The nations of the world might be deadlocked over what to do about greenhouse gases, but one important agreement reached 20 years ago seems to have produced tangible benefits for the atmosphere and Earth's inhabitants. Researchers tracking one of the chemicals that is most destructive to the ozone layer have found that its levels peaked in the early 1990s and have been declining steadily ever since. The finding reinforces conclusions that Earth's ozone layer is slowly returning to health.
Atmospheric concentrations of ozone block up to 99% of the cancer- and mutation-causing solar radiation. The gas forms when ultraviolet (UV) light strikes and splits oxygen molecules. The resulting free oxygen atoms quickly combine with other oxygen molecules to form ozone, or O3. And when more oxygen atoms are freed by UV light striking ozone molecules, they likewise quickly rebond into ozone. This cycle had been occurring for more than a billion years, until humans began manufacturing chemicals that rose into the atmosphere and started ripping apart ozone in a way that prevented it from easily reconstituting. In 1987, recognizing the dangers to ozone and to the biosphere from the buildup of those chemicals, 191 nations signed the Montreal Protocol that initially limited and then banned the manufacture of ozone-destroying molecules.
The changes in the ozone layer have been slow but steady ever since. Scientists charged with monitoring the protective layer's health reported earlier this year that the primary type of ozone-destroying chemicals--known as chlorofluorocarbons (CFCs)--had been declining since the mid-1990s (ScienceNOW, 5 March 2007). Now, two Arizona astronomers have analyzed data on hydrogen chloride (HCl) concentrations over the past 35 years. HCl, which comes from volcanoes and the breakdown of chemicals used to make plastics, rubber, and semiconductors, packs nearly as much ozone-destroying potential as CFCs, and its use was restricted by the Montreal Protocol. Utilizing instruments at the Kitt Peak National Observatory in Arizona, Lloyd Wallace of the National Optical Astronomy Observatories and William Livingston of the National Solar Observatory, both in Tucson, found that HCl levels had fallen by an average of about 1.8% per year since 1993. This compares with an average annual increase of 5.7% from 1971 to 1993, the authors report in the August issue of Geophysical Research Letters.
The findings show that "there is indeed good evidence that the chlorine loading in the atmosphere is going down as a result of the Montreal Protocol," says atmospheric chemist Christopher Cantrell of the National Center for Atmospheric Research in Boulder, Colorado. That doesn't mean the ozone layer is out of the woods, however, he says. The observations don't "tell us about the status of bromine levels," Cantrell says. Bromine is rarer in the atmosphere but is "much more effective at destroying ozone." Nevertheless, he says, "I think most everyone agrees that we should see [full ozone layer] recovery eventually," although probably not for at least 50 more years.