Study predicts oceans will start emitting ozone-depleting CFCs

The world’s oceans are a vast repository of gases, including ozone-depleting chlorofluorocarbons, or CFCs. They absorb these gases from the atmosphere and attract them to the depths, where they can remain sequestered for centuries and more.

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

Researchers predict that by 2075, the oceans will emit more CFC-11 into the atmosphere than they absorb, emitting detectable amounts of the chemical by 2130. Additionally, as the change increases climate, this change will occur 10 years earlier. Emissions of CFC-11 from the ocean will effectively prolong the average residence time of the chemical, causing it to persist five years longer in the atmosphere than it would otherwise. This could have an impact on future estimates of CFC-11 emissions.

The new findings may help scientists and policymakers better identify 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 will 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 what’s coming. out of the ocean, ”says Susan Solomon, study co-author, Lee and Geraldine Martin Professor of Environmental Studies in the Department of Earth, Atmospheric, and Planetary Sciences at MIT. “It’s an interesting prediction that will hopefully help future researchers avoid being wrong about what’s going on.”

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

An oversaturated ocean

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

Since its elimination, levels of CFC-11 in the atmosphere have been declining steadily, and scientists estimate that the ocean has absorbed about 5 to 10 percent of all manufactured CFC-11 emissions. However, as concentrations of the chemical continue to decline in the atmosphere, CFC-11 is predicted to supersaturate in the ocean, causing it to become a source rather than a sink.

“For a while, human emissions were so great that what went into the ocean was considered negligible,” Solomon says. “Now, as we try to get rid of human emissions, we find that we can no longer completely ignore what the ocean is doing.”

A weakening reservoir

In their new paper, the MIT team set out to determine when the ocean would become a source of the chemical and to what extent the ocean would contribute to CFC-11 concentrations in the atmosphere. They also sought to understand how climate change would impact 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 the northern and southern hemispheres. They added to this model the anthropogenic emissions of CFC-11 that had already been reported over the years, and then moved the model over time, from 1930 to 2300, to observe changes in the flow of chemicals between l ocean and atmosphere.

They then replaced the ocean layers in this simple model with the MIT General Circulation Model, or MITgcm, a more sophisticated representation of ocean dynamics, and performed similar simulations of CFC-11 over the same time period.

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

Uptake from the ocean in the 20th century and outgassing in the future also affects the effective residence time of the chemical in the atmosphere, decreasing it by several years during uptake and increasing it 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 2100, and found that climate change would advance the ocean’s passage to a 10-year source and produce detectable levels of CFC-11 by 2140.

“In general, a colder ocean will absorb more CFCs,” says Wang. “When climate change warms the ocean, it becomes a weaker reservoir and will also degas a little faster.”

“Even if there was no climate change, as CFCs disintegrate in the atmosphere, the ocean ultimately has too much of it for the atmosphere and it will come out,” Solomon adds. “Climate change, we believe, will make it even faster. But change does not depend on climate change.”

Their simulations show that the ocean’s displacement will occur slightly faster in the northern hemisphere, where large-scale ocean circulation patterns are expected to slow, leaving more gas in the shallow ocean to escape toward the sea. atmosphere. However, knowing the exact factors behind the ocean overturning will require 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 the models of change,” Scott says. “So far we’ve opened up some interesting new questions and given a hint of what we might see.”