'Nuclear Winter' Comes In From The Cold

“[Here we are on] the Halloween preceding 1984,” Carl Sagan solemnly told a gathering of scientists and reporters in Washington, D.C., “and I deeply wish that what I am about to tell you were only a ghost story, something invented to frighten children for a day. But unfortunately, it is not just a story.” Thus began a spellbinding tale of doom, delivered in inimitable Sagan fashion. Should nuclear war erupt between the world’s two superpow ers, warned the Corn

Bruce Fellman
Apr 30, 1989

“[Here we are on] the Halloween preceding 1984,” Carl Sagan solemnly told a gathering of scientists and reporters in Washington, D.C., “and I deeply wish that what I am about to tell you were only a ghost story, something invented to frighten children for a day. But unfortunately, it is not just a story.”

Thus began a spellbinding tale of doom, delivered in inimitable Sagan fashion. Should nuclear war erupt between the world’s two superpow ers, warned the Cornell University astronomer and popularizer of science, black smoke from cities set ablaze would blot out the sun and cause a long-term cataclysmic drop in the Earth’s temperature.

The scenario was quickly dubbed “nuclear winter,” and for a time it grabbed front-page head lines around the world before eventually fading from public view.

Now the theory is making a comeback. But the latest version is far from being a Halloween horror story. Instead, it’s founded on a body of scientific evidence that is linked to burgeoning research on global climate change.

The terrifying conclusion that Sagan originally presented was drawn from several sources. It relied on computer models that tried to measure the impact of a major nuclear exchange, new work by atmospheric chemists Paul Crutzen and John Birks on the climatic effects of forest fires, and speculations by Sagan and two of his former Cornell graduate students, James Pollack " and Brian Toon, of how dust storms cooled the surface of Mars.

The potential for a nuclear winter made for a fascinating science story that also had major implications for national policy makers. The apocalyptic hypothesis was quickly embraced by the disarmament movement, and just as quickly rejected by those who favored the preservation of our national nuclear stockpile.

But the often divisive scientific and political debate surrounding nuclear winter eventually subsided, and by 1986 the theory seemed to have disappeared. So reports late last year of a study by University of Maryland meteorologist Alan Robock that had implications for the current model of nuclear Winter caught many people by surprise. Wasn’t that theory passe, they wondered.

It turns out that the hypothesis Sagan and his colleagues wrought in 1983 is alive, well, and the focus of exciting atmospheric and computer science at government labs and universities around the country. Last year alone, the research area ceived more than $5 million from the Pentagon’s Defense Nuclear Agency. Moreover, the prospects for continued funding seem good: The questions that need to be answered to better understand nuclear winter, it turns out, are also the keys to such current hot topics as global warming trends and the depletion of ozone in the upper atmosphere.

Is Sagan upset that his hypothesis no longer sends people into the streets to protest the possibility of nuclear annihilation? Not at all. The fact that the theory no longer generates much controversy is, in his view, “part of the usual tradition of science. You start out with an original set of calculations, and there’s criticism. You rework what you have to, and then the whole thing settles out.” Sagan knows that criticism can also generate publicity, particularly when its target is a theory with a great deal of emotional and political appeal. While he certainly isn’t shy about drawing attention to issues about which he feels strongly, he is also content to watch advances in science take place in relative privacy, as participants quietly sift through the evidence and try to reach a consensus.

“The way I read the present situation is that, within the probable error of the calculations, everybody’s studies are in excellent agreement. And they’re close to our original model,” says Sagan.

To be sure, not everybody would agree with Sagan that the field is moving inexorably forward. Harvard policy analyst Russell Seitz is openly scornful of Robock’s work, which analyzed the impact on tomato plants of an intense but relatively small forest fire in a northern California community called Happy Camp.

“It’s utterly fascinating,” Seitz sneers. “A scientist discovers it’s cooler in the shade. At Happy Camp, there was 10 times the smoke, maybe more, than Sagan talked about as being curtains for the planet, but we see only one-eighth the predicied effect on the temperature. This fire is a joke played by nature on the whole global effects community.”

Curiously, the latest research shows that both men may be right. Recent computer models of the atmosphere and studies of the climatological effects of smoke have demonstrated that a significant cooling will follow a large-scale nuclear exchange. But the models predict that the post-bomb weather is more likely to turn-chilly than frigid—an autumn instead of a winter. In fact, says Michael C. MacCracken, an expert on computer modeling at Law- rence Livermore, anyone who read the original 1984 paper in the journal Science (volume 222, pages 1283- 92) could have drawn the same conclusion.

His implication is clear: The initial debate about nuclear winter was driven more by ideology than by science, with each side using the most extreme views of its opponents to score points. But what seems to be propelling current research on the topic is a genuine desire to learn more about the effects of massive disturbances in the earth's fragile atmosphere.

Of course, those pioneering researchers stoutly maintain that science was always uppermost in their minds. They note that the original paper by the so-called TTAPS group—Richard Turco, of UCLA; Toon, of NASA/Ames; Thomas Ackerman, of Penn State; Pollack, of NASA/Ames; and Sagan—offered a range of more than 50 nuclear war scenarios and their subsequent climatological outcomes. Using an admittedly simplified view of the atmosphere—a one-dimensional model—the scientists reported that smoke and dust generated by fire-storms from burning urban and industrial areas would cool off the intenors of northern hemisphere continents by an average of 10 to 20 degrees C. This meant frost was possible even in the summertime At worst, they calculated, the temperatures could plunge to nearly 50 degrees C. below zero.

But there were other scenarios, they admit; in which very little happened. Even the most dire predictions were bracketed by caveats, in part because one-dimensional models don’t take into account a number of mitigating factors, such as the temperature buffering capabilities of the oceans. TTAPS was a very impressive review, but there are many uncertainties,” notes MacCracken. “The numbers are in a state of flux.”

Scientists are hard at work trying to firm up their data. MacCracken’s team has worked with both two and three-dimensional models to provide a much more realistic depiction of the way the world really works. The extra detail also allows researchers to examine the complex interactions between a variety of atmospheric factors, such as sunlight, rain, thermal structure, and circulation. For example, in MacCracken’s latest model, smoke that enters the computer’s -atmosphere produces a summer cooling of "10 degrees C. on average, with periods below freezing.”

His findings are very similar to those of Robert C. Malone, a senior scientist at Los Alamos National Laboratory, and his colleagues. Malone’s 3-D model, a modification of the “General Circulation Model” developed at the National Center for Atmospheric Research (NCAR) in Boulder, Col., is widely considered to be the most sophisticated available.

“For a summertime war that resuIts in a large injection of smoke,” says Malone, “our model predicts temperature changes of between 10 to 15 degrees C. below normal in the interiors of large continents in the northern hemisphere.” The effect is “substantially less,” he adds, along the coasts and in the southern hemisphere. The impact would also be smaller if the war were to break out in the winter. The weaker sunlight of winter, he notes, lacks sufficient energy to propel the smoke into the upper troposphere and trigger the atmospheric cooling.

While the Doomsday experiment required to evaluate the models has not been done, recent examinations of the cooling effects of forest fire smoke provide support for MacCracken’s and Malone’s simulation of the post-nuclear holocaust climate. Last November, meteorologist Robock reported in Science (volume 243,. pages 911-913) that during the height of a 1987 Californiaforest fire, the average daily high temperature in the area dropped as much as 15 degrees C. An earlier investigation of a 1982 British Columbia fire, done in collaboration with Brian Toon, showed a less severe, but still significant, drop in temperature as the smoke plume drifted over the Midwest.

One intriguing aspect of Robock’s latest work is his discovery of a “positive feedback mechanism that enhanced the cooling.” During the monthlong Happy Camp fire, he explains, a thermal inversion (a section of the atmosphere in which temperature goes up rather than down as one moves higher) developed over the Klamath River Canyon area. “The inversion acted like a lid to trap the smoke,” he says. The surface cooled, and this made the inversion even stronger. It trapped more smoke, which cooled the surface even more. “Obviously, this is not the type of situation you’d expect on a global basis [the most effective inversions form over mountain valleys], but it is something that no one had predicted before,” he notes. “It’s important to realize that we can’t know what mechanisms we’ll discover.”

Still, the basic question remains: How bad is a nuclear winter likely to be? And it’s here that scientists are quick to distinguish between the data and the political rhetoric.

“It’s outrageous the things that went on,” declares George W. Rathjens, a political scientist and chemist at MIT, as he recalls the shocking images of a dark, frozen planet that accompanied initial reports of the theory (see above). “It was totally inexcusable to suggest that we’d see the end of life on Earth, with the implication that this was what would happen, rather than this was the worst possible case.”

Stanford University ecologist Paul R. Ehrlich doesn’t disagree with that view. But he sees the value of looking on the dark sided “I think it is very good science always to take the most pessimistic case,” says Ehrlich, who along with 16 colleagues, wrote a companion article to the TTAPS paper (“Long-term biological consequences of nuclear war, Science, volume 222, pages 1293- 1300, 1984). “The scientifically responsible thing to do is to say we have seen this possibility, so let’s take it real slow or stop until we see if it’s real.”

In Berkeley, Calif.

Ehrlich admits that the original TTAPS temperature projections were “overly pessimistic,” as were the well-publicized possible effects. “But it doesn’t make a damned bit of difference whether it drops to 40 below in Iowa during the growing season, or it just goes below freezing a couple of times and gets dim for a couple of weeks,” he says. “That’ll screw-up the corn crop just as well as freezing it right to the bottom.”

Malone notes that, while the models don’t necessarily predict frosts in July, “they don't rule them out either. They’re not able to handle small-scale, highly transient fluctuations in temperature.” He draws an analogy to the 19th century’s “year without a summer,” when a massive volcanic explosion caused freakish weather around the globe. “The average temperature in New England, for example, was not a large amount below normal, but there were isolated frosts throughout the summer, and they did in the crops,” notes Malone.

Regardless of the impact of a theoretical nuclear winter, many scientists involved in current research on climate change are glad that the subject has attracted so much attention. Everyone who’s been involved in the debate “has performed an important service for both humanity and science,” says NCAR climatologist Stephen Schneider says, “even if the earliest dramatic quantitative conclusions did not survive intact. Science is enriched by controversial hypotheses; and interdisciplinary science thrives on them.”

Bruce Fellman is a freelance writer based in Connecticut