The ocean is a vast natural sponge for carbon dioxide, taking in about 30 percent of the world’s annual carbon emissions. But certain human activities are rendering that sponge less absorbent, as described in new research. Bottom trawling for fish like sole and crustaceans and dredging to maintain harbors disrupts the seafloor, causing a cascade of chemical changes that reduce the ocean’s ability to sop up climate-warming carbon from the atmosphere, according to a study published March 28 in the journal Science Advances.
Together the impacts of dredging and trawling release between 2 and 8 teragrams (2-8 million metric tons) of carbon dioxide each year–equivalent to the annual energy emissions from up to 1 million U.S. homes, per the new analysis. Compared with all of the carbon emissions that result from burning fossil fuels globally (or even a single industrialized nation’s annual emissions), the amount of CO2 is, admittedly, small.
“First and foremost, we need to reduce our emissions. There’s no way around that,” Sebastiaan van de Velde, lead study author and a marine geochemist at the University of Otago in New Zealand, tells Popular Science. Yet when governments and corporate investors are pouring huge amounts of money and resources into moonshot carbon capture projects, the seafloor findings are significant, van de Velde says.
Shifting how and where trawling is done could cut hundreds of times more greenhouse gas emissions than all of the currently operational direct air carbon capture plants on Earth, he notes. Reducing trawling and dredging in a climate-conscious way could have carbon benefits on par with major mangrove and wetland restoration projects, at a fraction of the cost, he explains. That’s not to say we should stop restoring coastal ecosystems, which provide many benefits, like minimizing erosion and boosting storm resilience. But perhaps it means we should also consider changing how we fish.
Previous studies have found that trawling, and other seafloor disruptions, release greenhouse gases. However, this prior research has focused on the organic carbon: basically the decaying muck at the bottom of the ocean that keeps some sliver of carbon trapped in limbo. Trawling brings that muck back to the surface, where the stored carbon can be cycled through the microbe food web and re-released into the air as CO2.
The new study examines the other side of the coin: inorganic carbon and ocean acidity. Key chemical reactions involving dissolved rock particles and minerals unfold on the seafloor. These reactions help to turn the marine environment more alkaline (i.e. basic). Generating alkalinity is a central feature of ocean carbon cycles. It’s why the ocean can absorb carbon dioxide from the air in the first place–because CO2 dissolved in water becomes carbonic acid. If things get too acidic, and the seas get saturated, it’s unable to absorb any more. The alkaline seafloor processes counter acidification and prime the ocean to take in additional carbon dioxide.
Pulling a heavy set of nets and chains across the seafloor to catch fish or scooping up sediments through dredging throws those standard chemical cycles out of whack. Both activities introduce more oxygen to the ocean bottom than would normally be present. Oxygen reverses the reactions that produce alkaline molecules, instead creating more acidity and impeding seawater’s ability to absorb CO2 from the atmosphere.
To estimate the full impacts of dredging and trawling worldwide, van de Velde and his colleagues relied on real-world data cataloging seafloor carbon flux, oxygen levels, and pH. They built a global mathematical model of seafloor alkalinity, and then used separate stats on dredging and trawling to infer how much these activities disrupt the acid-base chemistry. They determined that muddy reaches of the coastal shelf are the most important type of seafloor surface for producing alkalinity, and thus the most impacted by the dredging and trawling.
The final value they came to (again, 2-8 teragrams of carbon dioxide produced per year) includes some uncertainty because data on dredging and trawling is limited, the ocean floor isn’t uniform, and the amount of sediment kicked up by each fishing boat or dredger depends on the type of equipment used and the location.
“They’re the best numbers we could get, but they’re not necessarily the [100 percent] correct numbers,” Van de Velde tells Popular Science. “There’s a lot of things we’re doing that we don’t really know the impacts of. And in the ocean, there’s a lot of unknown unknowns–things we don’t even know that we don’t know.” He hopes that his study represents the start of a better understanding, and leads to more exact future analyses.
In the meantime, he also hopes this first attempt at quantifying the inorganic carbon effects of trawling and dredging will spur marine industries and regulatory bodies to adjust. Advances in equipment can make trawling less harmful and fuel efficient, without imperiling the seafood supply. And in some regions, trawling could be restricted to 40 percent of the current fishing grounds or even less–setting aside 60 percent for conservation, and yields would only fall by 10 percent, according to a 2021 report from the International Council for the Exploration of Sea.
“To me, it seems like a no-brainer,” van de Velde says. It’s theoretically possible to have your fish and eat it too (with far fewer climate impacts)–but industry has to be willing to be swept up in the tides of change.
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