Two new studies credit a 1987 international agreement, which phased out harmful gases, for the improvement.
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.