17 December 2000

Ozone Recovery - Not Yet

by Kate Melville

Scientists expect that recovery of the Arctic ozone layer may be slower than previously expected because of unusually low stratospheric temperatures.

Low temperatures have recently increased ozone losses over the Arctic despite the phase out of chlorine-containing chlorofluorocarbons and halons. Researchers noted large losses of stratospheric ozone observed in the Arctic last spring and the highest local ozone loss ever observed at any given altitude in that region was recorded with losses at greater than 70%.

Researchers will be holding a press briefing on their findings at the American Geophysical Union's (AGU) Annual Fall Meeting on December 15 at 4:00 p.m. (Pacific Time) in room 112 at the Moscone Center in San Francisco. Their research based on new results from recent U.S. and European field campaigns studying Arctic ozone loss suggests that chlorine and bromine may destroy more ozone than expected if greenhouse gases continue to increase and stratospheric temperatures become colder. Paul A. Newman of NASA's Goddard Space Flight Center in Greenbelt, Md. will introduce panelists Mark R. Schoeberl, James G. Anderson and Dale Hurst at the press briefing.

These panelists have worked on the joint SAGE III Ozone Loss and Validation Experiment (SOLVE) and Third European Stratospheric Experiment on Ozone (THESEO 2000) and obtained comprehensive measurements of halogen compounds (chlorine and bromine) that have given them a better understanding of how human-produced compounds destroy the ozone layer. These observations have shown how factors other than CFCs and halons contribute to winter ozone decreases.

Deployed from Kiruna, Sweden, balloon and aircraft measurements along with satellite observations of organic and reactive halogen species and other long-lived compounds in the stratosphere were obtained within the Arctic vortex between December 1999 and March 2000. These observations show that chlorine levels in the stratosphere have peaked and are predicted to decrease throughout this century. Observations of large ozone losses last winter in the Arctic have given scientists a better understanding of how human-produced compounds destroy the ozone layer.

Newman said, "Chlorine and bromine destroy stratospheric ozone in the Arctic when they are converted into harmful forms on the surfaces of stratospheric cloud particles." Most of this chlorine and bromine comes from human-produced compounds such as CFCs and halons. Newman noted, however, that the observations also show that the total equivalent chlorine (including bromine and chlorine) levels in the stratosphere have peaked or nearly peaked at all levels in the stratosphere. Projections of future declines in the total amount of chlorine and bromine released in the atmosphere over the next decade will not slow down as rapidly as the past decade. Most of the improvement in chlorine was made-up by the decline of methyl chloroform, which will essentially disappear in the next few years. If these future projections hold true, it suggests that Arctic ozone losses will persist into the 2050-2070 period.

According to the panelists, Arctic ozone should recover as we progress through the next century and amounts of chlorine and bromine continue their decline, but other factors including greenhouse-induced cooling of the stratosphere could delay future recovery of ozone levels.