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. 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. 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. Collapse of AMOC ocean current may already be locked in 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. 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.
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