Regional Nuclear War Could Have Drastic Climate Impact, Experts Say at AAAS

Alan Robock (left) and Owen B. Toon

Research during the 1980s showed that an indirect consequence of a full-scale nuclear conflict between the United States and the Soviet Union would likely be a "nuclear winter" as soot and smoke from burning cities had a devastating impact on global climate.

While such concerns receded after the end of the Cold War, a new analysis suggests that even a small nuclear war involving regional powers such as India and Pakistan could disrupt world agriculture for decades and threaten starvation for a billion people or more.

Owen B. Toon, an atmospheric scientist from the University of Colorado, and Alan Robock, professor of climatology at Rutgers University, described the possible climate impacts of a small-scale nuclear war at a 12 June briefing for reporters organized by the AAAS Center for Science, Technology and Security Policy and the Center for Media and Security. Toon and Robock also discussed their findings at a 13 June Capitol Hill briefing organized by the AAAS Center.

The prospects of a nuclear winter had been widely discussed in the 1980s. Toon was co-author of a scientific paper that found for the first time that a nuclear war involving thousands of warheads could have catastrophic consequences far beyond the target cities.

Up to 150 million tons of black soot particles from burning cities would be sent aloft, initially to the upper troposphere (six to nine miles above the Earth's surface) where they would darken the sky and absorb heat from the sun.

That heating would cause the particles to move even higher. The new research shows that the particles eventually would be lofted into the upper stratosphere, where they would persist for a decade or more. The particles also would block sunlight from reaching the ground. The resulting cooling, according to Robock, would produce average surface temperatures colder than the last Ice Age, 18,000 years ago.

After the United States and Russia began sharp reductions in their nuclear arsenals and Cold War tensions eased, the threat of war and an associated nuclear winter passed. "It seemed like the problem had been solved," Toon said. But he said his interest in the issues was reawakened a few years ago when a reporter asked him what might happen if India and Pakistan were to have a nuclear conflict over the disputed territory of Kashmir.

Toon enlisted Robock in an effort to project the results of an exchange in which India and Pakistan each used 50 Hiroshima-size bombs on population centers. Such an exchange would use less than half of 1% of the current global nuclear arsenal, according to Toon and Robock. They project that as many as 21 million people would die in the nuclear war—about half as many as died in all of World War II—and 5 million tons of soot would be lofted into the stratosphere (compared to the projection of as much as 150 million tons of soot from a much larger nuclear conflict involving the Cold War superpowers).

Robock and his colleagues used computerized climate models that predict a rapid drop in average global surface temperatures of 1.25 degrees C, with temperatures several degrees colder for some major agricultural regions during summer. Growing seasons in mid-latitudes of both hemispheres would be shortened by weeks and rainfall would be much lower in some places.

Even in the best of times, there is only about a one-month backlog in food supply for the planet, Robock said. So the disruptions caused by a milder version of the nuclear winter scenario could pose a starvation threat for many millions of people, he said. "A war between India and Pakistan could threaten the rest of the world," Robock said. He and Toon said the lower temperatures in summer could persist at mid-latitudes for more than a decade.

Toon and Robock acknowledge that they have no expertise in nuclear strategy or predicting how a regional nuclear conflict might unfold. But they assume that India and Pakistan would target each other's population centers. Beyond the horror of the initial deaths due to the nuclear blasts, the most important factor, Robock said, would be the amount of smoke soot lofted from the burning cities and military facilities.

The researchers tested the fidelity of their projections by comparing them to what happens following large volcanic eruptions, which also spew tons of materials into the atmosphere and are natural analogues for what could happen following a regional nuclear conflict.

The greatest volcanic eruption of the past 500 years, the 1815 Tambora eruption in Indonesia, resulted in the "Year Without Summer" in the Northern Hemisphere in 1816. Killing frosts disrupted agriculture throughout New England that summer and there were widespread harvest failures in Europe. According to an essay by Robock in a publication of the International Pacific Research Center, the black soot produced by a regional nuclear conflict would have climatic effects "significantly greater and more persistent than those following the Tambora eruption. Moreover, the cooling in the decade following the injection of 5 million tons of soot into the atmosphere "is almost twice as large as the global warming of the past century (about 0.7 degrees C) and would lead to temperatures cooler than the pre-industrial Little Ice Age."

The computer models also suggest that the heating of soot particles in the upper atmosphere would accelerate chemical reactions that trigger depletion of the Earth's ozone layer, which protects surface life from harmful ultraviolet rays from the sun. An analysis by Toon and others in the 8 April issue of the Proceedings of the National Academy of Sciences, found that a regional nuclear conflict would produce massive ozone losses, exceeding 20% globally, 25 to 45% at the mid-latitudes and 50 to 70% at the high latitudes. The losses would persist for five years, with substantial losses continuing for another five years.

Toon and Robock are hoping to spur more interest in the topic of climatic effects of nuclear conflict. "So far we haven't detected much of an interest from policy makers," Robock said. The scientists said there may be some confusion in the public mind on whether the nuclear winter theory of the 1980s was correct. There was some debate on whether the result of a full-scale nuclear exchange might produce instead a "nuclear autumn" with less severe climate effects than first thought.

But Robock said the newer climate models, not available in the 1980s, have confirmed the baseline scenario from 25 years ago. The new simulations suggest that the use of several thousand warheads by each side would, indeed, have plunged continental areas of the North America and Eurasia into subfreezing temperatures for years. The models also suggest the effects would have lasted much longer than previously thought. (New analyses by Toon and Robock also show that the arsenals of the United States and Russia, even after reductions planned by 2012 under current treaties, would still be sufficient to trigger a nuclear winter if used.)

Toon and Robock call for testing of their results with other climate models to determine the detailed consequences of a regional nuclear exchange on agriculture, water supply, global trade, communications, travel, pollution and other human impacts.

"Policy makers need to know the bad effects of these weapons," Robock said. And, he added, "We need to get the world scientific community together to analyze this problem."

Earl Lane

23 June 2008