Nuclear winter

Nuclear winter (also known as atomic winter) is a hypothetical climatic effect of countervalue nuclear war. Models suggest that detonating dozens or more nuclear weapons on cities prone to firestorm, comparable to the Hiroshima bombing of 1945, could have a profound and severe effect on the climate causing cold weather and reduced sunlight for a period of months or even years by the emission of large amounts of the firestorms smoke and soot into the Earth's stratosphere.

Similar climatic effects can be caused by comets or an asteroid impact, also sometimes termed an impact winter, or by a supervolcano eruption, known as a volcanic winter.

Mechanism
The nuclear winter scenario assumes that if 100 or more city firestorms follow the nuclear explosions of a nuclear war, and the firestorms loft large enough amounts of sooty smoke into the upper troposphere and lower stratosphere, soot lifted by the conveyor belt offered by the pyrocumulonimbus clouds that form during a firestorm. At 10 - 15 km above the Earth's surface, the absorption of sunlight could further heat the smoke, lifting some, or all of it, into the stratosphere, where the smoke could persist for years, if there is no rain to wash it out. This aerosol of particles could block out much of the sun's light from reaching the surface, with this causing surface temperatures to drop drastically, and with that, it is predicted surface air temperatures would be akin to, or colder than, a given region's winter, for years on end.

Aerosol removal timescale
The exact timescale for how long this smoke remains, and thus how severely this smoke affects the climate once it reaches the stratosphere, is dependent on both chemical and physical removal processes.

The physical removal mechanisms affecting the timescale of smoke particle removal are how quickly the aerosol particles coagulate, and fall out of the atmosphere via dry deposition, and to a slower degree, the time it takes for solar radiation pressure to move the particles to a lower level in the atmosphere. Whether by coagulation or radiation pressure, once the aerosol of smoke particles are at this lower atmospheric level cloud seeding can begin, permitting precipitation to wash the smoke aerosol out of the atmosphere by the wet deposition mechanism.

The chemical processes that affect the removal are dependent on the ability of atmospheric chemistry to oxidize the smoke, via reactions with oxidative species such as ozone and nitrogen oxides, both of which are found at all levels of the atmosphere. Historical data on residence times of aerosols, albeit a different mixture of aerosols, in this case stratospheric sulfur aerosols and volcanic ash, from megavolcano eruptions, appear to be in the 1-2 year time scale.

Aerosol atmosphere interactions are still poorly understood.

Climatic effects
A study presented at the annual meeting of the American Geophysical Union in December 2006 found that even a small-scale, regional nuclear war could disrupt the global climate for a decade or more. In a regional nuclear conflict scenario where two opposing nations in the subtropics would each use 50 Hiroshima-sized nuclear weapons (about 15 kiloton each) on major populated centres, the researchers estimated as much as five million tons of soot would be released, which would produce a cooling of several degrees over large areas of North America and Eurasia, including most of the grain-growing regions. The cooling would last for years, and according to the research could be "catastrophic".

Ozone depletion
A 2008 study published in the Proceedings of the National Academy of Science found that a nuclear weapons exchange between Pakistan and India using their current arsenals could create a near-global ozone hole, triggering human health problems and causing environmental damage for at least a decade. The computer-modeling study looked at a nuclear war between the two countries involving 50 Hiroshima-sized nuclear devices on each side, producing massive urban fires and lofting as much as five million metric tons of soot about 50 mi into the mesosphere. The soot would absorb enough solar radiation to heat surrounding gases, causing a series of chemical reactions that would break down the stratospheric ozone layer protecting Earth from harmful ultraviolet radiation.

Recent modeling
Based on new work published in 2007 and 2008 by some of the authors of the original studies, several new hypotheses have been put forth.

A minor nuclear war with each country using 50 Hiroshima-sized atom bombs as airbursts on urban areas could produce climate change unprecedented in recorded human history. A nuclear war between the United States and Russia today could produce nuclear winter, with temperatures plunging below freezing in the summer in major agricultural regions, threatening the food supply for most of the planet. The climatic effects of the smoke from burning cities and industrial areas would last for several years, much longer than previously thought. New climate model simulations, which are said to have the capability of including the entire atmosphere and oceans, show that the smoke would be lofted by solar heating to the upper stratosphere, where it would remain for years.

Compared to climate change for the past millennium, even the smallest exchange modeled would plunge the planet into temperatures colder than the Little Ice Age (the period of history between approximately A.D. 1600 and A.D. 1850). This would take effect instantly, and agriculture would be severely threatened. Larger amounts of smoke would produce larger climate changes, and for the 150 teragrams (Tg) case produce a true nuclear winter (1 Tg is 1012 grams), making agriculture impossible for years. In both cases, new climate model simulations show that the effects would last for more than a decade.

2007 study on global nuclear war
A study published in the Journal of Geophysical Research in July 2007, "Nuclear winter revisited with a modern climate model and current nuclear arsenals: Still catastrophic consequences", used current climate models to look at the consequences of a global nuclear war involving most or all of the world's current nuclear arsenals (which the authors judged to be one the size of the world's arsenals twenty years earlier). The authors used a global circulation model, ModelE from the NASA Goddard Institute for Space Studies, which they noted "has been tested extensively in global warming experiments and to examine the effects of volcanic eruptions on climate." The model was used to investigate the effects of a war involving the entire current global nuclear arsenal, projected to release about 150 Tg of smoke into the atmosphere, as well as a war involving about one third of the current nuclear arsenal, projected to release about 50 Tg of smoke. In the 150 Tg case they found that:

"A global average surface cooling of –7 °C to –8 °C persists for years, and after a decade the cooling is still –4 °C (Fig. 2). Considering that the global average cooling at the depth of the last ice age 18,000 yr ago was about –5 °C, this would be a climate change unprecedented in speed and amplitude in the history of the human race. The temperature changes are largest over land ... Cooling of more than –20 °C occurs over large areas of North America and of more than –30 °C over much of Eurasia, including all agricultural regions."

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 one third 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."

Kuwait wells in the first Gulf War
Following Iraq's invasion of Kuwait, Carl Sagan and other scientists predicted that nearly 700 burning oil wells set ablaze by the retreating Iraqi army could cause environmental damage comparable to nuclear winter. The fires were not fully extinguished until November 6, 1991, eight months after the end of the war, and consumed an estimated six million barrels of oil daily.

According to a 1992 study from Peter Hobbs and Lawrence Radke, daily emissions of sulfur dioxide were 57% of that from electric utilities in the United States, emissions of carbon dioxide were 2% of global emissions and emissions of soot were 3,400 metric tons per day. However, pre-war claims of wide scale, long-lasting, and significant global environmental impacts were not borne out and found to be significantly exaggerated by the media and speculators, with climate models at the time of the fires predicting only more localized effects such as a daytime temperature drop of ~10 °C within ~200 km of the source. At the peak of the fires, the smoke absorbed 75% to 80% of the sun’s radiation. The particles were never observed to rise above 6 km and when combined with scavenging by clouds gave the smoke a short residency time in the atmosphere and localized its effects; Professor Carl Sagan of the Turco, Toon, Ackerman, Pollack, Sagan (TTAPS) study hypothesized in January 1991 that enough 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 can lift smoke into the stratosphere, and this is supported by recent evidence that it occurs far more often than previously thought. The study also suggested that the burning of the comparably smaller cities, which would be expected to follow a nuclear strike, would also loft significant amounts of smoke into the stratosphere: "Stenchikov et al. [2006b] conducted detailed, high-resolution smoke plume simulations with the RAMS regional climate model [e.g., Miguez-Macho et al., 2005] and showed that individual plumes, such as those from the Kuwait oil fires in 1991, would not be expected to loft into the upper atmosphere or stratosphere, because they become diluted. However, much larger plumes, such as would be generated by city fires, produce large, undiluted mass motion that results in smoke lofting. New large eddy simulation model results at much higher resolution also give similar lofting to our results, and no small scale response that would inhibit the lofting [Jensen, 2006]."

Eruption of Mt. Pinatubo
The eruption of the Philippines volcano Mount Pinatubo ejected roughly 10 km3 of magma and 17000000 t of SO2, mostly during the explosive Plinian/Ultra-Plinian event of June 15, 1991, creating a global stratospheric SO2 haze layer which persisted for six months. Despite introducing ten times as much SO2 as the Kuwaiti fires, global temperatures dropped by only about 0.5 C-change, and despite a several-month 10% drop in solar irradiation, there was no global impact to agriculture.

Early work
In June 1957, The Effects of Nuclear Weapons by Samuel Glasstone was published containing a section entitled "Nuclear Bombs and the Weather" (pages 69–71), which states: "The dust raised in severe volcanic eruptions, such as that at Krakatoa in 1883, is known to cause a noticeable reduction in the sunlight reaching the earth ... The amount of debris remaining in the atmosphere after the explosion of even the largest nuclear weapons is probably not more than about 1 percent or so of that raised by the Krakatoa eruption. Further, solar radiation records reveal that none of the nuclear explosions to date has resulted in any detectable change in the direct sunlight recorded on the ground."

In 1974, John Hampson suggested that a full-scale nuclear exchange could result in depletion of the ozone shield, possibly subjecting the earth to ultraviolet radiation for a year or more. In 1975, the United States National Research Council (NRC) reported on ozone depletion following nuclear war, judging that the effect of dust would probably be slight climatic cooling.

According to Dr. Vitalii Nikolaevich Tsygichko, a Senior Analyst at the Academy of Sciences, the author of the study, Mathematical Model of Soviet Strategic Operations on the Continental Theater, and a former member of the General Staff, military analysts discussed the idea of a "nuclear winter" (although they did not use that exact term) years before U.S. scientists wrote about it in the 1980s.

1982
In 1981, William J. Moran began discussions and research in the NRC on the dust effects of a large exchange of nuclear warheads. An NRC study panel on the topic met in December 1981 and April 1982.

As part of a study launched in 1980 by Ambio, a journal of the Royal Swedish Academy of Sciences, Paul Crutzen and John Birks circulated a draft paper in early 1982 with the first quantitative evidence of alterations in short-term climate after a nuclear war. In 1982, a special issue of Ambio devoted to the possible environmental consequences of nuclear war included a paper by Crutzen and Birks anticipating the nuclear winter scenario. The paper discussed particulates from large fires, nitrogen oxide, ozone depletion and the effect of nuclear twilight on agriculture. Crutzen and Birks showed that smoke injected into the atmosphere by fires in cities, forests and petroleum reserves could prevent up to 99% of sunlight from reaching the Earth's surface, with major climatic consequences: "The normal dynamic and temperature structure of the atmosphere would therefore change considerably over a large fraction of the Northern Hemisphere, which will probably lead to important changes in land surface temperatures and wind systems." An important implication of their work was that a "first strike" nuclear attack would have severe consequences for the perpetrator.

1983
In 1982, the so-called TTAPS team (Richard P. Turco, Owen Toon, Thomas P. Ackerman, James B. Pollack and Carl Sagan) undertook a computational modeling study of the atmospheric consequences of nuclear war, publishing their results in Science in December 1983. The phrase "nuclear winter" was coined by Turco just prior to publication. In this early work, TTAPS carried out the first estimates of the total smoke and dust emissions that would result from a major nuclear exchange, and determined quantitatively the subsequent effects on the atmospheric radiation balance and temperature structure. To compute dust and smoke impacts, they employed a one-dimensional microphysics/radiative-transfer model of the Earth's lower atmosphere (to the mesopause), which defined only the vertical characteristics of the global climate perturbation.

Around this time, interest in nuclear war environmental effects also arose in the USSR. After becoming aware of the work of the Swedish Academy and, in particular, papers by N.P.Bochkov and E.I.Chazov, Russian atmospheric scientist Georgy Golitsyn applied his research on dust-storms to the situation following a nuclear catastrophe. His suggestion that the atmosphere would be heated and that the surface of the planet would cool appeared in The Herald of the Academy of Sciences in September 1983. Upon learning of the TTAPS scenarios, Vladimir Alexandrov and G. I. Stenchikov soon published a report on the climatic consequences of nuclear war based on simulations with a two-level global circulation model, which produced results consistent with the TTAPS findings.

1986
In 1984 the WMO commissioned Georgy 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–$2 g$ being mostly likely, with a range of 0.2–$6.4 g$ (NAS; TTAPS assumed $2.25$). 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); 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 1987 P. M. Kelly of the University of East Anglia Climatic Research Unit stated that "although there are a handful of vociferous critics, the atmospheric community is united in its conclusion that the threat of nuclear winter is genuine".

1990
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:


 * 10 to 25% of soot injected is immediately removed by precipitation, while the rest is transported over the globe in 1 to 2 weeks


 * SCOPE figures for July smoke injection:
 * 22 °C drop in mid-latitudes
 * 10 °C drop in humid climates
 * 75% decrease in rainfall in mid-latitudes
 * Light level reduction of 0% in low latitudes to 90% in high smoke injection areas


 * SCOPE figures for winter smoke injection:
 * Temperature drops between 3 and 4 °C

Following 1 to 3 years:


 * 25 to 40% of injected smoke is stabilised in atmosphere (NCAR). Smoke stabilised for approximately 1 year.


 * Land temperatures of several degrees below normal


 * Ocean surface temperature between 2 and 6 °C


 * Ozone depletion of 50% leading to 200% increase in UV radiation incident on surface.

Criticism and debate
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 local nuclear war would lead to long lasting global cooling but in a less dramatic manner than a global nuclear war, while a 2007 study of the effects of global nuclear war supported the conclusion that it would lead to full-scale nuclear winter.

The original work by Sagan and others was criticized as a "myth" and "discredited theory" in the 1987 book Nuclear War Survival Skills, a civil defense manual by Cresson Kearny for the Oak Ridge National Laboratory. Kearny said the maximum estimated temperature drop would be only about by 20 degrees Fahrenheit (11 degrees Celsius), and that this amount of cooling would last only a few days. He also suggested that a global nuclear war would indeed result in millions of deaths from hunger, but primarily due to cessation of international food supplies, rather than due to climate changes.

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 than 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 above state that "because of the use of the term 'nuclear autumn' by Thompson and Schneider [1986], 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 above, 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."

Policy implications
During the early 1980s, Fidel Castro recommended to the Kremlin a harder line against Washington, even suggesting the possibility of nuclear strikes. The pressure stopped after Soviet officials gave Castro a briefing on the ecological impact on Cuba of nuclear strikes on the United States.

In an interview in 2000, Mikhail Gorbachev, 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," 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."

As the implications of nuclear winter began to be taken seriously in the late 1980s, military analysts turned their attention to the development of nuclear warheads that would explode at low altitudes and cause less thermal radiation ignited fires, thus reducing the likelihood of a nuclear winter. The TTAPS paper had described a 3000 Mt counterforce attack on ICBM sites; Michael Altfeld of Michigan State University and political scientist Stephen Cimbala of Pennsylvania State University argued that smaller, more accurate warheads and lower detonation heights could produce the same counterforce strike with only 3 Mt and produce less climatic effects, even if cities were targeted, as lower fuzing heights, such as surface bursts, would limit the range of the burning thermal rays due to terrain masking and shadowing, while also temporarily lofting far more radioactive soil into the atmosphere. Therefore as a consequence of attempting to limit the target fire hazard by reducing the range of thermal radiation with fuzing for surface bursts, this will result in a scenario where the far more concentrated, and therefore deadlier, local fallout that is generated following a surface burst forms, as opposed to the comparatively dilute global fallout created when nuclear weapons are fuzed in air burst mode. Altfeld and Cimbala also suggested that belief in the possibility of nuclear winter has actually made nuclear war more likely, contrary to the views of Sagan and others, because it has inspired the development of more accurate, and lower explosive yield, nuclear weapons.

Books
The Cold and the Dark: The World after Nuclear War A book co-authored by Carl Sagan in 1984 which followed his co-authoring of the TTAPS study in 1983.