Even a small nuclear conflict could have catastrophic environmental and societal consequences, extending the death toll far beyond the number of people killed directly by bombs, according to the first comprehensive climatic analysis of a regional nuclear war.
A few dozen modest Hiroshima-sized nuclear weapons exchanged between India and Pakistan, for example, could produce a globe-encircling pall of smoke, causing temperatures to fall worldwide and disrupting food production for millions, according to the analysis presented Monday at a meeting of the American Geophysical Union in San Francisco.
While a small nuclear exchange might not trigger a life-ending "nuclear winter," it could cause as much death as was once predicted for a nuclear war between the U.S. and the Soviet Union, said Owen B. Toon, an atmospheric researcher at the University of Colorado.
"These results are quite surprising," Toon said at a media briefing. Regional nuclear conflicts "can endanger entire populations" the way it was once thought only worldwide conflict could.
Toon and coauthor Richard Turco, a professor of atmospheric sciences at UCLA, were part of the team of scientists that developed the original concept of nuclear winter in the 1980s.
The analysis was presented in two papers that dealt with the climatic, atmospheric and social consequences of a regional exchange. The studies were published in the online journal Atmospheric Chemistry and Physics Discussions.
Since the 1980s, when the U.S. and Soviet Union began drawing down their nuclear stockpiles, the number of weapons around the world has declined by a factor of three, Toon said. There are now about 10,000 nuclear weapons, and that is expected to drop to 4,000 by 2012.
But the number of nations with the potential to possess nuclear arms has gone up dramatically. Toon said 40 countries now have the fissile material to build nuclear weapons. Japan, with its large nuclear power industry, could make 20,000 weapons.
Many of the countries that could build nuclear weapons are also unstable, or at some stage of discontent with their neighbors.
In conducting their research, the scientists looked at other global cataclysms, such as the 1815 eruption of the Tambora volcano in Indonesia. The eruption triggered what has come to be known as the Year Without a Summer, which caused killing frosts and crop losses in New England as well as crop failures and famine in Europe.
The authors said even a limited nuclear conflict would be much worse, killing as many as 17 million in China alone.
The most significant atmospheric impact from a nuclear exchange would be the accumulation of smoke and soot in the atmosphere, said team member Georgiy Stenchikov, a professor of environmental science at Rutgers University.
Stenchikov estimated that 5 million tons of soot could be thrown into the air by the explosion of about 100 15-kiloton nuclear weapons.
The smoke and soot would ascend into the stratosphere and stay there for up to 10 years, causing temperatures to fall several degrees, the researchers said.
In areas far removed from the site of the explosions, growing seasons could be reduced by 10 days to a month, said Alan Robock, an environmental sciences professor at Rutgers who worked on the analysis.
One factor increasing the danger in densely populated areas is the proliferation of plastics, which in a firestorm would increase the soot released into the atmosphere. The production of plastics in the developed world has doubled in just the last two decades, Turco said.
Instead of feeling content that the U.S. and Russia are drawing down their nuclear arsenal, people should realize that they "are at a perilous crossroads," Toon said.
"Nuclear proliferation and political instability form the greatest danger to human society since the dawn of mankind."