The scientific team report that the September ozone hole had shrunk by more than 4 million km2 since 2000, when ozone depletion was at its peak.
The team also showed for the first time that this recovery has slowed somewhat at times, due to the effects of volcanic eruptions from year to year. Overall, however, the ozone hole appears to be on a healing path, according to the report published in Science today.
The findings appear to contradict last year’s World Meteorological Organisation (WMO) report that the Antarctic ozone hole in October 2015 was amongst the largest ever observed with an area of 28.2 million km².
In this latest report, scientists from Massachusetts Institute of Technology (MIT), the Department of Earth, Atmospheric and Planetary Sciences, along with researchers at the National Center for Atmospheric Research in Boulder, Colorado, and the University of Leeds in the UK, believed they would get a clearer picture of chlorine’s effects by looking at September’s ozone levels, when cold winter temperatures still prevail and the ozone hole is opening up. The team showed that as the chlorine has decreased, the rate at which the hole opens up in September has slowed down.
Measurements have shown that ozone depletion starts each year in late August, as Antarctica emerges from its dark winter, and the hole is fully formed by early October.
Lead author Susan Solomon, Ellen Swallow Richards Professor of Atmospheric Chemistry and Climate Science at MIT, suggests that everyone has been too focused on October, when the ozone hole is at its widest, but when it is also subject to other variables, 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,” she says.
The authors used “fingerprints” of the ozone changes with season and altitude to attribute the ozone’s recovery to the continuing decline of atmospheric chlorine originating from CFCs.
Praising the measures taken under the Montreal Protocol, Susan Solomon said: “We can now be confident that the things we’ve done have put the planet on a path to heal.”
“It’s been interesting to think about this in a different month, and looking in September was a novel way,” Ivy says. “It showed we can actually see a chemical fingerprint, which is sensitive to the levels of chlorine, finally emerging as a sign of recovery.”
The team suggests that the 2015 spike in ozone depletion was due primarily to the eruption of the Chilean volcano Calbuco. While volcanoes don’t inject significant chlorine into the stratosphere, they do increase small particles, which increase the amount of polar stratospheric clouds with which the human-made chlorine reacts.
As chlorine levels continue to dissipate from the atmosphere, Solomon sees no reason why, barring future volcanic eruptions, the ozone hole shouldn’t shrink and eventually close permanently by midcentury.
“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 spurred 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.”