FOR IMMEDIATE RELEASE
Contact: Barbara McGehan
WILL THEY CONTINUE TO DECLINE?
Reduction in the atmosphere of a single chemical, methyl chloroform, is primarily responsible for the overall decline of ozone-depleting substances observed in the atmosphere during the past five years. Because methyl chloroform will soon become a much smaller part of the remaining ozone-depleting substances due to its rapid natural disintegration, further reductions in the threat to the ozone layer would have to come from reductions in other ozone-depleting gases, say researchers at the Commerce Department's National Oceanic and Atmospheric Administration.
In a paper appearing in the current issue of Nature, scientists at NOAA's Climate Monitoring and Diagnostics Laboratory in Boulder, Colo., say that ozone-depleting chemicals have declined about 3 percent overall since 1994, mainly due to the loss of methyl chloroform from the atmosphere. Methyl chloroform, a chemical previously used as a cleaning solvent, is destroyed much more rapidly in the atmosphere by natural processes than other ozone-depleting substances. Lead author Stephen A. Montzka says that "based on our understanding of atmospheric removal processes, the level of methyl chloroform will diminish to such an extent over the next 5-10 years that it will no longer influence the overall amount of ozone-depleting substances in the atmosphere."
Restrictions on the production of ozone-depleting
chemicals are outlined in the Montreal Protocol, an international
agreement that limits the production of ozone-damaging chemicals.
These restrictions are designed to ensure that the threat posed
by ozone-depleting substances diminishes over the next 50 years
and will allow a return to "pre-ozone hole" levels
sometime in the middle of the next century.
To discover the root causes of these trends, the authors computed emissions of ozone-depleting substances based on observations from NOAA's network of air-sampling sites around the globe. The results suggest that during the past 10 years, emissions of most chemicals have diminished substantially as a result of the Montreal Protocol. Emissions of the two most common CFCs, CFC-11 and CFC-12, have diminished to between one-half and one-third of the amount released during the late 1980s, which were the peak years. Despite these reductions, concentration increases continue for CFC-12 because it is extremely persistent in the atmosphere.
"Emissions of halon-1211 have not decreased despite the ban on production in developed countries since 1994," Montzka says. Some of the continued release of this chemical results because a substantial reservoir or "bank" exists in fire extinguishers that are still in use today in many developed countries. Production figures from the United Nations Environment Programme suggest that accelerated production of this halon in developing countries also accounts for a large portion of the continued increase.
Hydrochlorofluorocarbons (HCFCs) are also
increasing because they are used as interim replacements for
ozone-depleting substances. But, the authors say, these chemicals
are less efficient at destroying stratospheric ozone than CFC-12
and the halons, and at present, only account for about 2 percent
of the ozone-depleting substances in the atmosphere.
According to David Hofmann, director of NOAA's Climate Monitoring and Diagnostics Laboratory, "continued monitoring of atmospheric levels of ozone-depleting gases will be necessary to assess our progress towards recovery of the ozone layer as mandated by the Protocol."