31 August 2005

Ozone Layer Decline Halts

by Kate Melville

A study from the Cooperative Institute for Research in Environmental Sciences suggests that the ozone layer, while still severely depleted following decades of thinning, is no longer in decline. It has been nearly twenty years since the Montreal Protocol imposed controls on the use of chlorofluorocarbons, or CFC's, believed to be the main culprit behind the ozone layer depletion. The depletion has been most severe at the poles, with levels declining by as much as 40 percent on a seasonal basis. But there has also been as much as a 10 percent seasonal decline at mid-latitudes, where most of the Earth's population resides.

The researchers behind the study, appearing in the Journal of Geophysical Research, are cautiously optimistic. "The observed changes may be evidence of ozone improvement in the atmosphere," said Betsy Weatherhead. "But we will have to continue to monitor ozone levels for years to come before we can be confident. It most likely will be decades before the ozone layer recovers, and it may never stabilize at the levels measured prior to the mid-1970s."

The new study examined levels of total-column ozone, the ozone that exists between Earth's surface and the top of the atmosphere. Total-column ozone is a primary blocker of ultraviolet radiation in the atmosphere. The researchers analyzed data from satellites as well as ground monitoring stations in North America, Europe, Hawaii, Australia and New Zealand. Encouragingly, the study found that in some parts of the world, the ozone layer has increased a small amount in the past few years, although it is still well below normal levels.

Conrad C. Lautenbacher, of the National Oceanic and Atmospheric Administration who provided the study funding, said the new findings were an acknowledgment of the success of the Montreal Protocol. "These early signs indicate one of the strongest success stories of international cooperation in the face of an environmental threat."

But while the depletion may have halted, ultraviolet levels will remain high for some time and medical experts say that warnings about excessive exposure to ultraviolet radiation are still valid. "This news about the ozone layer is encouraging, but people should not get a false sense of security. Ultraviolet radiation is still dangerous and we urge people to be 'sun smart' when outdoors," said Clay J. Cockerell, president of the American Academy of Dermatology.

On a sad note, the lead author of the study, Greg Reinsel of the University of Wisconsin-Madison, passed away after the completion of the study. Reinsel was instrumental in identifying and quantifying the decline of the ozone layer some twenty years ago.

Source: National Oceanic and Atmospheric Administration, University of Chicago, European Space Agency