Efforts to curb pollution are paying off.
Refrigerators and aerosols like hairspray once emitted chemical compounds called chlorofluorocarbons (CFCs). By the mid-1980s, scientists had had linked the chlorine in these CFCs with the annual depletion of ozone above Antarctica.
The ozone layer plays a crucial role in reducing the amount of UV radiation from the sun that warms the Earth, so the discovery of a gaping hole prompted concerted efforts to ban the use of CFCs by virtually every country in the world.
Those efforts now appear to be paying off. Last week, researchers reported in the journal Science that the ozone hole over Antarctica is rapidly shrinking.
"We can now be confident that the things we've done have put the planet on a path to heal," says lead author Susan Solomon, the Ellen Swallow Richards Professor of Atmospheric Chemistry and Climate Science at MIT, in a press release.
Solomon first showed in 1986 that when the atmosphere is cold and light hits CFCs, they break ozone molecules apart. Ozone depletion starts each year in late August, as Antarctica emerges from its dark winter, and the hole is fully formed by early October.
Measurements of the ozone hole are traditionally taken in October, when it is at its largest. But Solomon and her colleagues decided to take their measurements earlier in the year, while temperatures were still cold and the ozone hole was still opening up.
"I think people, myself included, had been too focused on October, because that's when the ozone hole is enormous, in its full glory," Solomon says. "But October is also subject to the slings and arrows of other things that vary, like slight changes in meteorology. September is a better time to look because chlorine chemistry is firmly in control of the rate at which the hole forms at that time of year.”
The researchers tracked the yearly opening of the Antarctic ozone hole in the month of September from 2000 to 2015, through measurements taken from weather balloons and satellites. They also tracked sulfur dioxide emitted by volcanoes, which can enhance ozone depletion, and temperature and wind, which can shift the ozone hole back and forth.
Compared with model simulations that predict ozone levels based on the amount of chlorine present in the atmosphere from year to year, the team found that the ozone hole had shrunk by more than 4 million square kilometers in 2015 from its peak size in 2000. More than half the shrinkage was deemed to be the result of reduction in atmospheric chlorine.
If chlorine levels continue to dissipate from the atmosphere at the current rate, Solomon believes, barring future volcanic eruptions, that the ozone hole should shrink and eventually close permanently by mid-century.