UV rays strengthening in North America.

Grab your hat and maybe some sunblock too. Researchers report they have detected an increase in levels of harmful ultraviolet (UV) radiation in at least one part of North America during the last four years.

As chlorofiuorocarbons and other chemical pollutants eat away the stratospheric ozone layer, scientists have been expecting to see a strengthening in the intensity of UV rays reaching Earth's surface. But evidence of any UV changes has proved elusive, both because measuring radiation is extremely difficult and because clouds and pollution can absorb such rays, masking any trends. James B. Kerr and C. Thomas McElroy of Environment Canada in Downsview, Ontario, now report the first signs of a UV increase in North America.

In the Nov. 12 SCIENCE, the two researchers announced that total levels of UV radiation in the Toronto area climbed 35 percent per year during winter and 7 percent per year during summer between 1989 and 1993. Kerr and McElroy believe that ozone erosion caused part of these upswings because the UV levels rose most dramatically at shorter wavelengths, a part of the spectrum strongly absorbed by ozone molecules. Longer wavelengths of UV light, which pass through the atmosphere unaffected by ozone, did not intensify much during the four years. Ozone concentrations above Toronto dipped 4.1 percent annually in winter and 1.8 percent in summer during the four-year study.

To determine the health significance of their findings, Kerr and McElroy calculated the changes in the specific bands of UV light that cause sunburns. Levels of this radiation climbed 5.3 percent per year in winter and 1.9 percent per year in summer.

The new findings play into a debate launched by some conservative commentators who downplay the importance of ozone depletion. Pointing to the paucity of evidence of UV trends, they suggest that the planet does not face any threat from ozone loss. The measurements from Toronto show that UV levels have, in fact, climbed there and most likely have increased over other regions where ozone concentrations are dropping, Kerr says. "This question is no longer disputable. It's more or less what we expected to observe."

This doesn't mean the people of Toronto should lock themselves indoors. The greatest increase in ultraviolet light has occurred in winter, when UV radiation levels are naturally lowest. The city also sits at a latitude of 44 degrees N, where it receives much less ultraviolet light than do areas closer to the tropics. "People can cover up, they can take precautions," says Kerr. "But for vegetation it's a more serious problem. [Plants] are outside all day long and don't have a choice. There's a lot of uncertainty involved, but it's possible that they won't be able to cope with increases in UV intensity."