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Cold cuts: regional nuclear war could cool globe
21 DECEMBER 2006

More than 20 years ago, with the nuclear superpowers poised for Armageddon, scientists calculated that smoke from burning cities could block sunlight, leading to drastic global cooling, or "nuclear winter." Black stain spreads over sky of India and Pakistan.Now, with the Soviet Union gone, the mass launch of intercontinental ballistic missiles seems remote (ignoring, of course, the chance of some catastrophic boo-boo in some remote command center or missile silo).

Watch a GIF animation of smoke released by a hypothetical nuclear war between Pakistan and India. Note: An optical density of 0.1 means 90 percent of the light gets through. Courtesy Alan Robock, Rutgers University

But the rash of emerging nuclear states raises a different scenario: Instead of thousands of nukes, we could see much smaller numbers of bombs go off. One major hazard appears at the nuclear fault line between long-time rivals India and Pakistan.

Each country probably owns a few dozen warheads. What would happen if each country unleashed 50 bombs against the enemy's biggest cities? Upwards of five million people -- and perhaps many more -- would immediately die. Then, according to a new estimate, the resulting fires would lift about 5 million tons of carbonized grut into the atmosphere.

Within weeks, this soot could spread around the world, darkening the sun, shading the planet, and changing the climate. "A cooling of several degrees would occur over large areas of North America and Eurasia, including most of the grain-growing regions," says Alan Robock, a professor of environmental science at Rutgers, who lead the study of the climate impact of massive fires.

Areas of blue and white overlay a map of the world
June, July, and August air temperatures at the surface fall markedly in most parts of the globe, according to a new simulation of the effects of a small nuclear war on climate. Courtesy Alan Robock, Rutgers University

The ominous projection was based on a new report by Owen Toon, of the University of Colorado, which calculated how much wood, plastic and asphalt would burn in 100 Hiroshima-sized fires in India and Pakistan. Robock, who also worked on that study, says the smoke calculation presumed that each fire would be as big as the one at Hiroshima (13 square kilometers). "We did not include the firestorms seen in Dresden [in World War II], or in San Francisco after the earthquake; potentially firestorms could have much more impact."

Dense cloud in a mushroom shape rises over desert as men in coats look on
A U.S. nuclear test, May 5, 1955. Who worried much about radiation, let alone global cooling, in the height of the Cold War? Photo: National Archives

Smoke, harmful to your [planet's] health
Robock's research group then entered that amount of smoke into a computerized climate model, software more typically used to predict the effects of global warming, and calculated that the average cooling would be about 1.25° C. That is far milder than the "nuclear winter" expected to follow all-out superpower war, which could cool the continents by 30° C.

The bad news is that a black smudge would linger for years in the upper atmosphere, disrupting climate and farming the whole time.

The new study has several improvements over previous efforts to predict the climatic effects of nuclear war, says Robock. "We now have an estimate of how much smoke would be generated in the mega-cities that have grown in the subtropics, and we have a climate model that is capable of calculating up to 80 kilometers." And because the model also accounts for the effect of oceans on climate, "We have more confidence that we have done a good job in simulating the climatic response."

Graph charts the increasing temps Surface temperature after a 100-bomb nuclear war. A global average surface cooling of -1.25° C lasts for years. The temperature changes are largest over land, and quite distant from the warring countries. The cooling more than reverses the global warming of the past 100 years. Courtesy Alan Robock, Rutgers University

Trouble aloft
Because the smoke is black, Robock told us, it is warmed by absorbing a great deal of subtropical sunlight. The smoke is then lifted into the high atmosphere, far above the rainfall that rinses particles from the lower atmosphere. Five years after the war, one-third of the particles would still be present, and even after 10 years, Robock said, the sky would still look hazy.

The sun beats down on tall, brown corn stalksDrought is one predicted consequence of a limited nuclear war. Here, corn (maize) suffers from lack of water. Photo: USDA

As background for forecasting something that has never (thankfully) happened, Robock referred to volcanic eruptions, which climatologists consider "natural experiments." Eruptions are the largest single sources of high-atmosphere air pollution, but the white droplets of sulfuric acid they make absorb less solar energy and thus do not reach such high altitudes. Most of the droplets get rained out in a few years.

Still, the huge eruption of the Indonesian volcano Tambora in 1815 changed the weather enough to cause Europe's "year without a summer" in 1816. The current simulation anticipates that the black nuke-war smoke would have a similar effect on climate -- but last up to a decade.

Truth or consequences
Lower temperatures reduce the amount of water evaporating from the ocean, so the climate model predicted that a reduction in rainfall would accompany the cooling. This deadly duo of changes would make life miserable for farmers pretty much around the world, Robock says. "Agriculture everywhere in the world is tuned to current climate, precipitation and sunlight. No matter what the local temperature, an abrupt change in climate would be quite disruptive."

Map of the world, growing seasons highlighted in blue
After a year or two, the growing season (period between frosts) shortens in both hemispheres. Inevitably, this will cut farm production. Graph courtesy Alan Robock, Rutgers University

Nobody has fed these numbers into a model of food production, but with grain supplies already stretched tight, the consequences could be severe.

It's always possible to quibble with the assumptions of any projection: Why choose this specific pattern of attack versus another? For example, if the bombs hit military targets, rather than cities, they would make less smoke. Robock responds, "There are many possible scenarios. We chose one that could occur today, and this experiment should be repeated for several different ones, and with different assumptions and different climate models, so that we have a better understanding of the possible consequences of the environmental impacts of the use of even a small number of nuclear weapons."

Ideally, he adds, this type of sobering calculation could arouse public opinion, and perhaps even put a brake on nuclear proliferation. Earlier studies of nuclear winter, Robock says, helped induce Homo sapiens to think twice about the safety of setting off thousands of nuclear explosions on the home planet. Stay tuned.

— David Tenenbaum

• Regional Nuclear War Could Devastate Global Climate, A. Robock et al, Atmos. Chem. Phys. Discuss., 6, 11817-11843, 2006.
• Atmospheric effects and societal consequences of regional scale nuclear conflicts and acts of individual nuclear terrorism O. B. Toon et al, Atmos. Chem. Phys. Discuss., 6, 11745-11816, 2006.

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