Bottom Trawling Could Unleash Carbon Dioxide, Worsening Global Warming

A Controversial Fishing Method May Dredge Up a Climate Time Bomb

By Kate McMahon edited by Andrea Thompson

A heavy metal net is dragged across the seafloor at breakneck speed, churning up dark clouds of sediment and swallowing everything in its path. A blue-spotted stingray tries to flee, flailing its winglike pectoral fins as the trawl closes in from behind, but its efforts are in vain. This unprecedented footage—a scene in David Attenborough’s latest documentary Ocean—is the first time bottom trawling has been captured in high definition, exposing a practice rarely seen by the public.

Bottom trawling is a highly controversial fishing method, but it provides a quarter of the world’s seafood. It involves a vessel pulling a weighted net and other heavy gear, blindly and fast, along vast stretches of seabed—often in pursuit of only one or two commercially valuable species. It traps huge numbers of other organisms and bulldozes over fragile habitats, destroying centuries-old coral, scallop gardens and seagrass beds. “It’s hard to imagine a more wasteful way to catch fish,” Attenborough narrates somberly as viewers watch a pile of dead juvenile sharks and rays get swept off the deck of the fishing vessel in Ocean.

But ecological destruction is not the only concern. Emerging research points to another lesser-known problem with bottom trawling: its potential to unleash climate-warming gases by disturbing carbon stored in seafloor sediments.

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The ocean is the world’s largest single carbon sink, absorbing around 30 percent of the carbon dioxide that human activity pumps into the atmosphere. Phytoplankton—microscopic plants and algae drifting near the ocean’s surface—take in CO₂ during photosynthesis, converting it into organic carbon that gets stored in their tissues. Many sink to the seafloor when they die; sediments eventually bury them and the carbon they contain, effectively locking it away.

Bottom trawling stirs massive clouds of this carbon-rich sediment back into the water column. There the organic matter is exposed to oxygen and microbial activity that converts some of the carbon into other chemical forms, which can worsen ocean acidification or escape into the atmosphere as CO₂.

But how much of this dredged-up carbon is actually released into the atmosphere by bottom trawling—and how quickly—remains a matter of debate.

Some scientists argue that a significant portion of the carbon released by bottom trawling makes its way into the atmosphere—at levels that rival some of the world’s worst emitters. “If all the disturbed carbon entered the atmosphere, it would rival aviation’s emissions,” says Utah State University ecologist Trisha Atwood, who co-authored a 2021 study and a 2024 follow-up that quantified trawling emissions by using statistical models. “Our latest research shows that 55 to 60 percent of the resuspended carbon is actually released into the atmosphere over seven to nine years, which is around 340 [million] to 370 million metric tons of CO₂ annually.” That’s more than the entire annual emissions of countries such as Italy or Spain.

But other experts disagree with such high estimates, explaining that the ocean’s carbon cycle is governed by complex biogeochemical processes that naturally absorb, convert or sequester much of the carbon that gets resuspended in the water column. “It is important to note that some of the CO₂ released from organic matter mineralization [microbial decomposition] ends up as bicarbonate, which is dissolved in seawater and does not exchange with the atmosphere,” says Volker Brüchert, an associate professor of biogeochemistry at Stockholm University. This input of bicarbonate contributes to acidification and hinders some of the ocean’s ability to absorb additional CO₂, he says—“but direct corresponding data for such a large CO₂ emission from oceanic shelves into the atmosphere has not been demonstrated.”

There is, however, general consensus that frequent trawling makes it harder for carbon to remain sequestered in seafloor sediments. “It’s difficult to measure the exact scale of greenhouse gas emissions, but we know carbon on the seabed is more likely to be preserved if it’s not continually resuspended by bottom trawling activity. There is growing evidence to support this,” says William Austin, a paleooceanographer at the University of St. Andrews in Scotland and chair of the United Nations Ocean Decade Program for Blue Carbon.

Methane, a greenhouse gas far more potent than CO₂, adds another variable that is difficult to account for in seafloor disturbance. Most oceanic methane is stored as methane hydrates—icelike compounds formed under high pressure and low temperatures—which are typically scattered over seabed areas too deep for trawling to reach. But in regions with shallow, nearshore methane deposits (such as the Siberian Shelf), bottom trawling could pose a risk, particularly as receding sea ice leads to new fishing grounds above methane hotspots.

This possibility is drawing much scientific interest, especially in light of past climate events. Around 56 million years ago, during an interval called the Paleocene-Eocene Thermal Maximum (PETM), the planet warmed by up to eight degrees Celsius in under 200,000 years—a blink in geological time and one of the closest analogues researchers have to help them understand modern climate change. Though the exact cause of that warming event remains uncertain, one hypothesis is that warming waters destabilized and melted seafloor methane hydrates, triggering an unprecedented release of methane into the atmosphere. Whether modern, human-driven warming could trigger similar processes, even at a much smaller scale, is still unclear.

“If we’re going to start trawling the seabed in a rapidly warming and transitioning Arctic,” Austin says, “we may need to stop and think first.”

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