New scientific modeling shows that a regional nuclear conflict between countries such as India and Pakistan could spark devastating climate changes worldwide. “We are at a perilous crossroads,” said Owen Toon of the University of Colorado at Boulder’s Department of Atmospheric and Oceanic Sciences. “The current combination of nuclear proliferation, political instability and urban demographics form perhaps the greatest danger to the stability of society since the dawn of humanity.”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.”

“We have a trend where the build up of nuclear weapons in many countries of the world creates the situation where there are 20, 30, 40 nuclear states, all dangerous as the Soviet Union used to be,” Robock said.

“The results described in one of the new papers represent the first comprehensive quantitative study of the consequences of a nuclear conflict between smaller nuclear states,” said Toon and his co-authors. “A small country is likely to direct its weapons against population centers to maximize damage and achieve the greatest advantage,” Toon said. Fatality estimates for a plausible regional conflict ranged from 2.6 million to 16.7 million per country.

Alan Robock, a professor in the department of environmental sciences and associate director of the Center for Environmental Prediction at Rutgers’ Cook College, guided the climate modeling effort using tools he previously employed in assessing volcano-induced climate change. Robock and his Rutgers co-workers, Professor Georgiy Stenchikov and Postdoctoral Associate Luke Oman (now at Johns Hopkins University) generated a series of computer simulations depicting potential climatic anomalies that a small-scale nuclear war could bring about, summarizing their conclusions in the second paper.

“Considering the relatively small number and size of the weapons, the effects are surprisingly large. The potential devastation would be catastrophic and long term,” said Richard Turco, professor of atmospheric and oceanic sciences, and a member and founding director of UCLA’s Institute of the Environment. Turco once headed a team including Toon and Carl Sagan that originally defined “nuclear winter.”

While a regional nuclear confrontation among emerging third-world nuclear powers might be geographically constrained, Robock and his colleagues have concluded that the environmental impacts could be worldwide.

“We examined the climatic effects of the smoke produced in a regional conflict in the subtropics between two opposing nations, each using 50 Hiroshima-size nuclear weapons to attack the other’s most populated urban areas,” Robock said. The researchers carried out their simulations using a modern climate model coupled with estimates of smoke emissions provided by Toon and his colleagues, which amounted to as much as five million metric tons of “soot” particles.

“A cooling of several degrees would occur over large areas of North America and Eurasia, including most of the grain-growing regions,” Robock said. “As in the case with earlier nuclear winter calculations, large climatic effects would occur in regions far removed from the target areas or the countries involved in the conflict.”

When Robock and his team applied their climate model to calibrate the recorded response to the 1912 eruptions of Katmai volcano in Alaska, they found that observed temperature anomalies were accurately reproduced. On a grander scale, the 1815 eruption of Tambora in Indonesia – the largest in the last 500 years – was followed by killing frosts throughout New England in 1816, during what has become known as “the year without a summer.” The weather in Europe was reported to be so cold and wet that the harvest failed and people starved. This historical event, according to Robock, perhaps foreshadows the kind of climate disruptions that would follow a regional nuclear conflict.

But the climatic disruption resulting from Tambora lasted for only about one year, the authors note. In their most recent computer simulation, in which carbon particles remain in the stratosphere for up to 10 years, the climatic effects are greater and last longer than those associated with the Tambora eruption.

“With the exchange of 100 15-kiloton weapons as posed in this scenario, the estimated quantities of smoke generated could lead to global climate anomalies exceeding any changes experienced in recorded history,” Robock said. “And that’s just 0.03 percent of the total explosive power of the current world nuclear arsenal.”

The papers are: “Atmospheric Effects and Societal Consequences of Regional Scale Nuclear Conflicts and Acts of Individual Terrorism,” O. B. Toon, R. P. Turco, A. Robock, C. Bardeen, L. Oman and G. L. Stenchikov, and “Climatic Consequences of Regional Nuclear Conflicts,” A. Robock. L. Oman, G. L. Stenchikov, O. B. Toon, C. Bardeen and R. P. Turco. Both can be accessed on the Atmospheric Chemistry and Physics Discussions journal Web site, at http://www.cosis.net/members/journals/df/recent.php?j_id=1.