In the 1980s, work conducted jointly by Western and Soviet scientists showed that for a full-scale nuclear war between the United States and the Soviet Union, the climatic consequences, and indirect effects of the collapse of society, would be so severe that the ensuing nuclear winter would produce famine for billions of people far from the target zones.
There are several wrong impressions that people have about nuclear winter. One is that there was a flaw in the theory and that the large climatic effects were disproven. Another is that the problem, even if it existed, has been solved by the end of the nuclear arms race. But these are both wrong. Furthermore, new nuclear states threaten global climate change even with arsenals that are much less than 1% of the current global arsenal.
The nuclear winter scenario predicts that the huge fires caused by nuclear explosions (particularly from burning urban areas) would loft massive amounts of dark smoke and aerosol particles from the fires into the upper troposphere / stratosphere, at 10-15 kilometers (6-9 miles) above the Earth's surface, the absorption of sunlight would further heat the smoke, lifting it into the stratosphere, a layer where the smoke would persist for years, with no rain to wash it out, and would block out much of the sun's light from reaching the surface, causing surface temperatures to drop drastically.
Column ozone losses could exceed 20% globally, 25-45% at mid-latitudes, and 50-70% at northern high latitudes persisting for 5 years, with substantial losses continuing for 5 additional years. Column ozone amounts would remain near or below 220 Dobson units at all latitudes even after three years, constituting an extra-tropical “ozone hole”. Human health ailments like cataracts and skin cancer, as well as damage to plants, animals and ecosystems at mid-latitudes would likely rise sharply as ozone levels decreased and allowed more harmful UV light to reach Earth, according to the PNAS study. This study demonstrates that a small-scale, regional nuclear conflict is capable of triggering ozone losses even larger than losses that were predicted in the 1980s following a full-scale nuclear war. The missing piece back then was that the models at the time could not account for the rise of the smoke plume and consequent heating of the stratosphere.
Based on new work published in 2007 and 2008 by some of the pioneers of nuclear winter research who worked on the original studies, we now can say several things about this topic.
New Policy Implications:
The climatic effects of smoke from fires started by nuclear war depend on the amount of smoke. For 50 nuclear weapons dropped on two countries, on the targets that would produce the maximum amount of smoke, about 5 megatons (Tg) of black smoke would be produced, accounting for the amount emitted from the fires and the amount immediately washed out in rain. As the smoke is lofted into the stratosphere, it would be transported around the world by the prevailing winds.
Two scenarios of war between the two superpowers who still maintain large nuclear arsenals, the United States and Russia were calculated. In one scenario, 50 Tg of black smoke would be produced and in another, 150 Tg of black smoke would be produced. The number of nuclear weapons required to produce this much smoke depends on the targets, but there are enough weapons in the current arsenals to produce either amount. In fact, there are only so many targets. Once they are all hit by weapons, additional weapons would not produce much more smoke at all.
Even after the current nuclear weapons reduction treaty between these superpowers is played out in 2012, with each having about 2,000 weapons, 150 Tg of smoke could still be produced. These new results were made possible by the use of a state-of-the-art general circulation model of the climate.
For the first time a complete calculation of not only atmospheric but also oceanic circulation was conducted, including the entire atmosphere from the surface up through the troposphere, stratosphere, and mesosphere, to an elevation of 80 kilometers (50 miles). Previous calculations had not been run for the 10 year simulations here, and had not allowed the smoke to be lofted into the upper stratosphere, where it would persist for many years. The climate response to the above scenarios was calculated.
Compared to the global warming observed for the past century, all three scenarios show massive cooling. Compared to the climate change for the Northern Hemisphere for the past 1,000 years, the famous hockey stick diagram, the climate change from any of these scenarios is unprecedented.
Compared to climate change for the past millenium, even the 5 Tg case (a war between India and Pakistan) would plunge the planet into temperatures colder than the Little Ice Age (approximately 1600-1850). This would be essentially instantly, and agriculture would be severely threatened. Larger amounts of smoke would produce larger climate changes, and for the 150 Tg case produce a true nuclear winter, making agriculture impossible for years. In both cases, new climate model simulations show that the effects would last for more than a decade.
In addition, they found that this cooling caused a weakening of the global hydrological cycle, reducing global precipitation by about 45%. As for the 50 Tg case involving 1/3 of current nuclear arsenals, they said that the simulation "produced climate responses very similar to those for the 150 Tg case, but with about half the amplitude", but that "the time scale of response is about the same." They did not discuss the implications for agriculture in depth, but noted that a 1986 study which assumed no food production for a year projected that "most of the people on the planet would run out of food and starve to death by then" and commented that their own results show that "this period of no food production needs to be extended by many years, making the impacts of nuclear winter even worse than previously thought."
Cornell Professor Carl Sagan of the TTAPS study warned in January 1991 that so much smoke from the fires "might get so high as to disrupt agriculture in much of South Asia...." Sagan later conceded in his book The Demon-Haunted World that this prediction did not turn out to be correct: "it was pitch black at noon and temperatures dropped 4°-6°C over the Persian Gulf, but not much smoke reached stratospheric altitudes and Asia was spared."
The 2007 study discussed above noted that modern computer models have been applied to the Kuwait oil fires, finding that individual smoke plumes are not able to loft smoke into the stratosphere, but that smoke from fires covering a large area, like some forest fires or the burning of cities that would be expected to follow a nuclear strike, would loft significant amounts of smoke into the stratosphere:
The work on nuclear winter in the 1980s, and the realization that both direct and indirect effects of nuclear war would be a global catastrophe, led to the end of arms race and the end of the Cold War. In response to the comment "In the 1980s, you warned about the unprecedented dangers of nuclear weapons and took very daring steps to reverse the arms race," in an interview in 2000, Mikhail Gorbachev said "Models made by Russian and American scientists showed that a nuclear war would result in a nuclear winter that would be extremely destructive to all life on Earth; the knowledge of that was a great stimulus to us, to people of honor and morality, to act in that situation."
Since the 1980s, the number of nuclear weapons in the world has decreased to 1/3 of the peak number of more than 70,000. The consequences of regional-scale nuclear conflicts are unexpectedly large, with the potential to become global catastrophes. The combination of nuclear proliferation, political instability, and urban demographics may constitute one of the greatest dangers to the stability of society since the dawn of humans. The current and projected American and Russian nuclear arsenals can still produce nuclear winter. Only nuclear disarmament will prevent the possibility of a nuclear environmental catastrophe.
In 1982 a special issue of the journal Ambio was devoted to the possible environmental consequences of nuclear war; it included an article by Paul Crutzen and J. Birks presenting the rudiments of the nuclear winter scenario ("The atmosphere after a nuclear war: Twilight at noon"; Ambio, 11, 114-125). The issue re-assessed and re-affirmed the consequences for the ozone layer noted in the 1975 National Academies of Science report (up to 70% of the ozone layer might be destroyed) and first raised by Hampson in 1974; and drew attention for the first time to the likelihood that large amounts of smoke and dust would be created.
In 1982 astrophysicist Carl Sagan and his colleagues undertook a computational modeling study of the atmospheric consequences of nuclear war. The report was dubbed "TTAPS" study from the initials of the last names of its authors, R.P. Turco, O.B. Toon, T.P. Ackerman, J.B. Pollack, and C. Sagan.
In December 1983 the "TTAPS" study was published in Science . The study was partly inspired to write the paper both by the suggestions of one Dr. A.M. Salzberg (who, unlike the TTAPS authors, believed that the initial dust thrown into the air would be primarily responsible for the climate changes) and by cooling effects due to dust storms on Mars . To carry out a calculation of the effect they used a very simplified two-dimensional model of the Earth's atmosphere that assumed that conditions at a given latitude were constant. The model also assumed a solid, smooth Earth.
In 1984 the WMO commissioned G. S. Golitsyn and N. A. Phillips to review the state of the science. They found that studies generally assumed a scenario that half of the world's nuclear weapons would be used, ~5000 Mt, destroying approximately 1,000 cities, and creating large quantities of carbonaceous smoke - 1–2 × 1014 grams being mostly likely, with a range of 0.2 – 6.4 × 1014 grams (NAS; TTAPS assumed 2.25 × 1014). The smoke resulting would be largely opaque to solar radiation but transparent to infra-red, thus cooling by blocking sunlight but not causing warming from enhancing the greenhouse effect. The optical depth of the smoke can be much greater than unity. Forest fires resulting from non-urban targets could increase aerosol production further. Dust from near-surface explosions against hardened targets also contributes; each Mt-equivalent of explosion could release up to 5 million tons of dust, but most would quickly fall out; high altitude dust is estimated at 0.1-1 million tons per Mt-equivalent of explosion. Burning of crude oil could also contribute substantially.
The 1-D radiative-convective models used in these studies produced a range of results, with coolings up to 15-42 °C between 14 and 35 days after the war, with a "baseline" of about 20 °C. Somewhat more sophisticated calculations using 3-D GCMs (Alexandrov and Stenchikov (1983); Covey, Schneider and Thompson (1984); which would be considered primitive by modern standards) produced similar results: temperature drops of between 20 and 40 °C, though with regional variations.
All calculations show large heating (up to 80 °C) at the top of the smoke layer at about 10 km; this implies a substantial modification of the circulation there and the possibility of advection of the cloud into low latitudes and the southern hemisphere.
The report made no attempt to compare the likely human impacts of the post-war cooling to the direct deaths from explosions.
In 1990, in a paper entitled "Climate and Smoke: An Appraisal of Nuclear Winter" , TTAPS give a more detailed description of the short- and long-term atmospheric effects of a nuclear war using a three-dimensional model:
First 1 to 3 months:
Following 1 to 3 years:
The TTAPS study was widely reported and criticized in the media. Later model runs in some cases predicted less severe effects, but continued to support the overall conclusion of significant global cooling. Recent studies (2006) substantiate that smoke from urban firestorms in a regional war would lead to long lasting global cooling but in a less dramatic manner than the nuclear winter scenario, while a 2007 study of the effects of global nuclear war supported the conclusion that it would lead to full-scale nuclear winter.
Kearny, who was not a climate scientist himself, based his conclusions almost entirely on the 1986 paper "Nuclear Winter Reappraised by Starley Thompson and Stephen Schneider. However, a 1988 article by Brian Martin in Science and Public Policy states that although their paper concluded the effects would be less severe then originally thought, with the authors describing these effects as a "nuclear autumn", other statements by Thompson and Schneider show that they "resisted the interpretation that this means a rejection of the basic points made about nuclear winter". In addition, the authors of the 2007 study state that "because of the use of the term 'nuclear autumn' by Thompson and Schneider , even though the authors made clear that the climatic consequences would be large, in policy circles the theory of nuclear winter is considered by some to have been exaggerated and disproved [e.g., Martin, 1988]." And in 2007 Schneider emphasized the danger of serious climate changes from a limited nuclear war of the kind analyzed in the 2006 study below, saying "The sun is much stronger in the tropics than it is in mid-latitudes. Therefore, a much more limited war [there] could have a much larger effect, because you are putting the smoke in the worst possible place.
A 1986 article by Russell Seitz in The National Interest reported that prominent physicist Freeman Dyson said of the TTAPS study that it was "an absolutely atrocious piece of science, but I quite despair of setting the public record straight....Who wants to be accused of being in favor of nuclear war?" However, the Brian Martin article mentioned above reported that Dyson had no memory of making this comment, and had said "I don't believe I ever said what Russell Seitz said I said, but I can't prove it." Seitz also mentioned that the Jan. 23, 1986 issue of Nature included a comment that nuclear winter research "has become notorious for its lack of scientific integrity".