Summer ozone loss detected for first time.

A United Nations scientific panel announced last week that over the last two decades, the global ozone layer has thinned significantly during the spring and summer seasons, when people face the greatest danger from the surfs ultraviolet rays.

The panel also reported the unexpected finding that ozone thinning has occurred in the lower stratosphere. "This is a bit new We had anticipated that the losses would be in the upper stratosphere:" says panel co-chairman Daniel Albritton, an atmospheric scientist with the National Oceanic and Atmospheric Administration in Boulder, Colo. The new finding suggests that scientists need to reduce their estimates of future global warming from greenhouse gases.

The United Nations team made these discoveries while assessing the latest data on stratospheric ozone levels and ozone-destroying chemical pollutants. Government leaders will use the new findings when they meet next year to consider strengthening the Montreal Protocol, an international treaty adopted in 1988 to reduce certain chemicals that erode the ozone layer. Signatory nations made the treaty more stringent last year by agreeing to ban all chlorofluorocarbons CFCS) and halons by the end of this century.

The panel's review of ground-based measurements indicates that ozone levels have been decreasing everywhere except over the tropics. Over the northern midlatitudes - a region that includes the contiguous United States and most of Canada - spring and summertime ozone levels have fallen by 3.3 [ + or - ] 1.2 percent per decade since 1979.

This is the first time instruments have detected summertime ozone decreases over the midlatitudes. Ozone depletions are particularly dangerous during summer because ultraviolet radiation, which causes skin cancer, reaches its peak and people spend more time outdoors in that season.

Scientists think chlorine pollution is driving the long-term erosion of the global ozone layer, Albritton says. Previous work has confirmed that these chemicals cause the Antarctic ozone hole and ozone losses in the Arctic.

After the United Nations released its report, Du Pont, the world's largest producer of CFCs, announced it would phase out CFCs by the end of 1996 and halons by the end of 1994.

Sensors on balloons, satellites and ground stations all indicate that the global ozone decreases have occurred in the lower stratosphere, below about 25 kilometers in altitude, according to the report. Because of the way ozone molecules absorb and emit energy, ozone thinning high in the stratosphere would contribute to global warming. But theory suggests that losses in the lower stratosphere should cool the climate - in a sense canceling out part of the warming effect from greenhouse gases. Indeed, temperature records indicate that the lower stratosphere has cooled slightly over the last two decades, the panel noted in its report.

None of the sophisticated climate models used to predict global warming has taken into account the lower-stratosphere ozone loss, says James Hansen, a climate modeler at NASA's Goddard Institute for Space Studies in New York City 

It's fair to say that if it is indeed confirmed that the ozone changes cause a cooling, then most of the simulations somewhat overestimate the expected greenhouse warming," he told SCIENCE NEWS.

Including the new information on ozone loss may lower predictions of global temperature increases by about 20 percent, Hansen speculates. It's not a tremendous change, but it's significant," he says.