As climate change makes our world a bit warmer, the ozone layer stands to pay part of the bill. According to a new study, warmer average water temperatures will make ocean water release chlorofluorocarbons, or CFCs, a class of ozone-depleting chemicals that have so far been absorbed from the atmosphere by seas and oceans.
Marine CFCs can stay trapped in the ocean's depths for centuries or more, where they can't harm the ozone layer. However, we'll likely see growing levels of these gases in the atmosphere in the future, despite a global ban on their production and use. The source, according to a new paper from MIT, are the oceans; as they heat up, they lose their ability to hold CFCs. Climate change isn't the direct cause of this release, the authors explain, but it will make it take place much earlier than it otherwise would.
"The ocean will turn into a source of CFC emission no matter if there is climate change or not. Because as human emissions drop, the ocean will eventually become saturated with CFC and begin to outgas. In our study, we did a simulation with climate change and a simulation without climate change," the authors explained for ZME Science.
"We found that climate change will make the ocean outgassing process happen about 5 years faster based on our simulation."
Climate change stands poised to change average temperatures and circulation patterns in the ocean, thus facilitating this outgassing process. CFCs are more soluble when sea surface temperatures are low, the authors explain. Furthermore, changes to ocean currents mean that lower quantities of the absorbed CFCs will be transported to the deep ocean, where they're more easily stored.
The team estimates that the oceans will emit more CFCs into the atmosphere than they absorb by 2075, and that they will emit significant amounts of such compounds by 2130. Effectively, this means that the oceans will stop acting as a sink for CFCs, as they did up to now, and begin acting as a source. All in all, this stands to throw a wrench on our estimations of atmospheric CFC levels in the future. The team focused their research on CFC-11 (trichlorofluoromethane), as changes in oceanic levels of this gas have been shown to affect changes in its atmospheric concentrations.
CFCs are stringently controlled after the global community came together under the 1987 Montreal Protocol to protect the ozone layer. But around the "first half of the 22nd century", the paper reports, CFC levels coming out of the ocean will be so high that "it might look like someone is cheating on the Montreal Protocol". As such, the findings are meant to give us a better understanding of this process in the future, as well as warn policymakers of this upcoming source of CFCs.
For the study, the team used "a hierarchy of models" to analyze mixing patterns between the ocean and atmosphere, and within them. They started from a simple model of the atmosphere, along with the upper and lower levels of the ocean for both hemispheres. Data pertaining to (previously reported) yearly, man-made CFC-11 emissions was also fed in, before allowing the model to run from 1930 to 2300.
After noting the CFC flows between the ocean and atmosphere this model produced, they replaced the ocean model they used with MITgcm, which is a more sophisticated simulation of ocean dynamics, and ran their model again over the same period.
Both models produced estimations of atmospheric CFC-11 through to the present day that match with our records, suggesting they work well. These models predict that oceans will start being net contributors of CFCs sometime around 2075. By 2145, these emissions would be great enough that our current monitoring systems could pick up on them. The team's climate-change model assumed a 5 degrees Celsius increase by 2100. Under these conditions, the ocean will shift to becoming a net CFC contributor around 10 years earlier and will produce detectable amounts of the gas by 2140.
Does this spell doom for the ozone layer? Not really, the authors state.
"Current studies indicate that Antarctica's ozone layer will recover back to the 1980s level by around 2070s. The ocean is acting as a net sink also till the 2070s, so the effect of the ocean is actually making the healing of the ozone hole a bit faster, though based on our current simulation, this effect is not significant," they told me in an email.
"Our future work will look at the effect of the ocean on the healing of the ozone hole for a wider range of ozone-depleting substances. And when the ocean turns into a source of CFC emissions after the 2070s, the effects won’t be devastating for the ozone layer based on our current simulation."
They add that since CFCs naturally (albeit, slowly) degrade in the upper atmosphere through interactions with solar radiation, the oceans will eventually hold too much of these gases relative to the atmosphere, so outgassing processes are inevitable. Climate change is however expected to start impacting ocean circulation patterns in the Northern Hemisphere first, meaning outgassing will likely start here.
As our discussion was drawing to a close, I asked the team whether their findings are only bad news or if there's a silver lining to it all. They told me that this is definitely an example of how human activity can cast a very long shadow on the Earth and its workings, but that it's not "simply bad news".
"CFCs are almost conserved in the ocean (no natural production nor loss), and CFCs have been very useful tracers for oceanographers to study ocean circulation in the past, despite its devastating impact to the stratosphere ozone layer. The ocean is acting as a reservoir for CFCs and it will eventually return CFCs back to the atmosphere," they said.
"Our study indicates that even by 2300, it will still be emitting a small but detectable amount of CFCs back to the atmosphere. This amount won’t be devastating for the ozone layer, but it demonstrates how human emissions are making a really long-lasting impact on the Earth."
Perhaps the most positive finding of the paper is that the Montreal Protocol worked -- if atmospheric levels of CFC didn't drop, the ocean couldn't release them. But we shouldn't rest on our laurels. Oceanwater still holds lower levels of these substances than land reservoirs do, mostly in the form of old refrigeration units and other material disposed of in landfills. Disposing of or containing these sources would ensure that the ozone layer recovers properly.