Regional nuclear war could devastate large cities and disrupt the global climate

December 11, 2006

Even a small-scale, regional nuclear war could produce as many direct fatalities as occurred during all of World War II and disrupt the global climate for a decade or more, impacting nearly everyone on Earth.

These conclusions are reported by a team of scientists from UCLA, the University of Colorado at Boulder and Rutgers, The State University of New Jersey in two research articles posted online in the journal Atmospheric Chemistry and Physics Discussions and at a press conference today at the American Geophysical Union's annual meeting in San Francisco.

The new results represent the first comprehensive quantitative assessment of the consequences of a nuclear conflict between small or emerging nuclear states, said Richard Turco, professor in the UCLA Department of Atmospheric and Oceanic Sciences and a member and founding director of UCLA's Institute of the Environment.

The team of scientists reviewed the current status of nuclear weapons development, analyzed data on modern megacities and applied a state-of-the-science climate model. They calculated the local effects of individual "small," Hiroshima-size (15-kiloton) nuclear detonations in urban centers, including potential casualties from the blast and radioactive fallout, Turco said.

Even the smallest nuclear powers today and in the near future may have as many as 50 or more Hiroshima-size weapons in their arsenals, according to the scientists. Moreover, about 40 countries possess enough plutonium and uranium to construct substantial nuclear arsenals.

"Considering the relatively small number and sizes of the weapons -- perhaps less than one megaton in total yield -- the potential devastation would be catastrophic and long-term," Toon said. "A single low-yield nuclear detonation in an urban center could lead to more fatalities, in some cases by orders of magnitude, than occurred in major historical wars."

Megacities attacked with nuclear devices, through war or terrorism, would likely be abandoned indefinitely, inducing mass migration and long-term economic decline, Turco said. Turco in the 1980s headed a group -- whose members included Owen "Brian" Toon, a co-author on the current research, and the late Carl Sagan -- that originally defined the "nuclear winter" phenomenon, a phrase that Turco coined. For a regional-scale nuclear conflict, fatality estimates range from 2.6 million to 16.7 million, Turco said.

The scientists estimated the quantities of soot -- the highly absorbing component of smoke -- that would be generated in urban firestorms ignited by nuclear detonations. This effort was led by Toon, professor and chair of the department of atmospheric and oceanic sciences at the University of Colorado at Boulder, together with Turco and University of Colorado student Charles Bardeen. At Rutgers, Alan Robock, professor of environmental sciences and associate director of the Center for Environmental Prediction at Rutgers' Cook College, professor Georgiy Stenchikov and postdoctoral associate Luke Oman (now at Johns Hopkins University) employed a coupled atmosphere-ocean climate model to simulate the effects of the putative smoke emissions in perturbing the global climate system and causing regional climatic anomalies.

The amount of soot emitted by firestorms was found to exceed 5 million metric tons in many cases. Because so many people live in megacities, the quantity of black smoke generated per kiloton of explosive yield could be more than 100 times larger than previously estimated for a full-scale superpower nuclear exchange involving thousands of megatons, according to one of the journal papers.

While a regional nuclear confrontation among emerging nuclear powers might be geographically constrained, the environmental impacts could spread worldwide, Robock and his colleagues conclude.

"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 post-war climate simulations used soot emissions provided by Toon, Turco and Bardeen.

As had been suggested in earlier nuclear winter studies, and more recently by observations of large wildfire smoke plumes, Robock's calculations indicate that a large fraction of the nuclear soot could linger in the upper atmosphere for up to a decade, producing significant cooling and reduced precipitation, with the greatest changes occurring over land. The implications for global food supplies appear grim.

"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 was 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 calibrated their climate model against 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 starvation stalked most of the continent. This historical event, according to Robock, perhaps foreshadows the kind of climate disruptions that would follow a regional nuclear conflict.

The researchers emphasized that known climatic anomalies associated with major volcanic eruptions such as Tambora typically last for a year or so because volcanic particles tend to fall out of the atmosphere relatively quickly. By contrast, nuclear-generated soot particles may remain suspended in the upper troposphere and stratosphere for up to a decade as a result of the strong interactions between solar heat absorption by the smoke and wind patterns in the upper atmosphere. Consequently, the climatic effects can be significantly greater and longer lasting than those associated with any historical volcanic eruption.

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

In related research, researcher Michael Mills of the University of Colorado at Boulder led a broad team, including Toon and Turco, in defining the impacts of a regional nuclear conflict on the stratospheric ozone layer. Mills' results, based on detailed simulations with a two-dimensional global chemical-transport model, reveal average column ozone losses exceeding 20 percent worldwide and persisting for at least three to four years, with mid-latitude losses as large as 30 to 40 percent and polar reductions up to 70 percent.

Such ozone depletions would be unprecedented in human history and imply serious ecological and human consequences, Turco said. The primary effects on ozone are due to accelerated catalytic chemical cycles, which are caused by the heating of the stratosphere as injected soot absorbs sunlight, and to severely perturbed dynamics of the region, again owing to the soot heating.

Previous studies, carried out in the 1980s with less sophisticated models, had indicated comparable or smaller ozone losses for a full superpower nuclear exchange, Mills noted.

Turco said that a small nuclear state is likely to direct its weapons against population centers to maximize damage and achieve the greatest advantage, thus making such outcomes more plausible.

The research team concludes that the confluence of nuclear proliferation, political instability and urban demographics forms perhaps the greatest danger to the stability of human society since the dawn of civilization.

The research team assessed uncertainty at each step of their analysis. Because of the complexity of the problem and the limited data available to support calculations, they emphasize that further research is needed to improve the predictions described in their publications.

Nevertheless, the researchers note that the largest uncertainties are associated with the possible scenarios for a nuclear exchange. They conclude that the current buildup of nuclear weapons in a growing number of states points to scenarios in the next few decades that may be even more extreme than those considered in the present analysis.
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"j_id=1.

About UCLA

California's largest university, UCLA enrolls approximately 38,000 students per year and offers degrees from the UCLA College of Letters and Science and 11 professional schools in dozens of varied disciplines. UCLA consistently ranks among the top five universities and colleges nationally in total research-and-development spending, receiving more than $820 million a year in competitively awarded federal and state grants and contracts. For every $1 state taxpayers invest in UCLA, the university generates almost $9 in economic activity, resulting in an annual $6-billion economic impact on the Greater Los Angeles region. The university's health care network treats 450,000 patients per year. UCLA employs more than 27,000 faculty and staff, has more than 350,000 living alumni and has been home to five Nobel Prize recipients.

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