Earth's sunscreen appears poised for recovery after decades of assault from man-made chemicals.
After years of decline, global concentrations of ozone in a key region of the stratosphere have held steady for the past eight to nine years, according to two new, independent studies.
Scientists noted initial signs of this trend three years ago. But these latest efforts benefit from an additional three years of measurements. And they appear to be the first to specifically attribute the changes to the Montreal Protocol, a 1987 international treaty that phased out key chemicals known to destroy ozone.
Although a range of man-made gases can deplete ozone, the main targets so far have been chlorine-carrying compounds used as coolants in refrigerators and for fighting fires.
"It's clear that the Montreal Protocol has reduced the total amount of chlorine entering the atmosphere," notes Derek Cunnold, an atmospheric scientist at Georgia Institute of Technology in Atlanta and a member of one of the two research teams. Those reductions, he continues, should first arrest the decline, then allow the ozone layer to rebuild.
"We are now seeing that kind of a response," he says, referring to the stabilization of ozone levels.
Ozone concentrations are still quite low and are allowing historically high levels of damaging ultraviolet light to reach Earth's surface, researchers caution. Moreover, once the upswing begins, a recovery is likely to come only in fits and starts over 50 to 60 years. And scientists say it's highly uncertain whether the effort will overshoot or undershoot the protocol's target of returning stratospheric ozone concentrations to pre-1980 levels.
Global warming is expected to present ozone with a far different environment than the one that existed before 1980. Several factors - changing circulation patterns, temperatures, and the rise or fall of methane, nitrous oxides, and water vapor reaching the stratosphere - will affect the outcome.
Still, "we're on the cusp" of recovery, says Elizabeth Weatherhead, a researcher at the Cooperative Institute for Research in Environmental Sciences at the University of Colorado at Boulder and the lead author of the second study. She and colleague Signe Andersen published their review of the data in the May 4 edition of the journal Nature. Dr. Cunnold and his colleagues are publishing their results in an upcoming edition of the Journal of Geophysical Research.
Ozone is an atmospheric Janus. Made up of three oxygen atoms, ozone at ground level can trigger bad air days in urban areas across the planet. But in the stratosphere, ozone intercepts ultraviolet light from the sun, reducing the amount that reaches Earth's surface.
Both studies rely on ground-based and satellite measurements of ozone for trends, and to varying degrees on computer models for attributing trends to what the data reveal. The strongest evidence for the protocol's effect comes at altitudes ranging from 11 to 16 miles. There, chlorine has leveled off, and so has ozone decline, Cunnold's team reports.
However, both teams note unexpectedly large improvements in ozone concentrations in the part of the stratosphere below 11 miles. These improvements, though still relatively small, appear over mid-latitudes in the northern hemisphere, where the fastest recovery rates are expected. This surprise has less to do with the protocol, however, than with changes in atmospheric circulation patterns, both teams hold.
At the poles, by contrast, recovery is expected to take the longest, particularly over Antarctica, with its infamous ozone hole. There, total-column ozone - the amount over any one location - also appears to have halted its decline.
But Dr. Weatherhead offers that the decline has halted largely because the region has little stratospheric ozone left to lose. Recovery at both poles is expected to take far longer than at lower latitudes.
Even as they continue to monitor the protocol's impact on ozone levels, researchers are now struggling to factor the impact of climate change and natural variations of stratospheric ozone into estimates of how long recovery will take and how close to the mark it will reach.
For David Doniger, the climate-policy director for the Natural Resources Defense Council in Washington, these results indicate that when it comes to the environment, "global treaties work." But he holds that the battle to curb ozone-depleting chemicals isn't over.
Production of another potent ozone-eater - methyl bromide - initially was to have ended in 2000. Farmers use the compound as a fumigant and the Environmental Protection Agency continues to grant waivers for its production and use in the US.