The world’s biggest carbon sink may start emitting CO2 after net zero

The Southern Ocean draws down more carbon than any other single region, absorbing about 10 per cent of human emissions. But if the world reaches net-zero emissions and carbon dioxide concentrations fall, it will start releasing CO2 into the atmosphere. This massive release of carbon will prolong global warming for centuries, even if humanity artificially removes gigantic amounts of CO2 from the atmosphere, a study has found. The results show how difficult it will be to reverse climate change, even as countries place ever-larger hopes on carbon dioxide removal. “That impact can sustain much, much longer than we expected,” says Yechul Shin at Seoul National University in South Korea. “Let’s say we try to reduce the CO2… this sink-to-source transition in the Southern Ocean still acts as an obstacle to reduce the atmospheric CO2 concentration.” Advertisement The notoriously strong winds around the “roaring forties” and “furious fifties” latitudes of the southern hemisphere stir up waves and encourage the water to absorb CO2. The water cools and sinks, storing that carbon away in the deep ocean for centuries or millennia. This carbon uptake has been increasing as humanity pumps ever more greenhouse gas into the atmosphere, creating a higher concentration of CO2 in the air than in the surface water. But if the atmospheric CO2 concentration begins falling, that ratio will flip, and the Southern Ocean will start releasing rather than absorbing CO2. “It’s like you’re constantly pressing a piston into the ocean,” says Nicolas Gruber at ETH Zurich in Switzerland, who wasn’t involved in the study. “If you pull the piston up again, then the CO2 comes back out.” The researchers modelled a scenario in which the world reaches net zero in 2125 and the concentration of atmospheric CO2 starts to gradually decline from 700 parts per million. They also modelled one in which humanity drastically decreases atmospheric CO2 after net zero through carbon dioxide removal, a range of approaches ranging from widespread tree planting to machines that filter CO2 out of the air. In both scenarios, the Southern Ocean goes from absorbing about 3 grams of carbon per square metre per year now to emitting 8 to 9 grams of carbon per square metre by the year 2400. Under both scenarios, atmospheric CO2 is projected to fall, but this decline gradually levels off after 2200 as the Southern Ocean emits more CO2. In the carbon-removal scenario, atmospheric CO2 actually rises again for 50 years before levelling off at 425 parts per million – roughly the current concentration. Without carbon removal, it levels off at 590 ppm. The scenarios aren’t necessarily realistic, though. Many countries, including the UK and members of the European Union, aim to reach net zero as soon as 2050. Moreover, the amount of carbon dioxide removal remains minuscule compared with emissions, which are still increasing. But regardless of when humanity reaches net zero, the ocean as a whole is likely to become a net source of CO2 as it begins releasing all the carbon that major sinks like the Southern Ocean stored, according to Alberto Naveira Garabato at the University of Southampton, UK. “Anything that changes in terms of the way the ocean interacts with the atmosphere could be like adding a massive country to the world’s carbon budget,” he says. While scientists largely expect the Southern Ocean to begin emitting CO2 at some point, the new research finds that two major feedbacks may amplify this release, says Gruber. The first is driven by the enormous amount of heat that the Southern Ocean transfers to the depths alongside the carbon. Warm water can hold less gas. So, as that stored heat gradually returns to the surface, the Southern Ocean will lose more CO2, just like a glass of warm soda loses its fizz more rapidly than a glass of cold soda. The second feedback is essentially an increase in ocean acidification. Many of the plankton in the Southern Ocean form alkaline shells of calcium carbonate. When they die and sink, they transfer alkalinity to the deep ocean. But as the surface water warms, it will become less dense. As a result, it will mix less with cold, dense and more-alkaline deep water. With less alkalinity returning to the surface, less CO2 – which is slightly acidic – will be absorbed by the ocean. The sooner humanity reaches net zero and stops heating the Southern Ocean, the less CO2 these waters will ultimately emit, says Shin. This month, the UN said the world will fail to meet the Paris Agreement goal of keeping global warming below 1.5°C and should instead focus on ways to reduce global temperatures later on, including through carbon removal. But the new study shows this may be harder to do than we expect, says Gruber. “It’s like crashing a car into the wall, and then you back up the car. It’s not the same car any more,” he says. “We can’t just simply take out CO2. There are consequences, and some of these consequences are long-lasting.” Science DOI: 10.1126/sciadv.aee9228