Toon was one of the scientists who warned in the 1980s of a "nuclear winter" should the United States and Soviet Union engage in a nuclear conflict.

The demise of the Soviet Union has reduced such a threat, but using supercomputing analysis not available two decades ago, the team calculated a devastating impact from the exchange of 100 nuclear weapons -- an amount they said represented the potential of India and Pakistan.

"Regional scale nuclear conflicts can inflict casualties comparable to those predicted for a strategic attack between the United States and the USSR," Toon told the fall meeting of the American Geophysical Union in San Francisco. "The smoke produced can endanger the entire population of Earth through climate changes and ozone loss."

The study's authors warned of the spread of nuclear technologies to many nations and the risks to ever more concentrated urban centers with large fuel stockpiles that would feed massive fires.

"Owing to the confluence today of nuclear proliferation, migration into megacities and the centralization of economies within these cities, human society is extremely vulnerable," said Richard Turco of the Department of Atmospheric and Oceanic Sciences at the University of California, Los Angeles.

The scientists said that smoke from a regional conflict would spread across the entire world within weeks and even produce a cooling effect as the sun's rays are partially blocked.

"This is not a solution to global warming because you have to look at the devastating climate changes," said Alan Robock of the Department of Environmental Sciences at Rutgers, who has studied the impact of climatic change from regional nuclear war.

"The main point here is that while most people think that we are on a path of reduced probability of war with the build down of the superpowers and we are on a trend toward a peaceful century, we actually have the opposite situation going on."