The ozone hole over the Antarctic has begun to heal, according to a new study, more than 30 years after its discovery. The findings suggest that global efforts to phase out ozone-depleting chemicals have been effective, though scientists still aren't entirely sure about what's driving the ozone hole's recovery.

The study, published in the journal Science, combines data gathered from balloons and satellites to measure the area of the ozone layer over Antarctica from 2000 to 2015. Since 2000, the paper reports, the size of the ozone hole over Antarctica shrank by about 4 million square kilometers — an area equivalent to about half of the contiguous United States. Using computer simulations to account for changes in wind and temperature, the study's authors estimate that about half of its reduction can be attributed to a decline in levels of the ozone-depleting gases chlorine and bromine.

The stratospheric ozone layer protects Earth from the sun's harmful ultraviolet (UV-B) rays, which can cause skin cancer and cataracts in humans, and physiological damage in animals and plants. Faced with growing evidence of ozone depletion, governments in 1987 ratified the Montreal Protocol, a global treaty that aimed to phase out the production of harmful chemicals known as chlorofluorocarbons (CFCs). At the time, CFCs were used in hairspray, aerosol cans, and refrigerators. A single CFC molecule can last for between 20 and 100 years in the atmosphere, and can destroy100,000 ozone molecules.

"It's really a remarkable achievement for society."

Previous studies have shown that the rate of ozone depletion has declined since the Montreal Protocol went into effect, and a 2014 United Nations report showed that Earth's ozone layer has begun to heal. But the ozone hole over the Antarctic reached a record size in 2015, casting some doubt over claims of a recovery, and the UN report said it was still unclear whether healing in the Antarctic could be attributed to a decline in ozone-depleting gases. Susan Solomon, professor of atmospheric chemistry and climate science at MIT and lead author of the study published this week, says her findings suggest that the Montreal Protocol has in fact worked.

"We are beginning to see clear signs that actions that society took to phase out chlorofluorocarbons are actually having the intended effect of beginning to heal the Antarctic ozone layer," Solomon says, stressing that the recovery is still in its early phases. "It's really a remarkable achievement for society," she adds. "It's a global environmental problem, and we have put ourselves on a good trajectory."

The ozone hole appears over Antarctica every year and is usually at its largest during October, as the continent transitions from winter to summer. Previous studies used that month as a benchmark for measuring the hole's size, but Solomon and her colleagues focused instead on September because the Antarctic weather is historically less variable, making it easier to isolate the effect of chlorine and bromine, which deplete ozone. In the process, they found that the Antarctic ozone hole is opening later in the year, another sign that it is healing. They also found that seemingly anomalous observations of very large holes, as in 2015, could be attributed to volcanic eruptions, which deplete ozone by releasing sulfates into the atmosphere.

A satellite image of the ozone hole over Antarctica in October 2015, when it was at its largest. (NASA)

Paul A. Newman, chief scientist for atmospheric sciences at NASA's Goddard Space Flight Center, says the study raises important questions about what else is driving the recovery of Antarctica's ozone hole besides the decline in chlorine and bromine. He notes that the paper's ozone measurements still show a high level of variability, which Solomon and her colleagues attribute to weather effects. The authors say there is likely some interplay between declining chlorine levels and changes in Antarctic weather, though they have yet to quantify that relationship.

"You have to be able to explain the causes of the total trend," says Newman, who was not involved in the study. "Not just the half due to the ozone depleting substances."

Still, Newman believes that the study's findings provide further proof that experts were correct in sounding the alarm over ozone levels during the 1970s and 1980s. "We predicted this way back in the day — we scientists said this would happen, and we said that if you do something, things will eventually get better," he says. "And so this is the kind of thing that we've been looking for."

But the Montreal Protocol has had some negative consequences, as well. When countries began phasing out CFCs, manufacturers replaced them with hydrofluorocarbons (HFCs). HFCs don't deplete ozone, but they are potent greenhouse gases, which contribute to global warming. The challenge going forward, then, will be to develop new alternatives to HFCs — and to have the world adopt them, once again.

"I only wish that they could unify in the same way over the many issues that affect the climate today."

"That's going to be really interesting to see if the diplomatic community can manage that," Solomon says. "And what the technology community can do in terms of developing alternatives is obviously going to be very important."

Jonathan Shanklin, a meteorologist at the British Antarctic Survey, was one of three scientists who discovered the ozone hole over Antarctica in 1985. Their discovery spurred world leaders to ratify the Montreal Protocol two years later, and every country in the UN has now signed on to it. He says Solomon's findings are further proof that the treaty has been "astonishingly successful," though he never imagined that the work he did 30 years ago would lead to it.

"At the start, I didn't really think it was a terribly significant discovery," Shanklin says. "I thought it was some obscure fact of Antarctic meteorology and it might be interesting scientifically, but that would be the end of it."

"It has been really astonishing to me that that little discovery has unified the world's countries to really produce a measurable effect. And I only wish that they could unify in the same way over the many issues that affect the climate today."

Correction 3:10pm ET: A previous version of this article stated that the Ozone hole over Antarctica had shrunk by 4 million square miles. The correct measurement is in kilometers. The piece has been updated.

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