Ocean threatens to release ozone-depleting compounds

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While the ocean has so far absorbed part of our chlorofluorocarbon emissions (CFC), the situation threatens to be reversed in the second half of the century. A phenomenon until now ignored which would require to review the horizon of effective re-emission of the ozone layer. The results are published in the journal PNAS this March 23.

The destruction of stratospheric ozone caused by the release of compounds containing chlorine or bromine – CFC – was a major environmental concern in the second half of the 20th century. Thanks to the Montreal Protocol signed in 1987 and its amendments, the molecules in question have been gradually replaced by others less harmful to the ozone layer. Since then, the latter has shown signs of recovery. Also, it is estimated that the average level of the 1980s would be found in the second part of the 21st century.

However, new work is shaking up this prospect somewhat. Indeed, a group of researchers from Massachusetts Institute of Technology (WITH) discovered that the ocean may hinder the decrease in atmospheric concentrations of chlorofluorocarbons (CFC). In particular, that of CFC-11. But how is this explained?

CFC : when the ocean passes from well to source

When human activities release these compounds at a high rate, only some of them accumulate in the atmosphere and help destroy stratospheric ozone. The other is taken over by the ocean. This enrichment of water CFC has also been used as a tracer of oceanic circulation by physicists. Indeed, as the gases enter from above, they make it possible to follow the movement of water masses towards the depths. In total, nearly 10% of anthropogenic emissions from CFC-11 were absorbed by the ocean.

Oceanic content vertically integrated in CFC-11 in 1994. The observations are on the right and the reproduction by the researchers’ model on the left. Finally, the values ​​are in moles per km². Credits: Peidong Wang et al. 2021.

However, if emissions go down and atmospheric concentrations go down, the ocean finds itself overload compared to air. More precisely, the partial pressure of CFC-11 water is no longer in equilibrium with that of the air. Part of the gas then escapes from the ocean and slows down the drop observed in the atmosphere as well as the rate of re-emission of the ozone layer. Simulations performed by scientists show that the moment when the ocean emits more than CFC-11 that it absorbs is around 2075. The flow then becomes large enough to have significant implications in the early 2100s.

“By the time you get to the first half of the 22nd century, you’ll have enough flow coming out of the ocean to make it look like someone is cheating on the Montreal Protocol. But instead it could just be about what’s going out of the ocean ”, specifies Susan Solomon, co-author of the paper. “It’s an interesting projection which will hopefully help future researchers to avoid being wrong about what is going on”.

Climate change won’t help

Another point highlighted by the study is that climate change would advance by about 10 years the date from which the ocean becomes a net emitter. This is in the case of a scenario where nothing is done to reduce the surge in global temperatures. “In general, a colder ocean will absorb more CFCs”, explains Peidong Wang, lead author of the study. “When climate change warms the ocean, it becomes a weaker tank and will also degas a little faster. A hitherto unrecognized problem that now remains to be explored at a more detailed level. “So far, we have opened up some interesting new questions and given an idea of ​​what we might see”, points out Jeffery R. Scott, co-author of the paper.

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