Sulfur pollution from coal burning has a similar effect. Some studies suggest that China’s surge in coal consumption over the last decade partly offset the recent global warming trend (though coal does have a strong net warming effect).

It’s difficult to estimate how much the new rule could affect temperatures. We don’t know enough about cloud physics and the behavior of atmospheric particles, nor how diligently the shipping industry will comply with the new rule, says Robert Wood, a professor of atmospheric sciences at the University of Washington.

Another wrinkle is that ships emit other particles that can sometimes also stimulate cloud droplets to form, including black carbon, a major component of soot. Removing the sulfur from the fuel could alter the size and quantity of these particles, which could affect clouds as well, says Lynn Russell, a professor of atmospheric science at the Scripps Institution of Oceanography.

“So we can’t really say exactly what the change will be,” says Russell, though she adds that the rule change is “likely” to produce a warming effect on balance.

The upcoming change does offer a different way of thinking about intentional efforts to cool the climate, known as geoengineering, according to some proponents of research in this area. Rather than some radical experiment, deliberate geoengineering could instead be seen as a way of continuing to do what we’ve been doing inadvertently with ships, but in a safer way.

Sulfur emissions cool the planet in two ways, directly and indirectly. The direct way is that when sulfur dioxide is further oxidized in the atmosphere, it can form particles that reflect sunlight back into space. This happens in large volcanic eruptions, which can release tens of millions of tons of sulfur dioxide.

The indirect way is that sulfur particles can also act as nuclei around which cloud droplets form. Clouds, too, reflect more sunlight. You can see this in satellite images, which show lines of white clouds above the ocean along busy shipping lanes.

Geoengineering researchers have explored both processes, but with less toxic particles, as potential ways to alter the climate (see “Scientists Consider Brighter Clouds to Preserve the Great Barrier Reef”).

For instance, researchers with the Marine Cloud Brightening Project, centered at the University of Washington, have spent years studying the possibility of spraying tiny salt particles into the sky along coastlines to induce cloud droplets to form. The group has spent the last few years attempting to raise several million dollars to build the sort of sprayers that would be needed, in the hopes of carrying out small-scale field experiments somewhere along the Pacific coastline.