U.S. scientists have good news about the Earth's protective ozone layer. They report that its destruction in the upper atmosphere is slowing and the amount of industrial pollutants doing the destruction is going down. The findings show that a 16-year-old global ban on those pollutants is effective.
The industrial pollutants in question are the hard-to-pronounce chlorine compounds called chlorofluorocarbons, or CFC's for short. CFC's were once common as refrigerants and aerosol propellants.
Nearly 30 years ago, scientists learned they destroy the ozone layer that blocks harmful ultraviolet radiation from the sun. An increasing amount of this radiation reaching the ground has been linked to increased incidence of skin cancer and cataracts of the eye.
As a result, a 1987 international treaty called the Montreal Protocol and subsequent amendments phased out production of CFC's by 1996 and other ozone-depleting chemicals by 2000.
By the mid-1990s, researchers first noticed a drop in CFC's in the lower atmosphere where we live and breathe. Scientists now report that the compounds are declining much higher in the atmosphere - in the upper stratosphere, the topmost ozone layer far above where airplanes fly. At the same time, the rate of ozone destruction there is slowing.
"We have compelling evidence from global satellite observations that the Montreal Protocol is working," says University of Alabama atmospheric scientist Michael Newchurch. "The ozone layer is in its first stage of recovery in the upper stratosphere."
Mr. Newchurch stresses that the Earth is not yet gaining ozone. It is just losing it less quickly than before 1997. For 20 years, the rate of ozone decline was about eight percent a decade, for a total of 15 percent. According to the Alabama researcher, the latest rate of ozone loss is only four percent per decade and going down.
"However, full recovery is still decades away - on the order of four to six decades - about 50 years," he said.
The study, to appear in the Journal of Geophysical Research, finds the improvement only in the upper stratosphere. U.S. space agency atmospheric chemist Rich Stolarski notes it will take longer for the ozone situation to improve in the lower stratosphere.
"What we're waiting for is to be able to see it in the lower stratosphere where most of the ozone is and the thing that contributes most to total ozone," he said.
According to Mr. Stolarski, ozone is probably beginning to recover in the lower stratospheric layer. But he points out that observations cannot yet prove this because the amount of ozone varies too much from year to year to note a trend.
"There have been studies that say that to actually see the ozone increase in the lower stratosphere, you're going to need a decade or more of continued observations to really make sure that what you are seeing is what you think you are seeing," he explained.
The University of Alabama's Michael Newchurch says removing chlorine compounds from the upper stratosphere is not enough to solve the lower stratosphere problem. He notes that many factors affect that region, including the concentration of greenhouse gases that many scientists say is warming the Earth. While these gases warm us, they cool the stratosphere by radiating heat into space. This cooling changes wind and air mixing patterns in the lower stratosphere in a way that can increase ozone depletion there.
Yet Mr. Newchurch emphasizes that the Montreal Protocol banning CFC's was necessary, despite its limited atmospheric result.
"The scientific community knew that the Protocol would not solve the problem," he said. "But we also knew it was a very important first international geopolitical step to take."