The level of ozone-depleting chemicals in the atmosphere is falling, mostly because of reductions in one solvent, meaning further cuts can occur only if emissions of other chemicals are lowered, researchers say.
"We have made progress, but we're not out of the woods yet," said Stephen A. Montzka of the National Oceanic and Atmospheric Administration.
The researchers monitored levels of the chemicals at remote locations worldwide between 1992 and 1997. Levels were found to have declined about 3 percent. The findings were published in Thursday's issue of the journal Nature.
Production of a solvent called methyl chloroform was phased out between 1993 and 1996. Levels of the chemical have dropped almost in half since then because of production cuts and the substance's five-year lifespan in the atmosphere, the researchers said.
That decrease accounted for so much of the drop in ozone-depleting chemicals that if methyl chloroform were ignored, the overall level would have been increasing in 1997, the researchers said.
Other chemicals such as CFC-12, a refrigerant, and CFC-11, used to make foam, are still present at more than twice the peak levels of methyl chloroform. Although CFCs are regulated, CFC-12 levels actually rose, while CFC-11 declined slightly.
In addition, atmospheric levels of one firefighting chemical, H-1211, continued to increase despite the availability of alternatives. H-1211 is particularly troubling because it contains bromine, a potent ozone destroyer.
The ozone layer protects the Earth from excessive ultraviolet light, which has been linked to skin cancer. Ozone depletion has been the most severe over the Antarctic, where an "ozone hole" appears periodically.
In 1989, 165 nations signed the Montreal Protocol, an agreement to reduce levels of ozone-damaging chemicals.
The quick gains in reducing methyl chloroform, are not expected to be repeated with the CFCs. The two chemicals are still allowed to be made in developing countries on a limited basis until 2010. And once released, they spend a long time in the atmosphere -- 100 years for CFC-12 and 45 years for CFC-11. In addition, CFC-11 and CFC-12 continue to be released from junked air conditioners, foam and other products.
Ozone depletion can be halted in the next decade, Paul J. Fraser, of CSIRO Atmospheric Research in Victoria, Australia, and Michael J. Prather of the University of California at Irvine wrote in an accompanying commentary.
"But it will require a new level of global stewardship that still poses a substantial challenge to all parties to the Montreal Protocol," they said.
By Alex Dominguez