WASHINGTON -- Even if the world drastically curtails the use of ozone-destroying chemicals, a large volcano could touch off a catastrophic worldwide ozone loss sometime in the next 20 or 30 years, scientists warned last month.
A volcano similar in size to the 1982 El Chichon eruption in Mexico would accelerate chemical reactions between ozone and millions of tons of chlorofluorocarbon chemicals that will continue to float in the atmosphere for years after use of the substances has ended, said Guy F. Brasseur, a physicist at the National Center for Atmospheric Research in Boulder, Colo.
In effect, conditions similar to those that cause annual ozone "holes" to appear over Antarctica would develop over the entire planet, he said.
Within weeks, up to 12 percent of the protective gas would vanish from the stratosphere over populated areas of North America, Europe and Asia, Dr. Brasseur said.
The calculations assume that 95 percent of the use of chlorofluorocarbons, also known as CFCs, would be phased out by 2000, which is called for under the Montreal Accord, a treaty to end the use of the ozone-destroying chemicals.
Even with the phase-out, however, conditions have been primed for the complicated interactions among volcanic gas, ozone and chlorofluorocarbons that would remain in the atmosphere for years after emissions are stopped, Dr. Brasseur said. Therefore, no one can avert the ozone loss that would occur with a large volcano.
"There is nothing we can do," Dr. Brasseur said. "We can only hope that such a volcano does not occur."
Typically, an eruption the size of El Chichon occurs every few decades.
A loss of 12 percent of the ozone would be about twice as much depletion as scientists have recorded since the discovery that chlorine from the CFC chemicals was depleting ozone in the stratosphere.
In the stratosphere, ozone protects the Earth's surface from ultraviolet radiation from the sun. Additional ultraviolet radiation would lead to increased incidence of skin cancer, as well as damage to crops and fisheries, scientists say.
According to some calculations, a loss of 12 percent of the ozone would lead to increases of up to 48 percent in cases of melanoma, already one of the most rapidly increasing and most dangerous forms of cancer.
The concern about the effect volcanic eruptions could have on CFC-ozone reactions stems from the fact that particles of sulfuric acid that would be pumped into the stratosphere in huge quantities serve to drastically accelerate the chemical processes in which chlorine molecules destroy ozone.
Over Antarctica, crystals of ice that float in the stratosphere have the same effect in September, October and November each year, leading to the ozone "holes."
Dr. Brasseur said he based his calculations on predictions that CFC concentrations in the stratosphere will climb to as much as 4.5 parts per billion in 2010, even with the Montreal Accord phase-out.
After El Chichon erupted in 1982, there was an increased loss of ozone throughout the stratosphere that scientists have been unable to explain until recently.
At that time, the concentration of CFCs in the stratosphere was only 2.5 parts per billion.
Scientists say that a loss of 1 percent of the ozone in the stratosphere results in an increase of 2 percent in the amount of ultraviolet radiation that reaches the surface of the Earth.