Even if all the world's industrialized nations succeed in phasing out 95 percent of chlorofluorocarbons (CFCs) in the next 10 years, the Earth's ozone layer could still be imperiled by an event beyond humanity's control -- a major volcanic eruption.

A team at the National Center for Atmospheric Research has produced computer models to calculate how certain volcanic emissions -- chiefly airborne droplets of sulfuric acid -- might react chemically with man-made chlorine in the atmosphere to produce the kind of compounds that destroy ozone.

The group reports that aerosols from a single eruption on the scale of the 1982 explosion of El Chichon in Mexico could cause enough chemical reactions to deplete as much as 8 percent of the ozone layer -- which prevents much harmful ultraviolet light from striking the planet's surface -- for a year or more.

"CFCs have a very long lifetime, around 50 to 100 years," says Guy Brasseur, director of NCAR's atmospheric chemistry division and one of the report's authors. "In 1982, when El Chichon erupted, the amount of chlorine in the atmosphere was substantially lower than it is now or will be in, say, 2010. In 1980, it was probably 2 parts per billion. It's now 3.2 and could go up to 4.2 by 2010."