Since the discovery of the Antarctic ozone hole, scientists, policymakers, and the public have wondered whether we might someday see a similarly extreme depletion of ozone over the Arctic.

A new MIT study finds it isn't a big worry. In the 30 years of international efforts to limit ozone-depleting chemicals, ozone levels in the Arctic haven't yet sunk to Antarctica levels. Picking one solution and declaring it the savior may not be valid; in Canada, ozone-depleting chemicals dropped but ozone still went up, forcing policymakers to scramble and claim it must be coming from Asia.

Regardless, good news is good news, especially in cold regions. Frigid temperatures can spur ozone loss because they create prime conditions for the formation of polar stratospheric clouds. When sunlight hits these clouds, it sparks a reaction between chlorine from chlorofluorocarbons (CFCs), human-made chemicals once used for refrigerants, foam blowing, and other applications — ultimately destroying ozone. 

"While there is certainly some depletion of Arctic ozone, the extremes of Antarctica so far are very different from what we find in the Arctic, even in the coldest years," says Susan Solomon, Professor of Atmospheric Chemistry and Climate Science at MIT and lead author of a paper on it in the Proceedings of the National Academy of Sciences.

"A success story of science and policy"

After the ozone-attacking properties of CFCs were discovered in the 1980s, countries across the world agreed to phase out their use as part of the 1987 Montreal Protocol treaty. While CFCs are no longer in use, those emitted years ago remain in the atmosphere. As a result, atmospheric concentrations have peaked and are now slowly declining, but it will be several decades before CFCs are totally eliminated from the environment — meaning there is still some risk of ozone depletion caused by CFCs.

"It's really a success story of science and policy, where the right things were done just in time to avoid broader environmental damage," says Solomon, who made some of the first measurements in Antarctica that pointed toward CFCs as the primary cause of the ozone hole.

To obtain their findings, the researchers used balloon and satellite data from the heart of the ozone layer over both polar regions. They found that Arctic ozone levels did drop significantly during an extended period of unusual cold in the spring of 2011. While this dip did depress ozone levels, the decrease was nowhere near as drastic as the nearly complete loss of ozone in the heart of the layer seen in many years in Antarctica.

The MIT team's work also helps to show chemical reasons for the differences, demonstrating that ozone loss in Antarctica is closely associated with reduced levels of nitric acid in air that is colder than that in the Arctic.

"We'll continue to have cold years with extreme Antarctic ozone holes for a long time to come," Solomon says. "We can't be sure that there will never be extreme Arctic ozone losses in an unusually cold future year, but so far, so good — and that's good news."