Fish is better than pork and beef—not just for your body, but for the planet. That’s long been the thinking, anyway. But a new study has uncovered a hidden climate impact of the fishing industry—one that, ironically, will get worse as boats switch to cleaner fuels. In some cases, the effect could make fishing for tuna as hard on the climate as raising pork, and trawling for shrimp about half as bad as raising beef.

Hogs and cattle get a bad rap because they—and their manure—emit a lot of methane, a powerful greenhouse gas. But the fishing industry also contributes to climate change: mostly from the carbon dioxide (CO 2 ) from burned diesel fuel that persists in the atmosphere for hundreds of years. The boats produce other short-lived pollutants, such as sulfur oxides and black carbon, which have cooling and warming effects, respectively. But they have typically been neglected as unimportant in the grand scheme of things.

The new study may change that. Elliott Campbell and Brandi McKuin, environmental engineers at the University of California (UC), Merced, estimated fishing industry emissions by combining fisheries catch records with the amount of fuel typically needed to catch various species. Given the total burned fuel, they estimated the amounts of the pollutants, using information about engine types and fuel types. Black carbon, a form of soot that arises from incomplete combustion, has been underestimated by an order of magnitude in previous studies, the team reports in the Journal of Geophysical Research: Atmospheres.

“That was the real eye-opener for me,” says Peter Tyedmers, an ecological economist at Dalhousie University in Halifax, Canada, who was not involved in the research.

Black carbon warms the atmosphere because of its ability to absorb radiation from the sun, but its effect can be especially pernicious in polar regions, where, falling on bright ice, the soot diminishes the regions’ ability to reflect away heat. For now, however, in many parts of the world, Campbell and McKuin found that the warming effect of the black carbon is counteracted by the cooling effect of sulfur pollution. When burned, the sulfur-heavy fuels often used by fishing boats emit sulfur oxides, which are converted in the atmosphere into sulfate particles. These shiny aerosols reflect sunlight back to space, leaving Earth cooler. They also can seed the formation of clouds, which also reflect light.

But sulfur emissions, which are hazardous to human health and can lead to acid rain, are on the wane. The International Maritime Organization has adopted limits on sulfur emissions in places like the Baltic Sea, the North Sea, the Caribbean Sea, and along the coasts of North America. Eventually, Campbell says, sulfur-heavy fuels could disappear altogether, and for good reason. “I think the quest to remove sulfur from fuels is inevitable and the right way to go.”

There will be a drawback to more regulation, however. The sulfur-cooling effect will go away, and some of the warming from black carbon will remain. Already, in areas where sulfur-rich fuels are regulated, the researchers find a significant extra warming effect over 20 years. They find that fishing for large pelagic animals—species like tuna or swordfish—warms the climate, pound for pound, as much as raising pork.

Still, even with the extra warming, small species such as herring or sardines represent a source of protein with climate costs that are nearly as low as beans and other high-protein vegetables, Tyedmers says. That’s because fishing ships can efficiently trap schools of small fish in nets near the shore, without burning much fuel. However, chasing larger species, such as tuna, requires longer trips and bursts of ship speed that burn more fuel. Trawling for crustaceans burns the most fuel because of the large amounts of engine force required to drag thin-meshed nets along the sea floor.

One take-home message for consumers is that, just as milk doesn’t come from a carton, fish don’t come from a can. “People don’t realize how much time boats are spending trawling around,” says Jennifer Burney, an environmental scientist at UC San Diego who was not a party to the study, and she’s not surprised to see the high climate costs of crustaceans and large pelagic species. Yet overall, she says, emissions from fishing pale in comparison to industries on land, such as power generation from coal plants.

Burney is examining a similar trade-off between sulfur and black carbon in the United States, as coal-fired electricity plants shift to cleaner natural gas, which emits half as much CO 2 as coal per unit of electricity. Emissions of both sulfur and black carbon will go down with the switch, which means that the power industry will lose small short-term cooling and warming effects, in addition to gaining the larger long-term cooling effect of lower CO 2 emissions.

But which of the two small effects will dominate? In unpublished research, Burney says it’s the sulfur. “You’re taking away more cooling by removing the sulfate,” she says. So when you cook your fish on an electric stove, take a deep breath and be thankful for the clean, sulfur-free air—but know that your actions pack an extra climate wallop.