Scientific modeling has revealed that the oceans will stop absorbing chemicals that destroy the atmosphere's ozone layer and begin emitting them in less than 55 years, doing so in detectable amounts by 2145, and climate change will further accelerate the ocean's sink-to-source transition.

Led by atmospheric scientists at the Massachusetts Institute of Technology, the team behind the research noted that these emissions of chlorofluorocarbons, or CFCs, are not expected to affect the recovery of the Antarctic ozone hole but will be pertinent to future regulation, as they could be mistaken for illegal production of the banned compounds. The researchers also found that emission estimates are overlooking the air pollutants' varying lifetimes.

The study was published March 15 in Proceedings of the National Academy of Sciences.

CFCs were once a common ingredient in refrigerants and aerosol products, but by the 1970s scientists were blaming the group of long-lasting chemicals for depleting the atmosphere's ozone, which protects life from harmful ultraviolet light. They most notably punched a large hole in the ozone layer over Antarctica, and they are also potent planet-warming greenhouse gases.

To protect the ozone layer, all 196 countries in the United Nations signed the Montreal Protocol, which began a phaseout of CFCs through 2010, when all emissions were banned. The Antarctic ozone hole showed the first signs of healing in 2016, and illegal emissions of the air pollutants sharply dropped in 2019 to 52,000 metric tons following a brief rise in the 2010s.

The oceans have absorbed about 5% to 10% of all CFC emissions and are still taking them from the atmosphere — the compounds are harmless when dissolved in water — but MIT scientists were interested to see whether the waters will eventually spit them back into the air.

"We want to better quantify the effects of the ocean because in the past, because human emissions were so big, the effect of the ocean has been ignored," lead author Peidong Wang, an MIT Ph.D. student studying interactions between the ocean and atmosphere, said in an interview with The Academic Times.

Wang and his co-authors modeled the behaviors of the oceans and atmosphere to predict the future flows of CFC-11 and to some extent CFC-12, the most prevalent CFC compounds. The models showed that the oceans' absorption of CFC-11 will reverse around 2075 and begin adding the air pollutant back into the atmosphere, first becoming detectable in 2145 with outflows of up to 500 metric tons per year. The ocean will remain a significant source of CFC-11 through at least 2300, which was the cutoff for the modeling.

Although the predicted CFC-11 emissions will not notably interfere with the recovery of the Antarctic ozone hole, the researchers said, it could be relevant for monitoring illegal emissions in the future.

"By the time you get to the first half of the 22nd century, you'll have enough of a flux coming out of the ocean that it might look like someone is cheating on the Montreal Protocol, but instead, it could just be what's coming out of the ocean," said senior author Susan Soloman, a professor of environmental studies and chemistry at MIT.

In her previous work for the U.S. National Oceanic and Atmospheric Administration, Solomon led an Antarctic expedition that found conclusive evidence of CFCs causing the ozone hole, laying the groundwork for the Montreal Protocol.

For the new study, the scientists ran a different model that assumed a worst-case climate change scenario, which mildly sped up the transition. Under those conditions, the ocean would turn into a CFC-11 source about 10 years sooner and a detectable source five years sooner. The effect would be driven by warming water, which holds smaller amounts of dissolved CFC-11, and the continued weakeningof Atlantic Ocean currents that transport the compounds deep underwater for long-term storage.

But this climate change impact is relatively minor, according to Wang, and it also will not disrupt the ozone hole's recovery.

In addition, the researchers found that the oceans change the lifetime of CFC-11, which adds uncertainty to emissions estimates. It is assumed to persist in the atmosphere for a constant 55 years, but the ocean-atmosphere model revealed that it varied throughout time, from a lifetime of 50 years in 1950 to 60 years in 2250. This causes greater uncertainty and could mean some older estimates were as much as 10% too high, according to Wang.

"This might be important for the future policymakers to better pinpoint the exact emissions more precisely," Wang said. 

CFC-12 behaves similarly to CFC-11 in ocean-atmosphere systems and will follow a similar trend, the researchers wrote, but it is less soluble in water and will therefore be present in smaller amounts.

Wang said he and his colleagues are also interested in studying other ozone-depleting chemicals and how they move between the atmosphere and the oceans. These include methyl chloroform and carbon tetrachloride, which were also banned by the Montreal Protocol.

The study "On the Effects of the Ocean on Atmospheric CFC-11 Lifetimes and Emissions," published March 15 in Proceedings of the National Academy of Sciences, was authored by Peidong Wang, Jeffery Scott, Susan Soloman, John Marshall, Andrew Babbin, Megan Lickley and Ronald Prinn, Massachusetts Institute of Technology; David Thompson, Colorado State University; Timothy DeVries, University of California, Santa Barbara; and Qing Liang, NASA Goddard Space Flight Center.