Cloudy effects of ozone loss.

Thinning of the ozone layer may have hidden the temperature rise expected from greenhouse warming, according to scientists who studied the subtle climate role played by stratospheric ozone.

In addition to shielding Earth from damaging ultraviolet light, ozone is a greenhouse gas that helps keep the surface warm by trapping radiation before it leaks into space.

So a drop in ozone concentrations works to cool the globe -- an effect well known among scientists. Less obvious, however, is a link between ozone and clouds. Ralf Toumi of the Imperial College in London and his colleagues report that a thinning of the ozone layer should further cool climate by stimulating growth of sunlight-blocking cloud particles in the atmosphere.

In the Nov. 24 NATURE, Toumi explains that ozone loss in the stratosphere allows increased amounts of ultraviolet radiation to penetrate the troposphere. The radiation creates extra amounts of the hydroxyl radical by ripping apart tropospheric ozone molecules. The hydroxyl radicals, in turn, oxidize sulfur dioxide to form sulfuric acid, which serves as a nucleation site for cloud droplets. The bottom line: more ultraviolet light, more cloud droplets.

How much more, the researchers can't say. Their calculations suggest that the cloud connection could have minor cooling power, or it could dwarf the better-known direct effects of ozone loss on climate. If the ozone thinning does increase cloud cover dramatically, then it may have cooled climate enough to mask the warming from increasing levels of greenhouse gases, they suggest.