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Healing the ozone hole helps the Southern Ocean take up carbon, study reveals

New research suggests that the negative effects of the ozone hole on the carbon uptake of the Southern Ocean are reversible, but only if greenhouse gas emissions rapidly decrease.

The study, led by the University of East Anglia (UEA), finds that as the ozone hole heals, its influence on the ocean carbon sink of the Southern Ocean will diminish, while the influence of greenhouse gas (GHG) emissions will rise.

Relative to its area, the Southern Ocean takes up a disproportionate amount of carbon, which reduces the radiative effects of carbon in the atmosphere and strongly mitigates human-caused climate change. Therefore, knowing how much carbon it will absorb, and what controls this carbon uptake, is important.

Scientists from the UEA and the National Center for Atmospheric Science (NCAS), in the UK, looked at the relative role of ozone and GHG emissions in controlling the circulation of the Southern Ocean around Antarctica, focusing on how it would impact the carbon uptake.

They were interested in how the amount of atmospheric carbon taken up by the Southern Ocean has changed in the 20th century, and how it will change over the 21st century. Their findings are in the journal Science Advances.

Lead author Dr. Tereza Jarníková, of the Tyndall Center for Climate Change Research at UEA, said, "An interesting, and hopeful, highlight of this work is that the effects of human-caused ozone hole damage on the winds, circulation, and carbon uptake of the Southern Ocean are reversible, but only under a lower scenario of greenhouse gas emissions."

The Southern Ocean takes up lots of atmospheric carbon because of its unique circulation and properties. Winds have intensified in past decades due to the loss of stratospheric ozone, acting to reduce the uptake of carbon.

As the ozone hole recovers, however, the study shows this phenomenon could reverse. At the same time, increasing GHG emissions could also lead to stronger winds, so how the Southern Ocean circulation will behave in the future, and therefore how much carbon this ocean will take up, is uncertain.

"We found that in the past decades, the depletion of ozone led to a relative reduction of the carbon sink, in general, because of a tendency of the stronger winds to bring higher-carbon water from depth up to the surface of the ocean, making it less suitable for taking up atmospheric carbon," said Dr. Jarníková.

"This isn't true in the future: in the future, the influence of ozone on the winds, and therefore on the Southern Ocean, diminishes, and it's replaced by the increasing influence of greenhouse gas emissions, which also lead to strong winds."

The study also shows that in the future, changes in ocean circulation will have less influence on carbon uptake than they had in the past, because of the changing distribution of carbon between the surface and the deep ocean.

The team used an Earth system model (UKESM1) to simulate three sets of ozone conditions for the time period 1950-2100: a world where the ozone hole never opened; a realistic world where the ozone hole opened but started healing following the adoption of the 1987 Montréal protocol that banned ozone depleting substances; and a world where the ozone hole persisted at its 1987 size throughout the 21st century.

They also simulated two future greenhouse gas scenarios: a low emissions scenario and a high emissions scenario, and then calculated how the main physical features of the ocean change over the 150 simulated years, as well as how the amount of carbon taken up by the ocean changes in response to these physical changes.