Scientists at the Massachusetts Institute of Technology and elsewhere have identified the “first fingerprints of healing” of the Antarctic ozone layer.
In a study published on Thursday in the journal Science, the team found that the September ozone hole has shrunk by more than 4 million square kilometers 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 a news piece on the MIT website.
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 chlorofluorocarbons (CFCs). These chemical compounds were once emitted by dry cleaning processes, old refrigerators, and aerosols such as hairspray.
“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.
Ozone is sensitive not just to chlorine, but also to temperature and sunlight. Chlorine eats away at ozone, but only if light is present and if the atmosphere is cold enough to create polar stratospheric clouds on which chlorine chemistry can occur. 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.
Solomon and her colleagues believed they would get a clearer picture of chlorine’s effects by looking earlier in the year, at ozone levels in September, 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.
“It’s been interesting to think about this in a different month, and looking in September was a novel way,” says Diane Ivy, one of the paper's co-authors. “It showed we can actually see a chemical fingerprint, which is sensitive to the levels of chlorine, finally emerging as a sign of recovery.”
