Think you might escape the aftereffects of a limited nuclear war that happens on the other side of the globe from you? Think again.

Imagine that the long-simmering conflict between India and Pakistan broke out into a war in which each side deployed 50 nuclear weapons against the other country's megacities. Karachi, Bombay, and dozens of other South Asian cities catch fire like Hiroshima and Nagasaki did at the end of World War II.

Beyond the local human tragedy of such a situation, a new study looking at the atmospheric chemistry of regional nuclear war finds that the hot smoke from burning cities would tear holes in the ozone layer of the Earth. The increased UV radiation resulting from the ozone loss could more than double DNA damage, and increase cancer rates across North America and Eurasia.

"Our research supports that there would be worldwide destruction," said Michael Mills, co-author of the study and a research scientist at the University of Colorado at Boulder. "It demonstrates that a small-scale regional conflict is capable of triggering larger ozone losses globally than the ones that were previously predicted for a full-scale nuclear war."

Combined with the climatic impact of a regional nuclear war – which could reduce crop yields and starve hundreds of millions – Mills'

modeling shows that the entire globe would feel the repercussions of a hundred nuclear detonations, a small fraction of just the U.S. stockpile. After decades of Cold War research into the impacts that a full-blown war between the Soviet Union and the United States would have had on the globe, recent work has focused on regional nuclear wars, which are seen as more likely than all-out nuclear Armageddon. Incorporating the latest atmospheric modeling, the scientists are finding that even a small nuclear conflict would wreak havoc on the global environment (.pdf) – cooling it twice as much as it's heated over the last century – and on the structure of the atmosphere itself.

Mills' work, which appears online today in the Proceedings of the National Academies of Science, used a model from National Center for Atmospheric Research to look at the impact of throwing 5 million metric tons of black carbon, or soot, into the atmosphere. He found that when a cluster of cities are burning together, they end up creating their own weather, pumping soot 20,000 feet into the atmosphere. Once there, sunlight would heat the smoke, and drive it up 260,000 feet above the earth's surface.

Along the way, the hot soot would cause a variety of atmospheric changes with a net result of huge reductions in ozone, which in the stratosphere serves as sunblock for the earth. In the middle latitudes, the researchers found the ozone layer would be reduced by 25 to 45 percent, with the polar regions losing 50 to 70 percent of their ozone coverage. This thinning is known as a "hole" in the ozone layer, and would be many times the size of the famed hole over Antarctica.

According to research cited by the paper, the increase in ultraviolet light falling to earth at the 45-degree latitude – a little south of Portland, Oregon – would cause damage to DNA to increase 213 percent.

"It would have a dramatic effect on skin cancer and cataracts and be very damaging to crops and ecosystems," Mills said.

The reduced levels of ozone would persist for five years, with substantial reductions in ozone continuing for another five years after that.

Even if the cause of the war were local, its impacts would be felt across the globe.

"Pretty much everywhere [would be] affected," Mills concluded.

Photo: A nuclear bomb is detonated in a test blast at Mururoa atoll, French Polynesia, in 1971./Associated Press