The fillet of flounder sitting on your plate comes with a severe environmental cost. To catch it, a ship running on fossil fuels spewed greenhouse gases as it dragged a trawl net across the seafloor, devastating the ecosystems in its path. Obvious enough. But new research shows that the consequences extend even further: Trawl nets are hauling up both food and a huge amount of carbon that’s supposed to be sequestered in the murky depths.

In a paper publishing in the journal Frontiers in Marine Science, researchers have tallied up an estimate of how much seafloor carbon the bottom-trawling industry stirs into the water and how much of that is released into the air as CO₂ each year, exacerbating global warming. It turns out to be double the annual fossil fuel emissions produced by the entire world’s 4 million–vessel fishing fleet.

“At least 55 to 60 percent of the CO₂ created by trawling—scraping the seafloor—is going to come into the atmosphere within nine years,” says lead author and ecosystem ecologist Trisha Atwood, who focuses on carbon cycling at Utah State University and National Geographic’s Pristine Seas program. “It now suggests that countries should be looking at this industry, and that their carbon footprint goes a lot further than maybe they were thinking, just in terms of the amount of gas that they burned to get out to their fishing grounds.”

The oceans have gone a long way in saving humanity from itself. They’ve absorbed something like 90 percent of the extra heat our civilization has pumped into the atmosphere, helping naturally mitigate global warming. And they’re vast carbon sinks: Photosynthesizing phytoplankton absorb CO₂ as they grow at the surface, then die and sink to the seafloor, locking that carbon away from the atmosphere. Or little creatures known as zooplankton gobble up those phytoplankton and poop out pellets of carbon that also sink.

Either way, there’s a worldwide conveyor belt of carbon moving from the surface down into the depths, where it’s supposed to stay for a long, long time. “Once it gets buried under just a couple of centimeters, really, of sediment, it goes below the ‘active zone,’ as we call it,” says Atwood. “If it’s undisturbed—so it’s not mixed up or trawled up—that carbon can stay down there for tens of thousands of years.”

A huge, weighted trawl net obliterates all that. “They drag along the bottom and cut through everything in their wake,” says Max Valentine, campaign director of Oceana’s illegal fishing and transparency campaign in the United States, who wasn’t involved in the research. “We liken bottom trawling to clear-cutting of a forest. For example, hard corals in Alaska, which have been dated to hundreds of thousands of years old, can be destroyed in just a single swipe.” Anything caught up in the net that wasn’t the target food species—known as bycatch—gets hauled aboard the ship, often dead, and thrown back overboard.

The seafloor destruction also launches plumes of carbon into the water column. Microbes chow down on it, expelling CO₂ in the process. The dissolved gas then moves around on currents, both horizontally and vertically. While 40 to 45 percent of it remains in the water, the study found that the rest reaches the atmosphere within a decade.

The CO₂ that sticks around in the ocean is nothing to cheer, either. When CO₂ dissolves in the sea, it acidifies the water. That’s why the oceans have generally been acidifying, as humanity pumps plumes of the gas into the atmosphere. (It’s also why the carbonated water you drink is more acidic than regular tap water.) That extra acidity interferes with how organisms like crustaceans build their shells, and how corals build their calcium carbonate skeletons.

Trawling is adding still more CO₂ to seawater, and with it more acidity. “Where this needs to be looked at a little bit more carefully is in enclosed seas, places like the Mediterranean,” says Atwood. “Those are areas where it’s localized because acidification from trawling, from that CO₂, stays in the water and could become problematic.”

Scientists are still learning about the ocean’s seabeds and how they store carbon, so this sort of estimate could change with new discoveries, says Sarah Cooley, director of climate science at the Ocean Conservancy, who wasn’t involved in the research. “I’m happy this study contributes a bit more to help us consider all the outcomes of human activity in the deep ocean,” says Cooley. “There may be real climate impacts from trawling beyond fishing vessel fuel consumption, and fishery managers around the world should strive to consider the full impacts of bottom trawling when determining what sorts of fishing gear should be permitted.”

The tricky thing about regulating this kind of trawling is that every coastal nation sets its own rules for its waters. But with these new findings, it’s becoming ever-clearer that the practice is not only a threat to biodiversity, but the whole global climate. “Very few countries have fully outlawed bottom trawling,” says Valentine. “There are no global limitations. If people use their voices—if they yell out that they don’t want this type of fishing—it can really change the political landscape.”