International. The world's oceans are a vast reservoir of gases, including ozone-depleting chlorofluorocarbons, or CFCs. They absorb these gases from the atmosphere and drag them into the depths, where they can remain sequestered for centuries and beyond.
Marine CFCs have long been used as tracers to study ocean currents, but their impact on atmospheric concentrations was assumed to be negligible. Now, MIT researchers have discovered that ocean flows of at least one type of CFC, known as CFC-11, actually affect atmospheric concentrations. In a study that appeared 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 potent ozone-depleting chemical.
The researchers project that by 2075, the oceans will emit more CFC-11 into the atmosphere than they absorb, emitting detectable amounts of the chemical by 2130. In addition, with the increase in climate change, this change will occur 10 years earlier. EMISSIONS of CFC-11 from the ocean will effectively extend the average residence time of the chemical, causing it to remain five years longer in the atmosphere than it would otherwise. This may affect future estimates of CFC-11 emissions.
The new results may help scientists and policymakers better identify future sources of the chemical, which is now banned worldwide under the Montreal Protocol.
"By the time it reaches the first half of the twenty-first century, it will have enough flow coming out of the ocean that it might look like someone is cheating on the Montreal Protocol, but instead, it could simply be what lies ahead. out of the ocean," says study co-author Susan Solomon, a Lee and Geraldine Martin Professor of Environmental Studies in MIT's Department of Earth, Atmospheric and Planetary Sciences. It's an interesting prediction and will hopefully help future researchers avoid getting 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 of MIT; David Thompson of Colorado State University; Timothy DeVries of the University of California, Santa Barbara; and Qing Liang of NASA's Goddard Space Flight Center.
A supersaturated ocean
CFC-11 is a chlorofluorocarbon that was commonly used to make refrigerants and insulating foams. When emitted into the atmosphere, the chemical triggers a chain reaction that ultimately destroys ozone, the atmospheric layer that protects Earth from harmful ultraviolet radiation. Since 2010, the production and use of the chemical has been phased out worldwide under the Montreal Protocol, a global treaty that aims to restore and protect the ozone layer.
Since its removal, levels of CFC-11 in the atmosphere have been steadily declining and scientists estimate that the ocean has absorbed between 5 and 10 percent of all CFC-11 emissions manufactured. However, as concentrations of the chemical continue to fall in the atmosphere, CFC-11 is predicted to oversaturate in the ocean, pushing it to become a source rather than a sink.
"For some time, human emissions were so great that what went into the ocean was considered insignificant," 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 reservoir that weakens
In their new paper, the MIT team sought to determine when the ocean would become a source of the chemical and to what extent the ocean would contribute to CONCENTRATIONS of CFC-11 in the atmosphere. They also sought 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 mixing within and between the ocean and atmosphere. They started with a simple model of the atmosphere and the upper and lower layers of the ocean, both in the northern and southern hemispheres. They added to this model anthropogenic emissions of CFC-11 that had been previously reported over the years, then ran the model over time, from 1930 to 2300, to observe changes in chemical flow between the ocean and the atmosphere.
They then replaced the ocean layers of this simple model with MIT's general circulation model, or MITgcm, a more sophisticated representation of ocean dynamics, and ran similar simulations of CFC-11 over the same time period.
Both models produced atmospheric levels of CFC-11 to this day that matched the recorded measurements, giving the team confidence in their approach. When they looked at future projections from the models, they saw that the ocean began to emit more chemicals than it absorbed, starting around 2075. By 2145, the ocean would emit CFC-11 in quantities that would be detectable by current monitoring standards.
Ocean absorption in the twentieth century and degassing in the future also affects the effective residence time of the chemical in the atmosphere, decreasing it by several years during absorption and increasing it 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 cause the ocean to become a source in 10 years and produce detectable levels of CFC-11 by 2140.
"Generally, a colder ocean will absorb more CFCs," Wang explains. "When climate change warms the ocean, it becomes a weaker reservoir and also degasses a little faster."
"Even if there was no climate change, as CFCs break down in the atmosphere, eventually the ocean has too much relative to the atmosphere and will come back out," Solomon adds. "We think climate change will make that happen even sooner. But change does not depend on climate change."
Their simulations show that ocean change will occur slightly faster in the Northern Hemisphere, where large-scale ocean circulation patterns are expected to decrease, leaving more gases in the shallow ocean to escape back into the atmosphere. However, knowing the exact drivers of ocean inversion will require more detailed models, which the researchers aim to explore.
"Some of the next steps would be to do this with higher-resolution models and focus on patterns of change," Scott says. "For now, we've opened up some cool new questions and given an idea of what one might see."
This research was supported, in part, by the VoLo Foundation, the Simons Foundation, and the National Science Foundation.
Source: MIT.
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