Nuclear Winter Theorists Pull Back
By MALCOLM W. BROWNE
Published: January 23, 1990
SINCE 1983, scientists have been bitterly divided over whether a nuclear war is likely to result in a catastrophic global chilling. But the five scientists who introduced the term ''nuclear winter'' now acknowledge that they overestimated its severity, and their concession appears to have moderated the longstanding debate.
Scientists say the issues involved are as pertinent to human survival as ever, despite the new friendliness of Soviet-American relations. The strategic nuclear arsensals of both nations remain intact, they note, and could come into play if the current peaceful climate gives way to war.
The techniques developed to predict the effects of nuclear war on climate are also applicable to other climatic predictions, including the possibility that increased carbon dioxide in the air is leading to global warming, theorists say. The nuclear winter scenario is also closely related to the theory that dinosaurs became extinct when a giant meteor hit the earth and threw up a global dust cloud that caused catastrophic cooling.
The views of atmospheric scientists studying the nuclear winter theory still vary widely, although most of those interviewed said they believe a nuclear war could have some effect on climate. But most discounted the extreme view that global chilling of the atmosphere would be severe enough to be described as ''winter.'' Scientists specializing in such studies also generally reject the suggestion that a ''nuclear winter,'' in itself, could bring about the extinction of the human race. Even Dr. Richard P. Turco, the physicist who coined the phrase ''nuclear winter,'' discounts the idea.
Dr. Turco, a professor of atmospheric science at the University of California at Los Angeles, said in an interview that he had never believed that nuclear winter alone could wipe out humanity. ''That was a speculation of others, including Carl Sagan,'' he said. ''My personal opinion is that the human race wouldn't become extinct, but civilization as we know it certainly would.''
Dr. Sagan, a professor of astrophysics at Cornell University, was one of the scientists who collaborated with Dr. Turco in the article that ignited the nuclear winter dispute. The article, ''Nuclear Winter: Global Consequences of Multiple Nuclear Explosions,'' was published by the journal Science in 1983, and spawned a host of movies, plays and books predicated on the nuclear winter hypothesis.
The other authors of the article were Dr. Owen Brian Toon and Dr. James B. Pollack, both of the National Aeronautics and Space Administration's Ames Research Center, and Dr. Thomas P. Ackerman of Pennsylvania State University. Their paper became so famous and so frequently cited that other scientists have since referred to it by an acronym of the contributors' initials: TTAPS, pronounced ''tee-taps.''
That paper was not the first suggestion that global cooling might follow a nuclear exchange. A 1982 article by Dr. Paul J. Crutzen of the Max Planck Institute in West Germany and Dr. John W. Birks of the University of Colorado proposed the possibility of such an effect.
In a new paper published in Science, ''Climate and Smoke: An Appraisal of Nuclear Winter,'' the five TTAPS scientists review research conducted during the five years after their first joint paper. Drop in Temperatures While asserting that their general conclusions have been sustained, they say that a full-scale nuclear exchange in midsummer could reduce temperatures by an average of only 10 to 20 degrees centigrade (18 to 36 degrees Fahrenheit) in northern mid-latitudes. Compared with the reduction of 15 to 25 degrees centigrade (27 to 45 degrees Fahrenheit) predicted by their first paper, this chilling would be relatively mild, and in the view of some scientists, it deprives the phrase ''nuclear winter'' of realistic meaning.
Dr. Turco said his nuclear winter forecast had changed somewhat because he and his colleagues had been able to reduce the uncertainty inherent in some of the climatic effects involved. New experimental data and analyses from other groups also helped to refine his predictions, he said.
Dr. Stephen H. Schneider of the National Center for Atmospheric Research in Boulder, Colo., a long-standing critic of the extreme nuclear winter hypothesis, believes that a cooling of 10 to 20 degrees centigrade (18 to 36 degrees Fahrenheit) would not constitute the arrival of ''winter.''
''I would call it nuclear fall, not winter,'' Dr. Schneider said in an interview. ''But in any case, the TTAPS numbers have now more or less converged with ours, so I don't have a major problem with them anymore.''
In their latest paper, Dr. Turco and his associates say they have summarized and synthesized important experimental evidence and mathematical predictions made by other groups related to nuclear winter, thereby reducing the uncertainties inherent in their theory.
''Essentially,'' Dr. Turco said, ''what we say is that the basic physics we proposed turned out to be correct, although the magnitude of the effects has been moderated somewhat.'' Blocking of Sunlight The theory underlying nuclear winter is that if the Soviet Union and the United States were to wage an unlimited nuclear war, much of the resulting dust and smoke from fires, especially those of burning cities, would be spewed into the upper atmosphere, where it might remain for weeks or months. This would block sunlight, resulting in a sudden drop in atmospheric temperature.