Ozone-destroying CFCs could make late-21st-century comeback.

The oceans are ready to give back your 1980s hair spray.

The global oceans are gearing up to spray all that 1980s hair spray back in our faces. Chlorofluorocarbons (CFCs), the aerosol chemicals that tore a hole in Earth's protective ozone layer within years of their mass production, are set to make a comeback in the late 21st century, in a process accelerated by climate change, researchers say.

The Montreal Protocol banned the use of CFCs worldwide in 1987, after researchers discovered that CFCs had damaged the ozone layer that protects life on Earth from harmful ultraviolet radiation. And the Montreal Protocol has mostly worked — CFC levels in the atmosphere have dropped sharply in recent decades, and the ozone layer has begun to repair itself, as Live Science reported. But all those CFCs already released into the atmosphere had to go somewhere. And for many of those molecules, that somewhere was the world's oceans. 

Now, a new study projects that as CFC levels in the atmosphere drop and the oceans warm, some of those latent ozone-gobblers will end up back in the air — almost as if some country decided to begin emitting them again.

That's because the ocean and atmosphere tend to stay in balance. When the atmosphere has a lot of a water-soluble molecule, like a CFC, the oceans suck some of it up. And when the oceans have a lot of that same molecule but the atmosphere doesn't, they tend to release it back into the air. As the world has stopped producing CFCs, atmospheric CFC levels have dropped, and the oceans are absorbing less and less from the air. Eventually, the balance will tip, and the oceans will become net-emitters of CFCs. Climate change warms the oceans, which reduces the amount of CFCs a gallon of ocean water can hold, accelerating the process. This new study shows when all those factors should come together, and turn the oceans from CFC-sponges to CFC-emitters.

"By the time you get to the first half of the 22nd century, you'll have enough of a 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," study co-author and MIT environmental scientist Susan Solomon said in a statement.

CFCs are synthetic compounds that are made up of carbon atoms bonded with chlorine and fluorine atoms. Because they are inert, nonflammable and nontoxic, CFCs were used in refrigerants, aerosol cans, and other household and industrial goods in the second half of the 20th century, as Live Science previously reported. When first introduced, CFCs seemed like a safe alternative to poisonous ammonia and flammable butane. But researchers revealed that CFCs tend to break down after they're released into the atmosphere, emitting chlorine that reacts with ozone molecules — each made of three linked oxygen atoms — causing the ozone to break down. 

The slow repair of the ozone layer represents one of the greatest global environmental success stories of all time, environmentalists often say. But the researchers of the new study showed that such a grand success has led to a drop in atmospheric CFCs that could soon trigger the oceans to release the CFCs they have absorbed.

When the atmosphere fills up with a water-soluble chemical, like CFCs or even carbon dioxide, at much higher levels than those found in the ocean, the seas tend to absorb that chemical until the marine and air concentrations achieve balance. (The details of that balance vary from one compound to another.)

The authors of the new paper focused on CFC-11, one of several types of CFC covered by the Montreal Protocol. The authors estimated that about 5% to 10% of all CFC-11 ever manufactured and emitted ended up in the oceans. And because atmospheric CFC-11 levels have remained so much higher than oceanic CFC-11 levels to this point, despite the reductions due to the Montreal Protocol, most of what was absorbed has stayed put.

But by using careful models of ocean behavior and CFC production (real and expected) between 1930 and 2300, the researchers showed that as soon as the year 2075, atmospheric CFC-11 levels will fall so much that the oceans will release more than they absorb. And by 2145, the oceans will release so much CFC-11 that — if monitors didn't know better — it might look as though someone were breaking the Montreal Protocol.

Climate change will speed up that process. Assuming average global warming of 9 degrees Fahrenheit (5 degrees Celsius) by 2100, the study authors wrote, the oceans could flip from absorbing to emitting CFC-11 a decade earlier than expected. (Five degrees of warming would be higher than the targets set in international planning like the Paris Agreement, but is more is less in line with the course on which the planet appears to be headed.)

"Generally, a colder ocean will absorb more CFCs," said lead author and MIT researcher Peidong Wang. "When climate change warms the ocean, it becomes a weaker reservoir and will also outgas a little faster."

There's room to improve on this model, the researchers wrote. More powerful, higher-resolution models should offer a more precise picture of exactly what intensity of oceanic CFC emissions to expect and when to expect them. The CFC-11 hiding in the ocean is not enough on its own to wipe out the ozone layer, but it could prolong its repair.

The study was published March 15 in the journal Proceedings of the National Academy of Sciences.

23 March 2021

Live Science