When armed conflicts disrupted shipping routes in the Red Sea, few could have predicted that the consequences would reach as far as the atmosphere over the South Atlantic. For atmospheric scientist Michael Diamond from Florida State University, the sudden rerouting of commercial ships presented a unique opportunity to study a crucial climate question: how does the switch to cleaner fuels affect cloud formation?

The Impact of New Fuel Standards on Clouds

In a study published in the journal Atmospheric Chemistry and Physics, Diamond and graduate student Lilly Boss found that the 2020 regulations limiting sulfur content in ship fuel by about 80% also led to a roughly 67% reduction in the formation of cloud droplets compared to the use of older, more polluting fuels. According to Diamond, the unexpected shift in global shipping routes allowed for a quantitative assessment of the interaction between aerosols and clouds, helping to reduce one of the main uncertainties in global climate forecasts. He noted that such “laboratory” experiments in the atmosphere are extremely rare and provide valuable insights into Earth’s processes. The data obtained could help improve climate models and give scientists and policymakers more accurate tools for evaluating future environmental standards and public health measures.

How Lower Sulfur Content Affects Clouds

In January 2020, the International Maritime Organization (IMO) required a significant reduction in sulfur content in ship fuel to combat air pollution. Fine particles in ship exhaust, especially sulfate aerosols, play a key role in cloud formation and their reflectivity. These aerosols promote the creation of clouds with more, smaller droplets, which reflect more sunlight and enhance surface cooling. Historically, this cooling effect has offset about a third of the warming caused by greenhouse gases.

However, the impact of aerosols on climate remains highly uncertain. Unlike greenhouse gases such as CO2, which persist in the atmosphere for centuries, aerosols last only a few days or weeks. Their short lifespan and the natural variability of clouds make the interaction between aerosols and clouds the largest source of uncertainty in climate projections.

A Natural Experiment Triggered by Conflict

Since November 2023, attacks in the Bab-el-Mandeb Strait have sharply reduced ship traffic in the Red Sea, forcing vessels to reroute around the Cape of Good Hope. In the South Atlantic—a region with persistent low-level clouds that are sensitive to ship pollution—shipping activity surged dramatically.

Because these changes were caused by conflict rather than weather or new regulations, scientists had a rare chance to observe how clouds respond specifically to altered ship emissions. Such clear cause-and-effect scenarios are nearly impossible to replicate in laboratory conditions, making this a unique natural experiment for researchers.

Satellite Data and Results

Satellite measurements recorded a noticeable increase in nitrogen dioxide (NO2) concentrations over the southeastern Atlantic Ocean. NO2 is emitted by ship engines and was not affected by the IMO 2020 sulfur rules, making it a reliable indicator of increased shipping activity. This allowed researchers to directly compare cloud conditions before and after the introduction of fuel standards, under similar levels of ship traffic.

Even with about twice as many ships in the region in 2024, the formation of cloud droplets was only slightly weaker than before IMO 2020. By comparing NO2 levels and the number of cloud droplets—which depend on sulfur emissions—Diamond and Boss identified a 67% reduction in ships’ ability to influence cloud formation after the new fuel standards were implemented. This confirms that cleaner fuel has significantly reduced the impact of shipping on cloud properties and provides important data for refining future climate models.

Why This Matters

Understanding how clouds respond to changes in aerosol levels remains one of the most challenging problems in climate science. The new findings help narrow the uncertainty in Earth’s energy balance. More precise information enables policymakers to make better-informed decisions about environmental regulations, taking into account long-term climate goals. The study also highlights the complex trade-offs involved in improving air quality, as measures to protect public health can affect cloud-driven cooling.

While aerosols provide temporary cooling, they pose serious health risks. Sulfur particles are harmful pollutants linked to respiratory and cardiovascular diseases. Estimates suggest that IMO regulations have already prevented tens of thousands of premature deaths.