Wind flows tear at ozone layer

Climate change is giving scientists a new perspective on Northern Hemisphere ozone depletion. Ozone chemist Neil Harris says "it's now clear" that shifts in large-scale wind flows dilute the stratosphere's ozone and open mini ozone holes at northern mid-latitudes even when ozone-eating chemicals are not directly involved. He adds that this implies that we won't return to the pre-ozone-loss atmosphere even when the chemical culprits are gone.

International efforts to curb the use of such ozone-eaters as chlorine-containing refrigerants and bromine-containing fire suppressants are beginning to pay off. Their concentration in the atmosphere should gradually decline. But, as Dr. Harris explains, this won't eliminate concern about increased exposure to solar ultraviolet (UV) radiation for heavily populated parts of the Northern Hemisphere.

Harris's group at the University of Cambridge in England coordinates Europe's ozone research. It is one of several research centers studying this new twist on ozone depletion around the hemisphere.

Sunlight drives chemical reactions that form ozone high in the stratosphere. At the same time, other reactions destroy ozone. This competition reaches a balance where there generally is enough ozone present to absorb solar UV and protect the earth's surface from this dangerous radiation. Pollution by chlorine and bromine tilts this balance against ozone.

Their action opens the seasonal ozone holes over the North and South Poles. Now scientists are finding that changes in atmospheric circulation are thinning and tearing the ozone shield at Northern Hemisphere latitudes far south of the Arctic hole.

Last November, scientists working with data from the European Space Agency's ERS-2 satellite had a view of what can happen. For the first time, they could watch a mini-hole develop as it moved from Greenland to Scandinavia. What they found confirmed that this and other low ozone events over Europe "seem to be caused mainly by unusual air currents ... not by chemical breakdown of ozone," according to ESA's announcement.

Some of this may result from mixing ozone depleted Arctic air down into lower latitudes. That would be an indirect effect of chemical breakdown in the Arctic. But that's not the whole story.

J.R. Spackman and colleagues from Harvard University have analyzed several decades' worth of Northern Hemisphere ozone data. They find a 7 to 9 percent ozone decline per decade over the past 20 to 30 years for the mid-latitude lower stratosphere. They told a May meeting of the American Geophysical Union in Washington that this probably is caused by an influx of naturally ozone-poor air from the lower tropical stratosphere. No chemical ozone eaters involved.

Scientists following this research have not yet reached firm conclusions as to what is going on or what will happen in the future. They've documented long-term changes in Northern Hemisphere air circulation. They have correlated these with ozone depletion. They know that the wintertime occurrence of ozone mini-holes has increased over mid-latitudes during the past few decades. They don't know whether this is a natural climate fluctuation or the result of manmade global warming.

The point, they say, is to realize that curbing ozone-eating chemicals won't relieve Northern Hemisphere inhabitants of the need to learn to live safely with increased UV exposure.

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