In the mid 1980s the world made an important judgement call. CFCs, the chemical compounds in fridges, aerosols and dry cleaning products, had been boring a hole in the Earth's ozone layer over the polar regions which, if left unchecked, could cause grave public health and environmental problems. So pretty much every country signed up to ban the use of CFCs, a decision that is now paying big dividends with scientists reporting significant shrinkage of the hole and evidence of what looks to be a path to recovery.
Scientists first caught wind of rising CFC, or chlorofluorocarbon, levels in the atmosphere in the 1960s, with the first scientific paper predicting it would lead to ozone depletion published in the journal Nature in 1974. The chemical compounds eat away at the ozone in the atmosphere, but only where light is present and it is cold enough to form polar stratospheric clouds, which have the effect of introducing additional chlorine to the atmosphere in large amounts.
Over the preceding decades this extra chlorine has led to a seasonal hole in the ozone layer over Antarctica, which opens up as the icy continent exits a dark winter and closes again by the end of the local springtime.
The ozone layer plays the role of shielding us from ultraviolet light (UV), so when gaps start to appear and the world below is exposed to higher levels of it, that's a bit of a problem. When us humans are subjected to too much UV we are at a much higher risk of skin cancer. In fact, modeling has suggested that if ozone depletion was left to continue there would have been two million more skin cancer cases around the world each year. And this is to say nothing of the untold damage higher UV levels could cause to forests, the ocean, agriculture and the environment as a whole.
So the steps agreed upon first at the Montreal Protocol on Substances that Deplete the Ozone Layer in 1987, and then enacted in 1989, can be seen as quite pivotal to life on Earth today. But stopping an environmental trainwreck in its tracks is one thing, sending it back the other way is another. And it is here that an international team of scientists are reporting the "first fingerprints of healing" of the ozone layer over the Antarctic.
Because the hole over Antarctica swells to its largest point in October each year, this has largely been the focal point of efforts to track ozone depletion. But the process usually kicks off in late August, so the team figured that they might get a clearer understanding of the effects of chlorine buildup by studying ozone levels a little earlier, when the temperatures are still a little colder and the hole is still in its formative stages.
"I think people, myself included, had been too focused on October, because that's when the ozone hole is enormous, in its full glory," says Susan Solomon, Professor of Atmospheric Chemistry and Climate Science at MIT and co-author of the study. "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. That point hasn't really been made strongly in the past."
Solomon and her team monitored the opening of the Antarctic ozone hole in the month of September across 15 years, from 2000 to 2015. They found that the hole has shrunk four million square km (1,545,000 sq mi) since 2000, the year when ozone depletion peaked. After accounting for changes in weather patterns like temperature and wind that can move the ozone hole around, the team concluded that more than half of the hole's shrinkage was a direct result of lower levels of atmospheric chlorine.
"We can now be confident that the things we've done have put the planet on a path to heal," says Solomon. "Which is pretty good for us, isn't it? Aren't we amazing humans, that we did something that created a situation that we decided collectively, as a world, 'Let's get rid of these molecules'? We got rid of them, and now we're seeing the planet respond."
The team's 15-year tracking of the ozone hole did throw up one curveball, however. In 2015 the hole ballooned in contrast with the trend of declining atmospheric chlorine. But by poring over the data, Solomon and her colleagues zeroed in on the reason why.
When Chile's Calbuco volcano erupted in April of 2015, spewing ash clouds into the air and delaying many international flights, it increased the amount of small particles in the air. These then compounded the stratospheric clouds over the polar regions, and in turn spurred along the chlorine chemistry that causes the ozone to erode.
The team says this is the first time volcanic eruptions have been shown to put the brakes on the ozone hole's recovery. But aside from this little outlier the news is mostly good, with the researchers expecting the hole to continue shrinking and eventually close up for good by midcentury, unless of course more volcanic eruptions get in the way.
"What's exciting for me personally is, this brings so much of my own work over 30 years full circle," says Solomon, whose research into chlorine and ozone actually inspired the Montreal Protocol. "Science was helpful in showing the path, diplomats and countries and industry were incredibly able in charting a pathway out of these molecules, and now we've actually seen the planet starting to get better. It's a wonderful thing."
The research was published in the journal Science.