April 23, 2009 -- The Earth's ailing ozone layer will probably recover, but it will never look exactly like it used to.
That is the conclusion of a new study, which found that greenhouse gasses are interfering with ozone's rebound in complicated ways. The study predicts a patchy future for the ozone layer, with some sections becoming even thicker than they were before bans on ozone-damaging chemicals kicked in. Other sections, meanwhile, may remain sparse.
"This shows that greenhouse gas increases could have some surprising effects on ozone," said Feng Li, an atmosheric scientist at NASA's Goddard Earth Sciences and Technology Center in Greenbelt, Md.
The ozone layer lies in the stratosphere, the region of the atmosphere that stretches from about 10 miles to 30 miles above the planet's surface. Up there, ozone gas plays an important, even life-saving role. By absorbing most of the sun's ultraviolet rays, stratospheric ozone protects people from skin cancer and guards plants, animals, and ecosystems forms from the blistering effects of UV radiation.
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Lower down, ozone has a worse reputation -- as a major component of smog.
Stratospheric ozone was hard hit by emissions of chlorofluorocarbons (CFCs) and related chemicals, which used to be found in aerosol cans, refrigerants, aircraft, and other places. Then, in the late 1980s, as part of the international Montreal Protocol, many governments banned these chemicals.
Those bans, said Darryn Waugh, an atmospheric scientist at Johns Hopkins University in Baltimore, seem to have made a big difference: Chlorine, one component of CFCs, has peaked in the upper atmosphere, and scientists expect it to drop back to 1970s levels by 2060.
But will the decline in chlorine and other ozone-damaging substances lead directly to the revival of ozone? That's something researchers have been trying to figure out.
Scientists know that the accumulation of greenhouse gasses, including carbon dioxide, actually makes the stratosphere colder, even as it traps heats lower down. That's a good thing for the ozone layer, because the reactions that break down ozone happen more slowly in colder conditions.
But the increase in greenhouse gasses also speeds up the circulation of ozone through the atmosphere. Normally, ozone forms in the tropics, where it rises to the upper stratosphere and then moves toward the poles. Li wanted to know what a quickening of this process would do to the ozone layer.
With a computer model that looked at how climate change might affect ozone recovery over the next century, his team found that faster circulation through the stratosphere is depositing lots of ozone in mid- and high-latitude regions -- so much so that these areas are "super-recovering" to levels higher than in the pre-CFC era.
Faster circulation, however, is also bringing more ozone-poor air from the lower atmosphere into the stratosphere above the tropics. "This speeding up prevents the full recovery of tropical ozone," Li said. The study appeared in the journal Atmospheric Chemistry and Physics.
The findings may have policy implications, Waugh told Discovery News.
"If all that mattered was the amount of chlorine in the atmosphere, and chlorine is expected to go back to what it was in 1970, then you would expect ozone to go back to what it was in 1970," he said. "Now they're saying there are other things that are going to be impacting ozone."
In that way, he added, the ozone layer's healing process resembles a person's recovery from serious disease.
"If it takes you 30 years to recover from something, you're not going to recover to how you were when you were 20," Waugh said.
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