After 30 years, ozone is recovering

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.

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