The hole in the ozone layer was one of the most headline-hogging environmental issues of the 1980s and 90s, but an international ban on damaging chemicals has helped it recover in the 30 years since. It's long been known that the hole is shrinking, but a new study has provided greater insight into the improving health of the ozone layer by analyzing the chemicals around the hole over the last decade or so.

High up in the stratosphere, the ozone layer protects the surface of the Earth from the worst of the sun's ultraviolet light – so it was understandably alarming when scientists discovered a hole in that shield over Antarctica in the mid-80s. The culprit, it turned out, were chemical compounds called chlorofluorocarbons (CFCs), which drift into the stratosphere where UV radiation breaks them down into chlorine atoms, which then dissolve ozone molecules.

To fight back, the nations of the world rallied together to sign the Montreal Protocol in 1987, which banned the use of chemicals containing CFCs. Now decades later, the damage is slowly being undone, with scientists noticing that the ozone hole has shrunk by 4 million square km (1.5 million sq miles) overall since the year 2000.

But the size alone doesn't tell the whole story. In fact, the area of the hole fluctuates every year, peaking in size around October. That seasonal shift is due to the fact that CFCs can stay afloat in the atmosphere for up to a century, and increased UV radiation at certain times of year kickstart the ozone depletion process all over again.

The good news is that the average size of the hole has been steadily shrinking over the past few decades, but with other factors at play, scientists couldn't be completely sure that the Montreal Protocol could claim credit.

To check that the CFC ban is contributing to the recovery of the ozone layer hole, a team of researchers studied data gathered by a satellite-mounted instrument called the Microwave Limb Sounder (MLS), which measures trace gases in the atmosphere. Using MLS data gathered every winter between 2005 and 2016, the team was able to determine the daily changes in ozone levels during the entire winter season of each year – from early July to mid-September.

Sure enough, the researchers found that the rate of ozone loss decreased by 20 percent over that time period.

"This is very close to what our model predicts we should see for this amount of chlorine decline," says Susan Strahan, lead author of the study. "This gives us confidence that the decrease in ozone depletion through mid-September shown by MLS data is due to declining levels of chlorine coming from CFCs. But we're not yet seeing a clear decrease in the size of the ozone hole because that's controlled mainly by temperature after mid-September, which varies a lot from year to year."

As encouraging as the result is, we're not out of the woods yet. New CFCs aren't being pumped into the atmosphere, but the existing ones will still be around for a long time.

"CFCs have lifetimes from 50 to 100 years, so they linger in the atmosphere for a very long time," says Anne Douglass, co-author of the study. "As far as the ozone hole being gone, we're looking at 2060 or 2080. And even then there might still be a small hole."

The research was published in the journal Geophysical Research Letters.

Source: NASA