Research predicts that the oceans will start to emit ozone-depleting CFCs

The world’s oceans are a huge storehouse for gases, including ozone-depleting chlorofluorocarbons, or CFCs. They absorb these gases from the atmosphere and pull them into the depths, where they can remain in isolation for centuries and longer.

Marine CFCs have long been used as tracers to study ocean currents, but their impact on atmospheric concentrations was considered negligible. Now MIT researchers have found that the oceanic fluxes of at least one type of CFC, known as CFC-11, do indeed affect atmospheric concentrations. In a study published today in the Proceedings of the National Academy of Sciencesthe team reports that the global ocean will reverse its old role as a sink for the powerful ozone-depleting chemical.

The researchers predict that by the year 2075, the oceans will release more CFC-11 back into the atmosphere than they absorb, and that they will release detectable amounts of the chemical by 2130. Furthermore, with increasing climate change, this shift will occur 10 years earlier. The release of CFC-11 from the ocean will effectively extend the chemical’s average residence time, making it linger in the atmosphere for five years longer than it otherwise would. This may affect future estimates of CFC-11 emissions.

The new results could help scientists and policymakers better locate future sources of the chemical, which is now banned worldwide under the Montreal Protocol.

“By the time you get to the first half of the 22nd century, you’ve had enough of a current coming out of the ocean to make it look like someone is cheating on the Montreal Protocol, but instead it could just be what’s coming out of the ocean, ”said Susan Solomon, co-author of the study, the Lee and Geraldine Martin professor of environmental studies in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “It’s an interesting prediction and hopefully it will help future researchers not to get confused about what’s going on.”

Solomon’s co-authors include lead author Peidong Wang, Jeffery Scott, John Marshall, Andrew Babbin, Megan Lickley, and Ronald Prinn from MIT; David Thompson of Colorado State University; Timothy DeVries of the University of California Santa Barbara; and Qing Liang of NASA Goddard Space Flight Center.

An ocean, oversaturated

CFC-11 is a chlorofluorocarbon widely used to make refrigerants and insulating foams. When the chemical is released into the atmosphere, it sets off a chain reaction that eventually destroys ozone, the atmospheric layer that protects Earth from harmful ultraviolet rays. Since 2010, production and use of the chemical have been phased out worldwide under the Montreal Protocol, a global treaty that aims to restore and protect the ozone layer.

Since the phase-out, levels of CFC-11 in the atmosphere have steadily declined, and scientists estimate that the ocean has absorbed about 5 to 10 percent of all CFC-11 emissions produced. As concentrations of the chemical in the atmosphere continue to fall, CFC-11 is predicted to become supersaturated in the ocean, making it a source rather than a sink.

“Human emissions were so great for some time that what ended up in the ocean was considered negligible,” says Solomon. “As we try to get rid of human emissions, we find that we can no longer completely ignore what the ocean is doing.”

A weakened reservoir

In their new paper, the MIT team looked at when the ocean would become a source of the chemical and how much the ocean would contribute to CFC-11 concentrations in the atmosphere. They also tried to understand how climate change would affect the ocean’s ability to absorb the chemical in the future.

The researchers used a hierarchy of models to simulate the mixing in and between the ocean and the atmosphere. They started with a simple model of the atmosphere and the upper and lower layers of the ocean in both the Northern and Southern Hemispheres. They added to this model anthropogenic emissions of CFC-11 previously reported over the years, and then advanced the model over time from 1930 to 2300 to detect changes in the chemical’s flux between the ocean and the ocean. atmosphere.

They then replaced the ocean layers of this simple model with MIT’s general circulation model, or MITgcm, a more advanced representation of ocean dynamics, and performed similar simulations of CFC-11 over the same period.

Both models produced atmospheric levels of CFC-11 to this day that matched recorded measurements, giving the team confidence in their approach. When they looked at the models’ future projections, they saw that from 2075 onwards, the ocean began to emit more of the chemical than it absorbed. By 2145, the ocean would be emitting CFC-11 in amounts detectable by current monitoring standards.

Ocean uptake in the 20th century and future outgassing also impacts the chemical’s effective residence time in the atmosphere, reducing it by several years during uptake and by up to 5 years by the end of 2200.

Climate change will accelerate this process. The team used the models to simulate a future with global warming of about 5 degrees Celsius by the year 2100, and found that climate change will accelerate the ocean’s shift to a source by 10 years and detectable levels by 2140. of CFC-11.

“In general, a colder ocean will absorb more CFCs,” explains Wang. “When climate change warms up the ocean, it will become a weaker reservoir and will also outgass a little faster.”

“Even if there were no climate change, as CFCs decay into the atmosphere, the ocean will eventually have too much in relation to the atmosphere and will come out again,” adds Solomon. “We think that climate change will make this happen even sooner. But the switch is not dependent on climate change.”

Their simulations show that the ocean’s shift will occur slightly faster in the Northern Hemisphere, where large-scale ocean circulation patterns are expected to slow, leaving more gases in the shallow ocean to escape back to the atmosphere. Knowing the exact causes of the ocean’s reversal, however, requires more detailed models, which the researchers plan to explore.

“Some of the next steps would be to do this with higher resolution models and focus on change patterns,” says Scott. “For now, we’ve opened up some great new questions and gave you an idea of ​​what you might see.”