With India and Pakistan both holding arsenals of nuclear weapons, and the two nations locked in seemingly endless hostility over disputed Kashmir, a team of U.S. experts warns that even a limited nuclear war between them could cause a near-global threat to the Earth's atmosphere and the human life it protects.
An exchange of even small-scale atomic bomb attacks by the two nations, the experts say, would create a vast hole in the layer of ozone that fills the upper atmosphere and protects life below from damaging - even deadly - ultraviolet radiation.
The scientists, reporting Monday in the online edition of the Proceedings of the National Academy of Sciences, calculate that a nuclear war between the two countries involving about 50 relatively small nuclear warheads, each with the power of the Hiroshima bomb, would touch off violent chemical reactions in the stratosphere. Up to three-quarters of the ozone above the Earth's far northern latitudes and up to 40 percent above nations in the mid-latitudes, where most of the world's population lives, would be destroyed.
"We would see a dramatic drop in ozone levels that would persist for many years and it could have huge effects on human health and on our terrestrial, aquatic and marine environments," said Michael Mills of the Laboratory for Atmosphere and Space Physics at the University of Colorado in Boulder who led the research team.
Mills and four other leading atmospheric scientists base their calculations of ozone layer destruction on five widely used computer models that scientists use to study all aspects of the world's atmosphere.
Their basic conclusions: The explosions would cause major urban fires and send as much as 5.5 million tons of soot rising above the stratosphere 50 miles high, after a million tons of it fell back to Earth as black rain. The soot, heated by smoke from the fires below, would speed up 20 or more chemical reactions, involving oxygen, nitrogen, hydrogen, chlorine and bromine, and create immense amounts of nitrogen oxides that would swiftly destroy ozone.
The most widespread loss of ozone would persist for up to five years, the scientists calculated, and substantial losses would continue for at least five years beyond that.
While ozone can be a toxic gas for humans in the lower atmosphere, in normal times it lies in layers between 10 and 30 miles up and protects against solar radiation, preventing the potentially lethal destruction of DNA in plants and animals and skin cancers in humans.
More than 30 years ago, scientists discovered that widely used industrial chemicals known as chlorofluorocarbons were destroying ozone in the stratosphere over Antarctica. As the ozone hole widened and spread north, an international treaty banned the use of those and many other chemicals, and in recent decades the ozone layer has returned to normal.
Then in 1983, when the Cold War was at its most intense and the concept of "mutual assured destruction" animated both U.S. and Soviet military nuclear doctrine, a group of prominent senior scientists, led by the late Carl Sagan, produced a famous report warning that all-out nuclear war between the two enemies involving anywhere from 5,000 to 10,000 megatons of bombs and missiles would mean calamity for the world.
That kind of nuclear spasm would kill more than a billion people and injure a billion more from radiation, blast and fire, the scientists warned; it would annihilate major cities on both sides, and plunge the Earth's surface into crop-destroying darkness for months if not years. They called it a nuclear winter.
Two of that 1983 report's authors, Owen Toon of the University of Colorado and Richard Turco of UCLA, are among the new report's leading authors. The others are Douglas Kinnison and Rolando Garcia of the National Center for Atmospheric Research, a Boulder, Colo., facility supported by the government's National Science Foundation.
In an interview, Toon noted that his current team has had the advantage of two extremely sophisticated national computer models and three regional ones - all far faster and more accurate than the ones his team used for their 1983 report, he said.
"All the new models came up with the same results," Toon said, "and they gave us two surprises: One was the huge quantity of smoke that would be produced from even the limited nuclear war in our scenario, and the other was the conclusion that the smoke would remain dense above the stratosphere for as long as five years."
© 2008 The San Francisco Chronicle