Ozone decline levelling off

A new global study involving long-term data from satellites and ground stations indicates Earth's ozone layer, while still severely depleted following decades of thinning from industrial chemicals in the atmosphere, is no longer in decline.

Betsy Weatherhead, a researcher with the Cooperative Institute for Research in Environmental Sciences, a joint institute of the University of Colorado at Boulder and the National Oceanic and Atmospheric Administration and corresponding author of the study, said the team documented a levelling off of declining ozone levels between 1996 and 2002, and even measured small increases in some regions.

"The observed changes may be evidence of ozone improvement in the atmosphere," said 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, when scientists discovered human-produced chlorine and bromine compounds could destroy ozone and deplete the ozone layer, Weatherhead said.

The halt in the ozone decline follows the 1987 Montreal Protocol, an international agreement now ratified by more than 180 nations that established legally binding controls for nations on the production and consumption of halogen gases containing chlorine and bromine. Scientists say the primary source of ozone destruction is chlorofluorocarbons, or CFC's, which once were commonly used in refrigeration, air conditioning, foam-blowing equipment and industrial cleaning.

The new statistical study focused on levels of total-column ozone - ozone existing between Earth's surface and the top of the atmosphere. Total-column ozone is a primary blocker of UV radiation in the atmosphere.

The team analysed data from a cadre of NASA and NOAA satellites as well as ground stations in North America, Europe, Hawaii, Australia and New Zealand.

About 90% of total-column ozone is found between 10 miles to 20 miles above Earth's surface in the stratosphere, Weatherhead said. The ozone layer protects the planet from the harmful effects of UV radiation, including skin cancer and cataracts in humans and damaging effects on ecosystems.

Ozone depletion has been most severe at the poles, with levels declining by as much as 40% on a seasonal basis, said Weatherhead. But there also has been as much as a 10% seasonal decline at mid-latitudes, the location of much of North America, South America and Europe.