For the first time, scientists have uncovered what they see as unambiguous evidence that Earth's sunscreen, a tenuous shield of ozone in the stratosphere, is slowly beginning to recover from nearly 30 years of human-triggered loss.
In addition, new research is showing for the first time that a worrisome latecomer to the list of compounds threatening the ozone layer is vanishing worldwide from the lower atmosphere.
The findings are good news on the environment that, for some, underscore the effectiveness of global treaties at prodding countries to curb pollution. In this case the treaty in question is the 1987 Montreal Protocol and its amendments, which are credited with triggering these changes.
From a global standpoint, "this is the most significant environmental success story of the 20th century," says Michael Newchurch, an atmospheric chemist at the University of Alabama at Huntsville.
The Montreal Protocol first limited, and then banned, a group of chemicals known as chloroflurocarbons (CFCs). It was later expanded to include a wider range of ozone-threatening chemicals, including halons and, most recently, methyl bromide. CFCs were widely used in a range of products and technologies, while halons found broad use as fire suppressants.
At first blush, Dr. Newchurch's assessment may seem like excessive praise for something that affects such a tiny component of the atmosphere. On average, out of every 1 million molecules of air, only a few will be ozone. Some 10 percent of the atmosphere's ozone resides in the lower atmosphere, where a range of human activities and natural processes can generate ozone smog. High in the stratosphere, however, the remaining 90 percent of Earth's ozone absorbs much of the ultraviolet (UV) light coming from the Sun. Ozone is particularly good at absorbing wavelengths of UV light that can sever the chemical bonds of DNA, the biological molecule that carries the genetic blueprint for living organisms.
Until now, evidence that the Montreal Protocol is having its desired effect has come from measurements tracking the decline of ozone-destroying chemicals in the lower atmosphere and in the stratosphere.
In the current case for the protocol's impact, Exhibit A comes from satellite measurements of ozone itself in a region stretching from 22 to 28 miles above Earth's surface. Since 1979, instruments on a series of NASA satellites have measured the concentrations of a range of atmospheric gases by looking for their spectral fingerprints as the satellites slip behind Earth and catch sunlight passing through the thin veil of gases enveloping the planet.
An initial study of ground-based data published last year by Gregory Reinsel, a statistician at the University of Wisconsin at Madison, suggested that ozone destruction was declining.
When Dr. Newchurch's team, which included Dr. Reinsel, added the satellite data, "the evidence was so compelling," Newchurch says. The study has been accepted for publication in the Journal of Geophysical Research.
Exhibit B comes from data that, for the first time, show a global drop in bromine in the lower atmosphere.
While bromine appears in smaller concentrations than CFCs and has a shorter residence time in the atmosphere, researchers say, molecule for molecule it packs more ozone-destroying punch than CFCs. The first hints of the trend in bromine concentrations also came last year, when Japanese researchers crunched the numbers on data from one ground site and from aircraft measurements.
This year, Stephen Montzka, an atmospheric chemist at the National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Lab in Boulder, Colo., looked at bromine data from its monitoring stations around the world and found that bromine concentrations have fallen nearly 5 percent since 1998.
The decline "is substantial," he notes, "considering that bromine is 45 times more efficient" at depleting ozone than the chlorine in CFCs. His team's work is slated to appear in an upcoming edition of Geophysical Research Letters.
While this one-two punch of good news is encouraging, the ozone layer is hardly home free, researchers say.
In the upper stratosphere, where Newchurch's team saw its trends, chemical reactions between chlorine and ozone play the biggest role in determining how much ozone there is.
In the lower stratosphere, where 80 to 90 percent of the stratospheric ozone resides, factors such as changing temperatures can affect ozone depletion and restoration, says William Randel, an atmospheric scientist at the National Center for Atmo spheric Research in Boulder and a lead author of the United Nations Environment Program's "Scientific Assessment of Ozone Depletion: 2002."
Those temperatures, he says, can be affected by greenhouse gases - such as methane and water vapor - that creep into the stratosphere and cool it. Strato spheric cooling can slow the reactions that destroy ozone at mid latitudes, which helps recovery. But the cooling also has the perverse effect of accelerating the destruction of ozone at the poles.
Until such conflicting forces can be sorted out and correctly modeled, coming up with useful forecasts of when the rate of ozone decline gives way to ozone restoration will be difficult. Researchers say they expect any recovery to take decades.
"We need to stay diligent" and keep to the protocol's provisions to have any hope of restoring the ozone layer, says Elizabeth Weatherhead, an atmospheric scientist at the University of Colorado at Boulder.
Still, she adds, Newchurch's work represents "a valid first indicator of recovery."