The troublesome tear in Earth's protective blanket is getting stitched up.

A gaping hole in the ozone layer has been opening up over Antarctica each spring for decades. And now there are signs that the slow process of healing has begun, according to a study published Thursday in the journal Science.

Scientists credit this progress to the 1987 Montreal Protocol, an international treaty that phased out chemicals that eat away at the ozone layer, which shields our planet from deadly levels of radiation.

"The healing of the Antarctic stratospheric ozone level is the most significant environmental success story of the 20th century," Michael Newchurch, an atmospheric chemist at the University of Alabama at Huntsville who was not part of the study, tells The Christian Science Monitor.

The ozone layer in the atmosphere acts as a sort of sunscreen, blocking out harmful wavelengths of ultraviolet (UV) radiation that disrupt the very cells of living organisms.

"There would actually be no life on the planet's surface if we didn't have an ozone layer," study lead author Susan Solomon an atmospheric chemist at the Massachusetts Institute of Technology, tells the Monitor.

So "it was like a thunderbolt when the British Antarctic Survey discovered the ozone hole" in 1985, Dr. Solomon says. Those scientists had found that in October, which is spring in Antarctica, there was about half as much ozone present over their station as there had been just three decades earlier.

The shocking discovery spurred scientists and governments into action. Solomon herself went down to Antarctica in 1986 to figure out just what was happening in the atmosphere.

It turns out that when certain industrial chemicals react with the surfaces of the polar stratospheric clouds that hang over Antarctica in the spring, they become a form that damages stratospheric ozone. In 1987, an international treaty – the Montreal Protocol – phased out the production of chlorofluorocarbons (CFCs), which contain a chemical trigger for these reactions: chlorine. These compounds were commonly used as refrigerants and propellants for decades.

They didn't expect to see an immediate improvement. "These molecules live in the atmosphere for a very long time," between 50 to 100 years, Solomon says. "Even though we have stopped producing them, what's already there from your grandmother's refrigerator that she got rid of in the 1970s or the hairspray that you put on in 1972, some of it is still left."

And that's why the effect of that ban is just beginning to show now.

NASA

Solomon's new research finds that the ozone hole has shrunk by over 1.5 million square miles in September since 2000, which is about half of the area of the contiguous United States.

"This is further evidence that phasing out the CFCs and other ozone depleting chemicals is working to heal the ozone layer," David Doniger, director of the climate and clean air program at the Natural Resources Defense Council who was not part of the study, tells the Monitor.

"The bad news is that we really messed up the ozone layer," he says. "The good news is that we can save the ozone layer and we are restoring it by eliminating these manmade chemicals that are responsible for the damage."

This is not the first time scientists have reported that the ozone damage is on the mend. Dr. Newchurch's own research pointed out progress in 2003. But, Newchurch says, this new paper shows "we have gone through that first stage, we have hit bottom, and now we are starting to see signs of recovery. And that's pretty significant."

Scientists largely look at October, when the ozone hole is at its largest, to examine the damage done. But Solomon focused on September, when the effect is just getting going.

In October, weather and other factors contribute to the ozone hole. But in September, the chemistry is more clearly controlling the effect, Solomon says. So the team analyzed atmospheric chemistry data from weather balloons and satellites in September from 2000 to 2015, and used this data to create models of the expected effect these chemicals should have.

And the team found that the hole opens more slowly, by about 10 days, than it used to.

Not so simple

Although scientists are seeing improvement, it's not as simple as just eliminating the triggering chlorine. Other factors are at play.

"Last year we actually observed the biggest ever October ozone hole," Solomon says. "It was a shock to the scientific community. How can you have the biggest ozone hole now when it should be starting to heal?"

Through their models, Solomon and her team were able to find the answer: volcanoes.

The increase in the ozone hole was thanks to the eruption of the Andean volcano Calbuco. The small particles shot into the atmosphere by the erupting volcano provided just the right seeds for more clouds to form. And more clouds meant that the chemical reactions could happen more readily.

"It looks like we're starting to emerge toward the real healing of the Antarctic ozone hole," says Paul Newman, chief scientist for atmospheric sciences at NASA's Goddard Space Flight Center who was not part of the study.

But, "do we all adjourn to stage a big party tomorrow about the Montreal Protocol's success? Well, maybe not," Dr. Newman tells the Monitor.

Newman is hesitant to pin the shrinking ozone depletion to these human efforts because of a mismatch between Solomon's models of how much the hole should be shrinking due to the reduction in CFCs and what is actually happening.

"Nature always seems to be throwing us curve balls when it comes to figuring out if things are getting better," Newman says. And this puzzle needs to be sorted out so scientists can be sure the progress or a problem isn't being masked by another dynamic.

Still, "If nothing had been done and ozone depleting substances had continued to increase," Newman says, about two-thirds of the ozone layer would have been gone by 2065 worldwide.

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Newchurch says the Montreal Protocol is a good example of the scientific process in action. Scientists were able to determine what was triggering this ozone depletion and how humans could fix it. "It is the best process we know for solving real, physical problems," he says of the scientific process.

"It's wonderful to see that this most untouched part of our planet that we actually did touch in a very unusual way by creating the ozone hole is going back to its original state," Solomon says.