Investigating the Climate Impacts of Nuclear War  

Washington, D.C., October 30, 2024 - A 1983 study from scientists at the Department of Energy’s Lawrence Livermore Laboratory said that a full-scale nuclear war between the superpowers would “dramatically affect the atmosphere’s temperature, dynamic, precipitation, and chemistry,” reduce “the light reaching the surface in the Northern Hemisphere by 90% or more,” and could cause “a cooling of continental land areas by up to 30°C.”

The recently released internal study of the “Global-Scale Physical Effects of a Nuclear Exchange” is featured in an update to the National Security Archive’s 2022 Electronic Briefing Book on U.S. government thinking about the possibility of a “nuclear winter,” in which the fire effects of multiple nuclear detonations would produce enough smoke and soot to block sunlight and dramatically lower the Earth’s temperature.

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1983 was a year of increasing tensions between the Cold War superpowers, highlighted by the Soviet shootdown of a South Korean airliner (KAL 007) in September, a Soviet false alarm, and war scares on the Soviet side. It also saw massive protests against NATO missile deployments, and the nuclear freeze movement continued to gain momentum in the United States. At the same time, the scientific community was beginning to focus more intently on the risks of nuclear war. At the close of 1983, a group of scientists published a ground-breaking article in Science magazine asserting that the mass fires caused by nuclear war could inject enough smoke and soot into the atmosphere to obstruct sunlight and dramatically lower temperatures to freezing levels—nuclear winter.

A longer version of the report published in Science had been circulating privately for some months when scientists at Lawrence Livermore National Laboratory (LLNL), one of the two Department of Energy nuclear weapons labs, did its own internal study of the impact of nuclear war on the global climate. LLNL’s August 1983 report focused on the potential cooling effects of a nuclear war, finding that a superpower conflict involving the detonation of more than 6000 nuclear weapons could produce mass fires, both in urban areas and in forests. The fires would introduce a large amount of smoke and soot into the atmosphere, a phenomenon that could “reduce the light reaching the surface in the Northern Hemisphere by 90% or more” and that could “lead to a cooling of continental land areas by up to 30°C.”

The “reference scenario” used for the study posited a war where the total explosive yield of 5300 megatons was spread by some 6300 nuclear warheads. One third of the megatonnage was caused by U.S. warheads and two thirds by Soviet. About 2500 of the megatons were surface bursts with 85 percent of them from Soviet weapons. The authors did not explain why they assumed that the Soviets would detonate most of the surface bursts; it may have been an arbitrary assumption. Another scenario that was considered involved a “significant European theater component” with a total yield of 6500 megatons spread by 10,000 warheads, many of which were “tactical air burst[s].” The European scenario was not discussed further.

To analyze the climate impact of the reference scenario, the authors produced a complex and demanding report covering a wide variety of issues relating to the impact of thousands of nuclear detonations. They included the spread of radioactive debris (fission products), the impact of ultraviolet radiation on the ozone layers, the radiative effects of dust and nitrogen oxides, the fire effects of the nuclear detonations, and the effects of soot emissions on the troposphere. At the heart of the presentation is a section on “Global Climate,” where the authors suggested that the “burning of 13000 Tg [teragrams] of fuel in cities, urban areas, and forests” in the mass fires caused by thousands of nuclear detonations “can dramatically affect the atmosphere’s temperature, dynamic, precipitation, and chemistry.” (A teragram is a unit of mass in the metric system. 13000 teragrams is the equivalent of 13,000,000,000,000,000 grams or over 28 trillion pounds.)

According to the authors, the perturbations caused by the injection of so much soot and smoke into the atmosphere could “lead to modifications of the weather and climate” that could produce “reductions in average Northern Hemisphere land temperatures of 5-10° centigrade and perhaps several times more over mid-continental regions.” The cooling effects could take place within five to 10 days, and the “cooling of mid-continental regions could be substantially larger.” They also projected a decrease in the “precipitation rate of 20-30% over the cooled land region and by 5-10% over ocean region, where atmospheric warming has stabilized the upper troposphere.”

The authors believed that their line of approach involved many uncertainties and unverified assumptions and they pointed them out. For example, the degree of cooling would depend on the elevation of the smoke and the “effect of cloud cover changes.” Moreover, “unknown parameters” included the scale of the “mass loading [of soot and smoke] from fires” and how long particulates would reside in the atmosphere. In addition, the process that filters soot, smoke, and dust from the atmosphere and the mechanisms that transport them “are assumed to be those of an unperturbed atmosphere.” That assumption “was clearly not valid,” according to the study, but there were no “computational tools” at hand to overcome that problem. In addition, the “coupling” of the various climate effects of nuclear war or their “synergistic interactions” required serious analysis.

The authors noted that what stimulated their study and several related ones was a 1982 article by Paul J. Crutzen and John W. Birks titled, “The Atmosphere after a Nuclear War: Twilight at Noon,” that analyzed the impact of the soot produced by nuclear fire effects. The Crutzen-Birks article also inspired a report prepared by Richard P. Turco, Owen Brian Toon, Thomas P. Ackerman, the late James B. Pollack, and the late Carl Sagan (collectively known as TTAPS), who were circulating their March 1983 report, the main findings of which were published later in the year in Science. The National Academy of Sciences also had a study in the works. Although the Livermore report discussed “cooling” rather than freezing or subfreezing and emphasized the uncertainties, its authors found that their research results were in “qualitative accord” with the findings of TTAPS and the ongoing National Academy investigations.[A]

Edward Teller, the founder of LLNL and then its director emeritus, was a major influence in the nuclear weapons field and in the early 1980s was focused on the promotion of the Strategic Defense Initiative. The possibility that nuclear war could have a significant climate impact had never occurred to Teller, who was surprised by the TTAPS findings and wanted to find ways to refute or weaken them. Teller would criticize the nuclear winter concept in an article published in the journal Nature for which he received assistance from LLNL staffers.[B] Yet, Teller, like the LLNL staffers could “not simply discard [the] theory.” As he wrote, the “possibility of nuclear winter has not been excluded.”[C] Michael MacCracken, one of the authors of the 1983 LLNL study and a Teller Ph. D student, would move forward with studies of the cooling impact of nuclear war and the uncertainties of the nuclear winter thesis, although the latter remains highly influential.

In the original release, citing privacy grounds, the Department of Energy withheld the names of the project leaders and participants in this report. The National Security Archive challenged the excisions because so much time had passed since the publication of this report, and DOE duly released the names. The project team leader included MacCracken along with Joyce Penner, Robert Perrett, Bruce Tarter, Frederick Luther, and Joseph Knox.

Lawrence Livermore Laboratory, “LLN Study of the Global-Scale Physical Effects of a Nuclear Exchange: Preliminary Findings,” 15 August 1983, unclassified 
Aug 15, 1983

Source: FOIA release by Department of Energy

• nuclear winter
• Nuclear war
• Climate Change