Researchers from the Massachusetts Institute of Technology (MIT) are predicting that the world’s oceans, long regarded a stable repository of CFCs drawn from the atmosphere, will emit more CFC-11 back into the atmosphere than they absorb by 2075.
Although used as tracers to study ocean currents, marine CFCs have always been regarded as having negligible impact on atmospheric concentrations. The study, which appeared in the Proceedings of the National Academy of Sciences, reports that oceans may reverse their long-standing role as absorber of these ozone-depleting gases.
The MIT researchers discovered that the oceanic fluxes of at least one type of CFC, known as CFC-11, do in fact affect atmospheric concentrations. They are projecting that by the year 2075 the oceans will emit more CFC-11 back into the atmosphere than they absorb, emitting detectable amounts of the chemical by 2130.
The researchers believe this role reversal will occur 10 years earlier due to increasing climate change. The ocean’s CFC absorption in the 20th century and future emissions also affects the chemical’s effective residence time in the atmosphere – decreasing it by several years during uptake and increasing it by up to five years by the end of 2200.
“Generally, a colder ocean will absorb more CFCs,” says lead author Peidong Wang. “When climate change warms the ocean, it becomes a weaker reservoir and will also outgas a little faster.”
“By the time you get to the first half of the 22nd century, you’ll have enough of 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,” says the study’s co-author Susan Solomon, from 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 will help future researchers avoid getting confused about what’s going on.”
The researchers used a hierarchy of models to simulate the mixing within and between the ocean and atmosphere.
Studying the models’ future projections, they observed that the ocean will begin to emit more of the chemical than it absorbed, beginning around 2075. By 2145, the ocean would emit CFC-11 in detectable amounts based on current monitoring standards.
Using the models to simulate a future with global warming of about 5℃ by the year 2100, the research team found that climate change will advance the ocean’s shift to a source by 10 years and produce detectable levels of CFC-11 by 2140.
The simulations show that the ocean’s shift will occur slightly faster in the Northern Hemisphere. This is due to the team’s expectation that the region’s large-scale ocean circulation patterns would slow down and leave more gases in the shallow ocean to escape back to the atmosphere.
To find out the exact drivers of the ocean’s reversal will require more detailed models, say the researchers, who intend to continue their work.