How do you know that a cargo ship is polluting the environment? It makes clouds

If you have a habit of looking at satellite images of the world’s oceans – and who doesn’t? – you might get lucky and spot long thin clouds slashing across the sea like white slashes. In some regions, like off the west coast of the United States, the forward slashes can cross and create huge hash marks. This is a peculiar phenomenon known as ship tracks.

As cargo ships chug and spew sulfur into the atmosphere, they actually track their routes for satellites to see. That’s because these pollutants rise to low clouds and fill them up by acting as nuclei that attract water vapor, which also lightens the clouds. Contrary to intuition, these pollution-derived traces actually have a cooling effect on the climate, as lighter clouds reflect more solar energy back into space.

The Pacific Ocean off California is particularly heavily tagged with hash, due to the high level of shipping along this coast and the ideal atmospheric conditions for the trails to form. Well, at least it Second hand be. In 2020, a regulation from the International Maritime Organization (IMO) came into effect, severely limiting the amount of sulfur ships are allowed to emit. Shipping companies switched to low-sulphur fuel, which improved air quality, especially in busy ports. But in doing so, they reduced the number of ship tracks – which means fewer brightened clouds and therefore more warming.

On the map to the right you can see ship tracks highlighted in purple.

Figure: Yuan et al.

Write in your diary on Friday scientific advances, the researchers described how they used a new machine learning technique to quantify the clouds better than ever before and showed how the sulfur ordinance halved the number of ship tracks over major shipping lanes. This in turn had a moderate warming effect on these regions.

“The big takeaway is that the IMO’s proposed regulation in 2020 reduced global ship tracking numbers to their lowest level on record,” says Tianle Yuan, a climate scientist at NASA and the University of Maryland who led the research. (Yes, reduced economic activity during the pandemic lockdowns may have had a small impact, too. But shipping activity has remained low even as freight traffic has picked up again.) “We’ve had similar, but smaller, tough regulations before, and we can also see this impact,” he continues. “But there the effect is not global.”

In Europe and North America, for example, officials had already delineated so-called Emission Control Areas, or ECAs, which set local versions of the standards set by the 2020 global rule. “The number of tracks inside the ECAs, inside the control zones, has decreased dramatically, almost to the point of disappearing,” Yuan says. “But Outside we actually saw an increase in that because the shipping routes had shifted.”

The satellite images have caught ships doing something insidious. Outside the zones of control, where ships were not bound by sulfur regulations, they burned regular old fuel. Then, once in an ECA, their operators could switch to low-sulphur fuel to comply with pollution regulations. (Sulfur is a normal component of a fossil fuel, and it requires additional processing to remove it. Because low-sulfur fuel is more expensive, it’s more cost-effective for ship operators to spend as much time as possible outside of ECAs, burning old stuff. )

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