Reductions in CFCs and chlorine observed at ground level during the 1990s led Newchurch and his team to reassess the effect of these chemicals on the upper stratosphere, the region from 22 to 28 miles (35 to 45 kilometers) above Earth's surface.
Though most ozone is found in the lower stratosphere, the region from 6 to 22 miles (10 to 35 kilometers) above the Earth, ozone depletion in the upper stratosphere is more closely linked to chlorine chemistry alone, said Newchurch. As a result, scientists anticipated that reduced CFC levels and increased ozone levels tied he effect of the global CFC ban would be first observed in the upper stratosphere, Newchurch said.
The team analyzed data from three NASA satellites which have been taking measurements since the late 1970s and from ground-based atmospheric data collection stations in Japan, Switzerland, and the United States.
The results showed that between 1979 and 1997 ozone diminished at a rate of eight percent each decade in the upper stratosphere. However, between 1997 and 2002, the rate of depletion slowed to a projected average of just four percent per decade.
"We are starting to see a turnaround in the upper stratosphere," said Newchurch. "We are within a few years of the point when ozone levels will begin to climb again," he said.
Stephen Montzka, an atmospheric chemist with the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado, said the results are significant. "We are seeing the beginnings of a recovery at the edge of the atmosphere," he said.
However, Montzka warned that changes in the upper stratosphere should not yet be interpreted as evidence of recovery in the entire ozone layer.
There is not yet compelling data to indicate that ozone depletion is slowing in the lower stratosphere, "but there's no reason to believe the situation isn't improving," said Newchurch. His team is now looking for hints of ozone layer recovery there also. Ozone depletion is more complicated in the lower stratosphere due to a greater influence of weather effects.
Montzka said the finding in the upper stratosphere is nevertheless important "because we can now have more confidence that the changes to the ozone layer will follow our predictions."
Some predictions suggest that the ozone layer will have recovered to pre-industrial levels by the late 21st century. Though a total recovery could happen within 50 years, Montzka said.
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