Bottom Trawling Could Be Producing More Carbon Dioxide Than Flying

The damage trawler nets do to the environment is well known, but the carbon they stirred up when sediments get dragged has been an important overlooked environmental issue. Image Credit: Sineenuch J/Shutterstock.com

It seems there is almost no part of the planet in which human activities are not contributing to mucking up the climate. Astonishingly, trawling the ocean floor could be a bigger factor than stratospheric fossil fuel burning by airplanes. A solution exists, however, and would bring many associated benefits.

Benthic trawling is the process of dragging a net so deeply behind a boat it scrapes along the ocean bottom. The practice has attracted plenty of criticism for the devastation it causes to marine ecosystems – but there's another problem it causes that has hardly been noticed.

The bottom of the ocean is the world's major carbon sink, with living things falling to it and some of the carbon being sequestered, locking it away from the ocean/atmosphere for thousands or millions of years. Finding a way to accelerate the rain of organic material to the sea bed has been a major idea for tackling global heating. However, that only works if what settles stays there. A paper in Nature points out this isn't the case if human activity churns up ocean sediments.

In fact, Dr Enric Sala of the National Geographic Society and 25 co-authors calculate the amount of carbon released when nets rip their way across the sea bed. Combining satellite data on the extent of annual trawling and calculations of the carbon stirred up under different conditions the paper estimates 1.47 gigatonnes of carbon dioxide each year if all areas are fresh, falling to 0.58 Gt if it is assumed all areas have been trawled frequently.

By comparison, the total contribution of all flying in a non-lockdown year is nearly 1 Gt, although contrails amplify the effects.

 Although stirred sediment carbon does not directly reach the atmosphere, its presence in the water reduces the ocean's capacity to draw CO2 out of the atmosphere. Depending on how well mixed the deep waters are with the surface – an ongoing question – the bottom trawling could be a major climate killer.

Grim as this news is, Sala remains upbeat. Certain areas of the ocean are much more sensitive than others, and if these are protected the environmental benefits will be enormous, including for Greenhouse emissions.

“Ocean life has been declining worldwide because of overfishing, habitat destruction and climate change. Yet only 7% of the ocean is currently under some kind of protection," Sala said in a statement. "In this study, we've pioneered a new way to identify the places that -- if protected --will boost food production and safeguard marine life, all while reducing carbon emissions.” The claim of increased food production from restricting fishing is counter-intuitive, but protected zones have been shown to boost fish populations in the areas around them, often increasing overall yields.

Areas already marine protected areas (blue) and areas that are priorities for 1, 2, or 3 of the goals of carbon storage, biodiversity protection, and food production. Sala et al/Nature

If the paper is right, protecting 30 percent of the ocean – including protection from bottom trawling – will bring financial and environmental rewards far beyond the cost. Unfortunately, it won't do so immediately, so governments need to be willing to invest in the future.

For those willing to do so, Sala and co-authors have offered multiple maps that show which areas will bring the greatest advantages based on different criteria. "There is no single best solution to save marine life and obtain these other benefits,” said co-author Juan Mayorga of emLab. However, some areas are priorities on any measure.

China's ocean territories account for by far the largest emitter, releasing more than twice the carbon of the next nine countries combined. However, relative to population Denmark is much higher, unusually for a nation that prides itself on its climate-saving achievements.

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