ACTING like a sort of celestial washing machine, the stratosphere will cleanse itself of ozone-depleting compounds over the course of several decades - if activity on Earth doesn't interrupt the cycle.
The ozone layer, which encircles the globe six to 30 miles above Earth's surface, has been damaged by humanity's use of certain industrial chemicals, many scientists believe. Now that international treaties are in place to curb production of the major ozone-destroying chemicals, scientists say two other factors could impede the natural healing process: volcanic eruptions and global warming.
Why ozone in the sky matters
As scientists try to pin down how long it will take to restore the ozone layer, they note that both factors emerged as wild cards. The ozone layer protects Earth from much of the sun's ultraviolet radiation. Without this protective layer, scientists warn, severe damage to living organisms would occur.
Sherwood Rowland of the University of California at Irvine says the international agreements to phase out production and use of the worst of the chemical culprits has ''put a cap on ozone [layer] damage.'' These chemicals primarily are the chlorofluorcarbon (CFC) compounds used as refrigerants and halon compounds used mainly in fire extinguishers. CFCs break down in the stratosphere and release chlorine. Halons release bromine. Dr. Rowland shared a Nobel prize last year for research showing how such releases destroy ozone.
Speaking at the American Association for the Advancement of Science's annual meeting here, Rowland said the concern for the next two decades is that a large volcanic eruption will activate more ozone destruction than otherwise would occur.
Patrick McCormick of the NASA Langley Research Center in Hampton, Va., explains that sulfuric acid aerosol particles from volcanoes act as catalysts to speed the breakdown of CFCs and halons, releasing a flood of ozone-destroying chlorine and bromine. This was seen after the eruption of Mt. Pinatubo in the Philippines in 1991. Consequently, it is not enough just to monitor the actual decline in the influence of CFCs and halons; keeping track of volcanic aerosols is also required.
Global warming a player, too
A change in global climate as a result of the ''greenhouse effect'' could also affect the pace of ozone-layer healing, the scientists say. The atmosphere sandwiched between Earth and the ozone layer has been incrementally warming, the result of burning fossil fuels to power an industrial economy. This warming of the lower atmosphere, however, would actually cool the stratosphere where ozone resides. This, too, could enhance ozone destruction.
Richard Stolarski of the NASA Goddard Space Flight Center in Greenbelt, Md., said this new understanding of the complexity of the ozone story does not change scientists' expectation that the ozone decline will stop. But it does mean that it will take time to detect a substantial change in ozone destruction.