Ozone hole on the mend
IN RECOVERYScientists may have already spotted the annual Antarctic ozone hole, shown here in September 2006, getting better thanks to an international environmental treaty.NASA
Scientists may have spotted Antarctica’s ozone hole on the road to recovery, at least a decade sooner than they thought healing would be noticeable.
In 1989, an international agreement called the Montreal Protocol began phasing out chemicals that have gnawed away at Earth’s protective ozone layer. Most researchers thought it would take until at least 2023 to detect the hole’s slow recovery, but researchers in Australia now claim to have seen ozone ticking upward since the late 1990s.
“The key is to account for large year-to-year fluctuations that have obscured a gradual increase in the long-term evolution of ozone,” says atmospheric scientist Murry Salby of Macquarie University in Sydney. His team published its findings online May 6 in Geophysical Research Letters.
First spotted in 1985, the Antarctic ozone hole was quickly linked to chemicals called chlorofluorocarbons, emitted mainly in the Northern Hemisphere but concentrated over the South Pole by atmospheric circulation patterns. Chlorine atoms from these CFCs react with ozone molecules, seasonally destroying the layer that shields Earth from cancer-causing and crop-damaging ultraviolet radiation.
Scientists had predicted that ozone loss would bottom out and start recovering by now. They just didn’t think they would be able detect that change yet, since ozone levels vary dramatically from year to year because of complex atmospheric processes, sometimes by almost as much as the magnitude of the ozone hole itself.
To better understand these year-to-year ozone fluctuations, Salby’s team looked at “dynamical” influences such as waves that ripple through the planet’s atmosphere much like the ocean’s swells. The researchers found that winter dynamical factors closely tracked how much ozone was depleted the following spring. In essence, these processes control how much chlorine breaks away from CFCs each winter, which determines how much ozone will later disappear.
Knowing what caused these year-to-year changes, the scientists could then subtract them out, unmasking the long-term signal of ozone. After plummeting since the analysis began in 1979, that signal leveled off and began creeping up after 1996, Salby says.
Using different analytical techniques, other scientists have reported seeing a slowdown in the rate of ozone decline, but not an actual recovery.
Not all experts are convinced by the new work. The link between year-to-year dynamics and ozone levels seems strong now but could change on further analysis, says Darryn Waugh, an atmospheric scientist at Johns Hopkins University. “I expect Antarctic ozone to be slowly recovering,” he says, “but would have thought that we need several more years of data to statistically show this.”
Atmospheric chemist Paul Young of the National Oceanic and Atmospheric Administration Earth System Research Laboratory in Boulder, Colo., agrees. “My response is slightly cautious,” he says. “It’s a bold claim.”
Salby says he’s confident the ozone uptick represents a real trend; it has gone on almost as long as the decline observed during the 1980s and 1990s. He now wants to develop a way to better predict future ozone levels.
Whether or not the ozone hole has been spotted recovering, its long-term prognosis calls for many more decades of sick leave; it is not expected to heal fully before 2070. The northern pole is also having its own ozone issues; this spring, ozone thinned over the Arctic more than scientists have ever seen (SN: 4/9/11, p. 12).