How Do You Know a Cargo Ship Is Polluting? It Makes Clouds

Big vessels spew sulfur, which brightens clouds to produce long “ship tracks.” These emissions cause environmental damage—but also help cool the planet.

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.

In the map at right, you can see ship tracks highlighted in purple.

Illustration: Yuan et al.

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.)

“Our technique can help to validate whether a ship is using clean fuel or not,” says Yuan, “because we can observe indirectly how much pollution they're putting into the air.”

Illustration: Yuan et al.

To do all this, Yuan and his colleagues first gathered satellite data, which humans manually combed for ship tracks. They then fed this data into deep learning models, training the algorithms to recognize ship tracks on their own. It’s the same idea behind training an algorithm to recognize cats: If you show it enough pictures, it’ll get the general idea of what a cat looks like. So even though no two ship tracks look the same, the models could generalize well enough to identify them around the world. (You can see the ship tracks as the model saw them in the image above.) 

The researchers could then feed the models more NASA satellite data, covering all the world’s oceans, so the algorithms could identify the ship tracks and how their numbers changed over the years. 

Illustration: Yuan et al.

As you can see in the image above, there are a number of ship-track hot spots around the world, represented in a gradient that runs from red (high) to white (medium) to blue (low). As the red smudge in the upper left shows, the Pacific Ocean near Southern California and Mexico is particularly prone to ship tracks. Whether clouds form in a given area depends on a number of factors, like the stability and moisture content of the atmosphere, how polluted the air may already be, and the amount of ship traffic. 

The dark green lines around Asia, on the other hand, are estimates of emitted sulfur dioxide—not visible ship tracks—showing busy shipping lanes. These vessels don’t produce ship tracks like they do off western North America, mainly because the air is already polluted—that is, there are already lots of particulates getting into clouds, so the extra sulfur from ships doesn’t do much. 

Notice the white blob down between 60 and 0 degrees W on the map, halfway between the tips of South America and Africa. That’s near Antarctica, where hardly any ships venture. “It turns out it's a volcano,” says Yuan. “That provided us kind of an independent check, because there you don't expect any shipping activity at all, yet it's a hot spot.” That’s because volcanoes also spew sulfur aerosols, which seed clouds, brightening them in the same way ship tracks do. 

Getting a better handle on the prevalence of ship tracks has a two-fold utility. For one, the clouds betray a ship’s emissions: A captain might lie to regulators about what kind of fuel they’re burning, but the sky above won’t. “If we can measure individual ship tracks, and we can attach that ship track to an individual ship, then we can know if a ship is emitting a lot of pollution,” says Yuan. “Then we know that probably it's not burning clean fuel.”

And two, pollution plays a large—and largely understudied—role in climate change. Ship emissions are terrible for the environment because they destroy air quality, but ricocheting some of the sun’s energy back into space is actually a benefit. Interestingly, this is also the idea behind stratospheric aerosol injection, a proposed form of geoengineering in which planes would spray sulfur to deflect sunlight. Researchers are also playing with cloud brightening techniques, in which they’d spray sea salt to brighten low-lying clouds, just like ship pollution does. 

But not all kinds of pollution deflect solar energy, as sulfur does; some trap it. Other forms, like microplastics, have loaded the atmosphere with particulates that may have both cooling and heating effects on the planet. Plane contrails seem to largely play a warming role (although one that can be ameliorated by flying at certain altitudes). And both carbon dioxide and methane basically serve as insulating blankets, warming the planet. 

These pollutants are often intermingled, so cutting one can have a complex effect. This is a paradox of climate action: By reducing air pollution, including the aerosols that deflect solar energy, one recent study estimates that humanity may be boosting warming from carbon dioxide by 15 to 50 percent. 

In fact, the influence of aerosols remains one of the most uncertain areas in climate science, says Hailong Wang, who studies these dynamics at the Pacific Northwest National Laboratory. “Many, many models are still struggling to get the accurate representation of those effects in order to predict future climate change,” says Wang, who wasn’t involved in the new ship-tracks paper. “At some point, if we significantly reduce those aerosol emissions, we do expect some side effects of additional warming.”

Modeling how that’ll play out, though, is difficult, in part because air pollution isn’t homogeneous around the world—it varies significantly by region, and it can change rapidly due to weather patterns, and on longer timescales due to air-quality regulations. But even though this study looked just at ship tracks, researchers can use the new data to validate climate models, Wang says—for instance, to see if they can accurately represent what happens when local aerosol pollution suddenly plummets. 

Ships switching to low-sulfur fuel isn’t going to create a huge, planet-wide drop in emissions, because it's still a fossil fuel that burns carbon. (And don’t get it wrong—the bottom line is we absolutely have to stop burning fossil fuels for the good of the climate at large.) But it offers a little preview of what a reduction in one specific type of pollution might do for warming—and how complicated solving this puzzle will be.

In the meantime, as the 2020 regulation works its magic, ship tracks will continue to fade around the world. If you get lucky and spot one in new satellite imagery, you may have found yourself an outlaw.

How Do You Know a Cargo Ship Is Polluting? It Makes Clouds

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