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 atmospheric carbon dioxide concentrations fall, the ocean will start releasing CO2 into the air.
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.
鈥淭hat impact can sustain much, much longer than we expected,鈥� says at Seoul National University in South Korea. 鈥淟et鈥檚 say we try to reduce the CO2鈥� this sink-to-source transition in the Southern Ocean still acts as an obstacle to reduce the atmospheric CO2 concentration.鈥�
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The notoriously strong winds around the 鈥渞oaring forties鈥� and 鈥渇urious fifties鈥� latitudes of the southern hemisphere stir up waves and encourage the water to absorb CO2. Through exposure to the cold Antarctic air,聽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.
鈥淚t鈥檚 like you鈥檙e constantly pressing a piston into the ocean,鈥� says at ETH Zurich in Switzerland, who wasn鈥檛 involved in the study. 鈥淚f you pull the piston up again, then the CO2 comes back out.鈥�
Shin and his colleagues 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聽as Earth鈥檚 natural carbon sinks remove some of the gas from the atmosphere. They also modelled one in which humanity drastically decreases atmospheric CO2 after net zero聽鈥� again reached in 2125 鈥撀爐hrough 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 鈥� roughly the current concentration. Without carbon removal, it levels off at 590 ppm.
The scenarios aren鈥檛 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 at the University of Southampton, UK.
鈥淎nything that changes in terms of the way the ocean interacts with the atmosphere could be like adding a massive country to the world鈥檚 carbon budget,鈥� 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,聽there is less potential for the water to react with slightly acidic CO2 in a process that increases the water鈥檚 capacity to pull more CO2 from the air.
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掳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.
鈥淚t鈥檚 like crashing a car into the wall, and then you back up the car. It鈥檚 not the same car any more,鈥� he says. 鈥淲e can鈥檛 just simply take out CO2. There are consequences, and some of these consequences are long-lasting.鈥�
Science Advances
