Increased ultraviolet in Argentina.

The thinning of Earth's protective ozone layer in the stratosphere should allow more ultraviolet (UV) light to reach the planet's surface. But clouds and pollution can complicate that relationship by absorbing UV radiation, making it hard to detect the effect of ozone loss from the ground. U.S. and Argentine researchers now report finding the first evidence of large increases in UV light over a populated part of the world.

Since 1988, atmospheric scientists have monitored UV levels at a station in Ushuaia, Argentina, on the southern tip of South America. While UV levels remained near normal for some years, biologically harmful UV frequencies grew significantly stronger during the austral summers of 1990 and 1992. In one month, December 1990, UV levels averaged 45 percent above the expected levels, says John E. Frederick of the University of Chicago. He and his colleagues describe their results in the May 20 JOURNAL OF GEOPHYSICAL RESEARCH.

They suggest that the UV surges stem from the annual breakup of the ozone hole over Antarctica. As the ozone hole forms during September, it is surrounded by a swirling band of winds that herds the ozone-poor air, keeping it concentrated mostly over the Antarctic. But the vortex of winds eventually loses strength, allowing blobs of ozone-depleted air to break away. Frederick believes that some of these passed over South America during December of 1990 and 1992.

The surges came at a time of year when UV radiation is naturally strongest. But researchers lack the measurements to tell whether the changes actually harmed plants, animals, or people in southern South America. Nor can they say whether the ozone-poor blobs remained intact long enough to drift over more northern regions and cause increases in UV radiation there. Argentina is now installing a network of stations to track such changes across the entire country.