to make more informed decisions as they weigh environmental protection and long-term climate goals. The results also highlight the complex trade-offs involved in air-quality regulations. Actions that protect human health can also influence the planet’s climate system in unexpected ways.

Although aerosols temporarily cool the atmosphere, the pollutants responsible for this effect pose significant health risks. Sulfur particles are linked to respiratory and cardiovascular disease, and the IMO regulation is estimated to have already prevented tens of thousands of premature deaths.

Reference: “Conflict-induced ship traffic disruptions constrain cloud sensitivity to stricter marine pollution regulations” by Michael S. Diamond and Lili F. Boss, 21 November 2025, Atmospheric Chemistry and Physics. DOI: 10.5194/acp-25-16401-2025

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