If you have a habit of perusing satellite imagery of the world’s oceans—and who doesn’t, really?—you might get lucky and spot long, thin clouds, like white slashes across the sea. In some regions, like off the West Coast of the United States, the slashes might crisscross, creating huge hash marks. That’s a peculiar phenomenon known as a ship track.

As cargo ships chug along, flinging sulfur into the atmosphere, they actually trace their routes for satellites to see. That’s because those pollutants rise into low-level clouds and plump them up by acting as nuclei that attract water vapor, which also brightens the clouds. Counterintuitively, these pollution-derived tracks actually have a cooling effect on the climate, since brighter clouds bounce more of the sun’s energy back into space.

The Pacific Ocean off of California is particularly hash-marked because there’s a lot of shipping along that coast, and ideal atmospheric conditions for the tracks to form. Well, at least it used to be. In 2020, an International Maritime Organization (IMO) regulation took effect, which severely limited the amount of sulfur ships are allowed to spew. Shipping companies switched to low-sulfur fuel, which improved air quality, especially around busy ports. But in doing so, they reduced the number of ship tracks—which means fewer brightened clouds, and thus more warming.

Writing Friday in the journal Science Advances, researchers described how they used a new machine-learning technique to quantify the clouds better than ever before, showing how the sulfur regulation cut the amount of ship tracks over major shipping lanes in half. That, in turn, has had a moderate warming effect on those regions.

“The big finding is the regulation in 2020, put forward by the IMO, has reduced the global ship-track numbers to the lowest point on the 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 influence too. But ship-track activity has remained low even as cargo traffic has picked back up.) “We’ve had similar but smaller-scale, strict regulations before, and we can also see that impact,” he continues. “But there, the effect is not global.”

In Europe and North America, for instance, officials had already sectioned off what are known as emission control areas, or ECAs, which established local versions of the standards set by the 2020 global rule. “The number of tracks within the ECAs, within the control zones, reduced dramatically, to the point of almost disappearing,” Yuan says. “But outside of it, actually we saw some increase because the shipping routes had shifted.”

The satellite imagery caught ships doing something sneaky. Outside of control zones, where the vessels weren’t bound by sulfur regulations, they burned regular old fuel. Then once inside an ECA, their operators could switch to low-sulfur fuel, coming in line with the pollution rules. (Sulfur is a normal component of a fossil fuel, and it takes extra processing to remove it. Because low-sulfur fuel is more expensive, it’s more cost-effective for ship operators to spend as much time outside of ECAs as possible, burning the old stuff.)