Washington, 01 Sep 2006
A new study using NASA and U.S. National Oceanic and Atmospheric Administration (NOAA) data finds consistent evidence that Earth's ozone layer is on the mend.
A team led by Eun-Su Yang of the Georgia Institute of Technology analyzed 25 years of independent ozone observations at different altitudes in Earth's stratosphere, which lies between 9.6 and 50 kilometers above the surface, according to an August 30 NASA press release.
The observations were gathered from balloons, ground-based instruments and NASA and NOAA satellites.
"At the current recovery rate,†Yang said, “the atmospheric modeling community's best estimates predict the global ozone layer could be restored to 1980 levels -- the time that scientists first noticed the harmful effects human activities were having on atmospheric ozone -- some time in the middle of this century." (See related article .)
The stratosphere is Earth's second-lowest atmospheric layer. It contains about 90 percent of all atmospheric ozone.
The researchers concluded the Earth's protective ozone layer above the Polar Regions stopped thinning around 1997. Ozone in these areas declined steadily from 1979 to 1997.
The abundance of human-produced ozone-destroying gases such as chlorofluorocarbons peaked at about the same time -- 1993 in the lowest layer of the atmosphere, 1997 in the stratosphere.
Use of such substances was phased out after the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer came into force. The international agreement, to which the United States is a party, called for phasing out production and consumption of compounds that deplete ozone in the stratosphere -- chlorofluorocarbons, halons, carbon tetrachloride and methyl chloroform.
"Our study is unique because it measures changes in the ozone layer at all heights in the atmosphere,†said Ross Salawitch, a senior research scientist at NASA's Jet Propulsion Laboratory in California, “then compares the data with models as well as observations from other instruments that measure variations in the total amount of ozone in the atmosphere."
To measure ozone at different altitudes in the stratosphere, the team combined data from balloons and independent ground-based observing networks with monthly averaged satellite data. The satellite data came from five independent NASA and NOAA instruments.
Measurements were compared with computer predictions of ozone recovery that considered actual measured variations in human-produced ozone-destroying chemicals. The calculations accounted for other factors that can affect ozone levels, such as sunspot cycle behavior, seasonal changes and stratospheric wind patterns.
The researchers concluded that about half the observed ozone change was in the region of the stratosphere above 18 kilometers and the rest in the lower stratosphere from 9.6 to 18 kilometers. The researchers attribute the ozone improvement above 18 kilometers almost entirely to the Montreal Protocol.
"Scientists expected the Montreal Protocol to be working in the middle and upper stratosphere and it is," said co-author Mike Newchurch of the University of Alabama. "The real surprise of our research was the degree of ozone recovery we found at lower altitudes, below the middle stratosphere.â€
There, he added, ozone levels are improving faster than expected, seemingly due to changes in atmospheric wind patterns, the causes of which are not yet well understood.
“Until the cause of the recent ozone increase in the lowermost stratosphere is better understood,†Newchurch said, “making high-accuracy predictions of how the entire ozone layer will behave in the future will remain an elusive goal.â€