If naval traffic changes route, there are climatic repercussions thousands of kilometers away, even in the formation of clouds.
When militia attacks in the Bab al-Mandab Strait forced cargo ships to avoid the Red Sea starting in November 2023, few could have predicted that those geopolitical upheavals would have climate repercussions thousands of kilometers away, over the South Atlantic.
For researcher Michael Diamond, an atmospheric expert at Florida State University, that unexpected detour was a historic opportunity: to study how marine fuels influence cloud formation.
The research. In a study published in Atmospheric Chemistry and PhysicsDiamond and student Lilli Boss (Department of Earth, Ocean and Atmospheric Sciences, FSU) show that the drastic reduction of sulfur in naval fuels – about 80% less than in the past – has led to a drop of almost 67% in the number of droplets present in the clouds, compared to previous levels.
“The unexpected change in global shipping routes gave us a unique opportunity to measure aerosol-cloud interactions,” explains Diamond. «When your “laboratory” is the atmosphere, it’s not every day you can conduct such clear experiments. It’s an invaluable opportunity to better understand how our planet works.”
The clouds change their face. Since 1 January 2020, the International Maritime Organization (IMO) has imposed a more severe limit on the sulfur content in ship fuels: from a maximum of 3.5% it has gone to 0.50% m/m for journeys outside emission control areas. This measure – known as “IMO 2020” – aims to reduce air pollution, in particular emissions of sulfur oxides (SOₓ), which are harmful to human health and the environment.
The aerosols resulting from these emissions, especially sulfates, play a key role in modifying the properties of clouds: they favor the formation of more numerous and smaller droplets, making the clouds brighter and more reflective. This has a cooling effect on the planet, which has historically “masked” a significant portion of greenhouse gas warming. However, precisely because aerosols remain in the atmosphere for relatively short times (days or weeks), and clouds are highly variable, these interactions are a major source of uncertainty in global climate models.
An experiment in the clouds. As more ships diverted around the Cape of Good Hope, the South Atlantic region – known for its persistent low clouds and sensitivity to ship emissions – saw a sudden increase in shipping traffic.
This allowed scientists to observe how clouds change in response to emissions alone, without interference from weather or political changes.
Thanks to satellite data, the research team detected a strong increase in nitrogen dioxide (NO₂) in that area. Since NO₂ is emitted by ship engines, but is not affected by IMO sulfur regulations, it has served as a reliable marker of ship activity. By comparing changes in NO₂ to the number of droplets in clouds (which instead depend on sulfur), scientists quantified a reduction of about 67% in ships’ ability to alter clouds after the rules went into effect.
Implications in climate models. Diamond and Boss’ data are particularly valuable because they help reduce a significant source of uncertainty in climate models: how clouds react to changes in aerosols. Preparing more precise models means offering politicians better tools to plan interventions on a global scale.
Additionally, this research highlights the fine balance between air quality and climate objectives. On the one hand, IMO regulations have improved human health by reducing emissions of sulfur – a pollutant linked to respiratory and cardiovascular diseases. On the other hand, by changing the composition of emissions, they have also changed the way clouds form and reflect sunlight, potentially altering the Earth’s energy balance.
How large is the affected fleet? To give an idea of the order of magnitude: today global maritime trade is served by tens of thousands of ships. According to recent data, over 50,000 merchant ships are active right now in the world. The impact of the new IMO rules affects a significant portion of this fleet: many shipowners have had to choose between using low sulfur fuels, installing emissions scrubbers, or – for some – converting ships to alternative fuels.
