thumb|[[Pyrocumulonimbus cloud formed by the firestorm following the atomic bombing of Hiroshima, 1945. Nuclear winter effects are triggered by at least a hundred such city firestorms. to occur after widespread urban firestorms following a large-scale nuclear war. The hypothesis is based on the fact that such fires can inject soot into the stratosphere, where it can block some direct sunlight from reaching the surface of the Earth. It is speculated that the resulting cooling, typically lasting a decade, would lead to widespread crop failure, a global nuclear famine, and an animal mass extinction event.

Climate researchers study nuclear winter via computer models and scenarios. Results are highly dependent on nuclear yields, how many cities are targeted, their flammable material content, and the firestorms' atmospheric environments, convections, and durations. Firestorm case studies include the World War II bombings of Hiroshima, Tokyo, Hamburg, Dresden, and London, and modern observations from large-area wildfires such as the 2021 British Columbia wildfires.

Studies suggest that a full-scale nuclear war, expending thousands of weapons in the largest arsenals in Russia and the United States, could cool global temperatures by more than 5 °C, exceeding the last ice age. According to these models, five billion people would die from famine within two years, and 40–50% of animal species would go extinct. Studies of a regional nuclear war involving hundreds of weapons, such as between India and Pakistan, could also cause cooling of a few degrees, threatening up to two billion people and making 10–20% of animal species extinct. However, many gaps remain in the understanding and modeling the effects of nuclear war.

General

"Nuclear winter", or as it was initially termed, "nuclear twilight", began to be considered as a scientific concept in the 1980s after it became clear that an earlier hypothesis predicting that fireball generated NOx emissions would devastate the ozone layer was losing credibility. In these model scenarios, various soot clouds containing uncertain quantities of soot were assumed to form over cities, oil refineries, and more rural missile silos. Once the quantity of soot is decided upon by the researchers, the climate effects of these soot clouds are then modeled. The term "nuclear winter" was a neologism coined in 1983 by Richard P. Turco in reference to a one-dimensional computer model created to examine the "nuclear twilight" idea. This model projected that massive quantities of soot and smoke would remain aloft in the air for on the order of years, causing a severe planet-wide drop in temperature.

After the failure of the predictions on the effects of the 1991 Kuwait oil fires that were made by the primary team of climatologists that advocate the hypothesis, over a decade passed without new published papers on the topic. More recently, the same team of prominent modellers from the 1980s have begun again to publish the outputs of computer models. These newer models produce the same general findings as their old ones, namely that the ignition of 100 firestorms, each comparable in intensity to that observed in Hiroshima in 1945, could produce a "small" nuclear winter. These firestorms would result in the injection of soot (specifically black carbon) into the Earth's stratosphere, producing an anti-greenhouse effect that would lower the Earth's surface temperature. The severity of this cooling in Alan Robock's model suggests that the cumulative products of 100 of these firestorms could cool the global climate by approximately 1 °C (1.8 °F), largely eliminating the magnitude of anthropogenic global warming for the next roughly two or three years. Robock and his collaborators have modeled the effect on global food production, and project that the injection of more than 5 Tg of soot into the stratosphere would lead to mass food shortages persisting for several years. According to their model, livestock and aquatic food production would be unable to compensate for reduced crop output in almost all countries, and adaptation measures such as food waste reduction would have limited impact on increasing available calories.

[[File:How would a nuclear war between Russia and the US affect you personally? - Future of Life Institute.webm|thumb|Simulation of a nuclear war between Russia and the US based on Xia et al.

A much larger number of firestorms, in the thousands, was the initial assumption of the computer modelers who coined the term in the 1980s. These were speculated to be a possible result of any large scale employment of counter-value airbursting nuclear weapon use during an American-Soviet total war. This larger number of firestorms, which are not in themselves modeled, This cooling would be produced due to a 99% reduction in the natural solar radiation reaching the surface of the planet in the first few years, gradually clearing over the course of several decades. it has been known that firestorms could inject soot smoke and aerosols into the stratosphere, but the longevity of this slew of aerosols was a major unknown. Independent of the team that continue to publish theoretical models on nuclear winter, in 2006, Mike Fromm of the Naval Research Laboratory, experimentally found that each natural occurrence of a massive wildfire firestorm, much larger than that observed at Hiroshima, can produce minor "nuclear winter" effects, with short-lived, approximately one month of a nearly immeasurable drop in surface temperatures, confined to the hemisphere that they burned in. This is somewhat analogous to the frequent volcanic eruptions that inject sulfates into the stratosphere and thereby produce minor, even negligible, volcanic winter effects.

A suite of satellite and aircraft-based firestorm-soot-monitoring instruments are at the forefront of attempts to accurately determine the lifespan, quantity, injection height, and optical properties of this smoke. Information regarding all of these properties is necessary to truly ascertain the length and severity of the cooling effect of firestorms, independent of the nuclear winter computer model projections.

Currently, from satellite tracking data, it appears that stratospheric smoke aerosols dissipate in a time span under approximately two months. are ignited by nuclear explosions,

Although it is common in the climate models to consider city firestorms, these need not be ignited by nuclear devices; more conventional ignition sources can instead be the spark of the firestorms. Prior to the previously mentioned solar heating effect, the soot's injection height is controlled by the rate of energy release from the firestorm's fuel, not the size of an initial nuclear explosion. it is estimated by those with strategic bombing experience that as the city was a firestorm hazard, the same fire ferocity and building damage produced at Hiroshima by one 16-kiloton nuclear bomb from a single B-29 bomber could have been produced instead by the conventional use of about 1.2 kilotons of incendiary bombs from 220 B-29s distributed over the city.

While the firestorms of Dresden and Hiroshima and the mass fires of Tokyo and Nagasaki occurred within mere months in 1945, the more intense and conventionally lit Hamburg firestorm occurred in 1943. Despite the separation in time, ferocity and area burned, leading modelers of the hypothesis state that these five fires potentially placed five percent as much smoke into the stratosphere as the hypothetical 100 nuclear-ignited fires discussed in modern models. and falls out of the atmosphere via gravity-driven dry deposition, and which also occur at greater concentrations when air is heated to high temperatures.

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 one-to-two-year time scale, however aerosol–atmosphere interactions are still poorly understood.

Soot properties

Sooty aerosols can have a wide range of properties, as well as complex shapes, making it difficult to determine their evolving atmospheric optical depth value. The conditions present during the creation of the soot are believed to be considerably important as to their final properties, with soot generated on the more efficient spectrum of burning efficiency considered almost "elemental carbon black," while on the more inefficient end of the burning spectrum, greater quantities of partially burnt/oxidized fuel are present. These partially burnt "organics" as they are known, often form tar balls and brown carbon during common lower-intensity wildfires, and can also coat the purer black carbon particles. However, as the soot of greatest importance is that which is injected to the highest altitudes by the pyroconvection of the firestorm – a fire being fed with storm-force winds of air – it is estimated that the majority of the soot under these conditions is the more oxidized black carbon.

Consequences

thumbnail|left|upright=1.6|Diagram obtained by the [[CIA from the International Seminar on Nuclear War in Italy 1984. It depicts the findings of Soviet 3-D computer model research on nuclear winter from 1983, and although containing similar errors as earlier Western models, it was the first 3-D model of nuclear winter. (The three dimensions in the model are longitude, latitude and altitude.) The diagram shows the models predictions of global temperature changes after a global nuclear exchange. The top image shows effects after 40 days, the bottom after 243 days. A co-author was nuclear winter modelling pioneer Vladimir Alexandrov. Alexandrov disappeared in 1985. As of 2016, there remains ongoing speculation by friend, Andrew Revkin, of foul play relating to his work.]]

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 kilotons each) on major population centers, 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", disrupting agricultural production and food gathering in particular in higher latitude countries.

A 2008 study by Michael J. Mills et al., published in the Proceedings of the National Academy of Sciences, 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-modeled 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 into the stratosphere. The soot would absorb enough solar radiation to heat surrounding gases, increasing the breakdown of the stratospheric ozone layer protecting Earth from harmful ultraviolet radiation, with up to 70% ozone loss at northern high latitudes.

Nuclear summer

A "nuclear summer" is a hypothesized scenario in which, after a nuclear winter caused by aerosols inserted into the atmosphere that would prevent sunlight from reaching lower levels or the surface, has abated, a greenhouse effect then occurs due to carbon dioxide released by combustion and methane released from the decay of the organic matter such as corpses that froze during the nuclear winter.

Another more sequential hypothetical scenario, following the settling out of most of the aerosols in 1–3 years, the cooling effect would be overcome by a heating effect from greenhouse warming, which would raise surface temperatures rapidly by many degrees, enough to cause the death of much if not most of the life that had survived the cooling, much of which is more vulnerable to higher-than-normal temperatures than to lower-than-normal temperatures. The nuclear detonations would release CO<sub>2</sub> and other greenhouse gases from burning, followed by more released from the decay of dead organic matter. The detonations would also insert nitrogen oxides into the stratosphere that would then deplete the ozone layer around the Earth. As charted, yields at least in the megaton range are required to lift dust/fallout into the stratosphere. Ozone reaches its maximum concentration at about 25 km (c. 82,000 ft) in altitude. US high-yield upper atmospheric tests, Teak and Orange were also assessed for their ozone destruction potential.<br /> 0 = Approx altitude commercial aircraft operate<br />1 = Fat Man<br />2 = Castle Bravo]]

In 1952, a few weeks prior to the Ivy Mike (10.4 megaton) bomb test on Elugelab Island, there were concerns that the aerosols lifted by the explosion might cool the Earth. Major Norair Lulejian, USAF, and astronomer Natarajan Visvanathan studied this possibility, reporting their findings in Effects of Superweapons Upon the Climate of the World, the distribution of which was tightly controlled. This report is described in a 2013 report by the Defense Threat Reduction Agency as the initial study of the "nuclear winter" concept. It indicated no appreciable chance of explosion-induced climate change.

The implications for civil defense of numerous surface bursts of high yield hydrogen bomb explosions on Pacific Proving Ground islands such as those of Ivy Mike in 1952 and Castle Bravo (15 Mt) in 1954 were described in a 1957 report on The Effects of Nuclear Weapons, edited by Samuel Glasstone. A section in that book entitled "Nuclear Bombs and the Weather" 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 [soil or other surface] debris remaining in the atmosphere after the explosion of even the largest nuclear weapons is probably not more than about one 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." The US Weather Bureau in 1956 regarded it as conceivable that a large enough nuclear war with megaton-range surface detonations could lift enough soil to cause a new ice age.

The 1966 RAND corporation memorandum The Effects of Nuclear War on the Weather and Climate by E. S. Batten, while primarily analysing potential dust effects from surface bursts, notes that "in addition to the effects of the debris, extensive fires ignited by nuclear detonations might change the surface characteristics of the area and modify local weather patterns ... however, a more thorough knowledge of the atmosphere is necessary to determine their exact nature, extent, and magnitude."

In the United States National Research Council (NRC) book Long-Term Worldwide Effects of Multiple Nuclear-Weapons Detonations published in 1975, it states that a nuclear war involving 4,000 Mt from present arsenals would probably deposit much less dust in the stratosphere than the Krakatoa eruption, judging that the effect of dust and oxides of nitrogen would probably be slight climatic cooling which "would probably lie within normal global climatic variability, but the possibility of climatic changes of a more dramatic nature cannot be ruled out".

In the 1985 report, The Effects on the Atmosphere of a Major Nuclear Exchange, the Committee on the Atmospheric Effects of Nuclear Explosions argues that a "plausible" estimate on the amount of stratospheric dust injected following a surface burst of 1 Mt is 0.3 teragrams, of which 8 percent would be in the micrometer range. The potential cooling from soil dust was again looked at in 1992, in a US National Academy of Sciences (NAS) report on geoengineering, which estimated that about 10<sup>10</sup> kg (10 teragrams) of stratospheric injected soil dust with particulate grain dimensions of 0.1 to 1 micrometer would be required to mitigate the warming from a doubling of atmospheric carbon dioxide, that is, to produce ~2&nbsp;°C of cooling.

In 1969, Paul Crutzen discovered that oxides of nitrogen (NOx) could be an efficient catalyst for the destruction of the ozone layer/stratospheric ozone. Following studies on the potential effects of NOx generated by engine heat in stratosphere flying Supersonic Transport (SST) airplanes in the 1970s, in 1974, John Hampson suggested in the journal Nature that due to the creation of atmospheric NOx by nuclear fireballs, 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, Hampson's hypothesis "led directly" In 1976, a study on the experimental measurements of an earlier atmospheric nuclear test as it affected the ozone layer also found that nuclear detonations are exonerated of depleting ozone, after the at first alarming model calculations of the time. Similarly, a 1981 paper found that the models on ozone destruction from one test and the physical measurements taken were in disagreement, as no destruction was observed.

In total, about 500 Mt were atmospherically detonated between 1945 and 1971, peaking in 1961–1962, when 340 Mt were detonated in the atmosphere by the United States and Soviet Union. During this peak, with the multi-megaton range detonations of the two nations nuclear test series, in exclusive examination, a total yield estimated at 300 Mt of energy was released. Due to this, 3 × 10<sup>34</sup> additional molecules of nitric oxide (about 5,000 tons per Mt, 5 × 10<sup>9</sup> grams per megaton) are believed to have entered the stratosphere, and while ozone depletion of 2.2 percent was noted in 1963, the decline had started prior to 1961 and is believed to have been caused by other meteorological effects. However, Martin ultimately concludes that it is "unlikely that in the context of a major nuclear war" ozone degradation would be of serious concern. Martin describes views about potential ozone loss and therefore increases in ultraviolet light leading to the widespread destruction of crops, as advocated by Jonathan Schell in The Fate of the Earth, as highly unlikely.

Science fiction

The first published suggestion that cooling of the climate could be an effect of a nuclear war, appears to have been originally put forth by Poul Anderson and F. N. Waldrop in their story "Tomorrow's Children", in the March 1947 issue of the Astounding Science Fiction magazine. The story, primarily about a team of scientists hunting down mutants, warns of a "Fimbulwinter" caused by dust that blocked sunlight after a recent nuclear war and speculated that it may even trigger a new Ice Age. Anderson went on to publish a novel based partly on this story in 1961, titling it Twilight World. However, "the computer models were so simplified, and the data on smoke and other aerosols were still so poor, that the scientists could say nothing for certain". launched in 1980 by Ambio, a journal of the Royal Swedish Academy of Sciences, Paul J. Crutzen and John W. Birks began preparing for the 1982 publication of a calculation on the effects of nuclear war on stratospheric ozone, using the latest models of the time. However, they found that as a result of the trend towards more numerous but less energetic, sub-megaton range nuclear warheads (made possible by the march to increase ICBM warhead accuracy), the ozone layer danger was "not very significant". published in the same edition of Ambio that carried Crutzen and Birks's paper "Twilight at Noon", Soviet atmospheric scientist Georgy Golitsyn applied his research on Mars dust storms to soot in the Earth's atmosphere. The use of these influential Martian dust storm models in nuclear winter research began in 1971, when the Soviet spacecraft Mars 2 arrived at the red planet and observed a global dust cloud. The orbiting instruments together with the 1971 Mars 3 lander determined that temperatures on the surface of the red planet were considerably colder than temperatures at the top of the dust cloud. Following these observations, Golitsyn received two telegrams from astronomer Carl Sagan, in which Sagan asked Golitsyn to "explore the understanding and assessment of this phenomenon". Golitsyn recounts that it was around this time that he had "proposed a theory to explain how Martian dust may be formed and how it may reach global proportions." an employee in Golitsyn's institute, developed a model of dust storms to describe the cooling phenomenon on Mars. Golitsyn felt that his model would be applicable to soot after he read a 1982 Swedish magazine dedicated to the effects of a hypothetical nuclear war between the USSR and the US. Having gained this committees approval, in September 1983, Golitsyn published the first computer model on the nascent "nuclear winter" effect in the widely read Herald of the Russian Academy of Sciences.

On 31 October 1982, Golitsyn and Ginsburg's model and results were presented at the conference on "The World after Nuclear War", hosted in Washington, D.C. had been interested in the cooling on the dust storms on the planet Mars in the years preceding their focus on "nuclear winter". Sagan had also worked on Project A119 in the 1950s–1960s, in which he attempted to model the movement and longevity of a plume of lunar soil.

After the publication of "Twilight at Noon" in 1982, the TTAPS team have said that they began the process of doing a 1-dimensional computational modeling study of the atmospheric consequences of nuclear war/soot in the stratosphere, though they would not publish a paper in Science magazine until late-December 1983. The phrase "nuclear winter" had been coined by Turco just prior to publication. In this early paper, TTAPS used assumption-based estimates on the total smoke and dust emissions that would result from a major nuclear exchange, and with that, began analyzing the subsequent effects on the atmospheric radiation balance and temperature structure as a result of this quantity of assumed smoke. To compute dust and smoke effects, they employed a one-dimensional microphysics/radiative-transfer model of the Earth's lower atmosphere (up to the mesopause), which defined only the vertical characteristics of the global climate perturbation.

Interest in the environmental effects of nuclear war, however, had continued in the Soviet Union after Golitsyn's September paper, with Vladimir Alexandrov and G. I. Stenchikov also publishing a paper in December 1983 on the climatic consequences, although in contrast to the contemporary TTAPS paper, this paper was based on simulations with a three-dimensional global circulation model.

The 1-D radiative-convective models used in these studies produced a range of results, with cooling up to 15–42&nbsp;°C between 14 and 35 days after the war, with a "baseline" of about 20&nbsp;°C. Somewhat more sophisticated calculations using 3-D GCMs produced similar results: temperature drops of about 20&nbsp;°C, though with regional variations.

All calculations show large heating (up to 80&nbsp;°C) at the top of the smoke layer at about ; this implies a substantial modification of the circulation there and the possibility of advection of the cloud into low latitudes and the southern hemisphere.

1990

In a 1990 paper entitled "Climate and Smoke: An Appraisal of Nuclear Winter", TTAPS gave a more detailed description of the short- and long-term atmospheric effects of a nuclear war using a three-dimensional model:

First one to three months:

  • 10–25% of soot injected is immediately removed by precipitation, while the rest is transported over the globe in one to two weeks
  • SCOPE figures for July smoke injection:
  • 22&nbsp;°C drop in mid-latitudes
  • 10&nbsp;°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&nbsp;°C

Following one to three years:

  • 25–40% of injected smoke is stabilised in atmosphere (NCAR). Smoke stabilised for approximately one year.
  • Land temperatures of several degrees below normal
  • Ocean surface temperature between 2 and 6&nbsp;°C
  • Ozone depletion of 50% leading to 200% increase in UV radiation incident on surface.

Kuwait wells in the first Gulf War

thumb| The [[Kuwaiti oil fires were not just limited to burning oil wells, one of which is seen here in the background, but burning "oil lakes", seen in the foreground, also contributed to the smoke plumes, particularly the sootiest/blackest of them.]]

thumb|Smoke plumes from a few of the [[Kuwaiti Oil Fires on April 7, 1991. The maximum assumed extent of the combined plumes from over six hundred fires during the period of February 15 – May 30, 1991, are available. Only about 10% of all the fires, mostly corresponding with those that originated from "oil lakes" produced pure black soot filled plumes, 25% of the fires emitted white to grey plumes, while the remaining emitted plumes with colors between grey and black.

Following Iraq's invasion of Kuwait and Iraqi threats of igniting the country's approximately 800 oil wells, speculation on the cumulative climatic effect of this, presented at the World Climate Conference in Geneva that November in 1990, ranged from a nuclear winter type scenario, to heavy acid rain and even short term immediate global warming.

In articles printed in the Wilmington Morning Star and The Baltimore Sun newspapers in January 1991, prominent authors of nuclear winter papers – Richard P. Turco, John W. Birks, Carl Sagan, Alan Robock and Paul Crutzen – collectively stated that they expected catastrophic nuclear winter like effects with continental-sized effects of sub-freezing temperatures as a result of the Iraqis going through with their threats of igniting 300 to 500 pressurized oil wells that could subsequently burn for several months.

As threatened, the wells were set on fire by the retreating Iraqis in March 1991, and the 600 or so burning oil wells were not fully extinguished until November 6, 1991, eight months after the end of the war, and they consumed an estimated six million barrels of oil per day at their peak intensity.

When Operation Desert Storm began in January 1991, coinciding with the first few oil fires being lit, Dr. S. Fred Singer and Carl Sagan discussed the possible environmental effects of the Kuwaiti petroleum fires on the ABC News program Nightline. Sagan again argued that some of the effects of the smoke could be similar to the effects of a nuclear winter, with smoke lofting into the stratosphere, beginning around above sea level in Kuwait, resulting in global effects. He also argued that he believed the net effects would be very similar to the 1815 eruption of Mount Tambora in Indonesia, which resulted in the year 1816 being known as the "Year Without a Summer".

Sagan listed modeling outcomes that forecast effects extending to South Asia, and perhaps to the Northern Hemisphere as well. Sagan stressed this outcome was so likely that "It should affect the war plans." Singer, on the other hand, anticipated that the smoke would go to an altitude of about and then be rained out after about three to five days, thus limiting the lifetime of the smoke. Both height estimates made by Singer and Sagan turned out to be wrong, albeit with Singer's narrative being closer to what transpired, with the comparatively minimal atmospheric effects remaining limited to the Persian Gulf region, with smoke plumes, in general,<!--The reference for this is a hardcopy transcript of the episode, excerpts on the Fred Singer talk page.-->

Sagan and his colleagues expected that a "self-lofting" of the sooty smoke would occur when it absorbed the sun's heat radiation, with little to no scavenging occurring, whereby the black particles of soot would be heated by the sun and lifted/lofted higher and higher into the air, thereby injecting the soot into the stratosphere, a position where they argued it would take years for the sun-blocking effect of this aerosol of soot to fall out of the air, and with that, catastrophic ground level cooling and agricultural effects in Asia and possibly the Northern Hemisphere as a whole. In a 1992 follow-up, Peter V. Hobbs and others had observed no appreciable evidence for the nuclear winter team's predicted massive "self-lofting" effect and the oil-fire smoke clouds contained less soot than the nuclear winter modelling team had assumed.

The atmospheric scientist tasked with studying the atmospheric effect of the Kuwaiti fires by the National Science Foundation, Peter V. Hobbs, stated that the fires' modest impact suggested that "some numbers [used to support the Nuclear Winter hypothesis]... were probably a little overblown."

Hobbs found that at the peak of the fires, the smoke absorbed 75 to 80% of the sun's radiation. The particles rose to a maximum of , and when combined with scavenging by clouds the smoke had a short residency time of a maximum of a few days in the atmosphere.

Pre-war claims of wide scale, long-lasting, and significant global environmental effects were thus not borne out, and found to be significantly exaggerated by the media and speculators, with climate models by those not supporting the nuclear winter hypothesis at the time of the fires predicting only more localized effects such as a daytime temperature drop of ~10&nbsp;°C within 200&nbsp;km of the source.

thumb|This satellite photo of the south of [[United Kingdom|Britain shows black smoke from the 2005 Buncefield fire, a series of fires and explosions involving approximately 250,000,000 litres of fossil fuels. The plume is seen spreading in two main streams from the explosion site at the apex of the inverted 'v'. By the time the fire had been extinguished the smoke had reached the English Channel. The orange dot is a marker, not the actual fire. Although the smoke plume was from a single source, and larger in size than the individual oil well fire plumes in Kuwait 1991, the Buncefield smoke cloud remained out of the stratosphere.]]

Sagan later conceded in his book The Demon-Haunted World that his predictions obviously did not turn out to be correct: "it was pitch black at noon and temperatures dropped 4–6&nbsp;°C over the Persian Gulf, but not much smoke reached stratospheric altitudes and Asia was spared."

The idea of oil well and oil reserve smoke pluming into the stratosphere serving as a main contributor to the soot of a nuclear winter was a central idea of the early climatology papers on the hypothesis; they were considered more of a possible contributor than smoke from cities, as the smoke from oil has a higher ratio of black soot, thus absorbing more sunlight. 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:

However, the above simulation notably contained the assumption that no dry or wet deposition would occur.

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 1600 and 1850 AD). This would take effect instantly, and agriculture would be severely threatened. Larger amounts of smoke would produce larger climate changes, making agriculture impossible for years. In both cases, new climate model simulations show that the effects would last for more than a decade. The authors used computational models developed by NCAR to simulate the climatic effects of a soot cloud that they suggest would be a result of a regional nuclear war in which 100 "small" (15 Kt) weapons are detonated over cities. The model had outputs, due to the interaction of the soot cloud:

<blockquote>...global ozone losses of 20–50% over populated areas, levels unprecedented in human history, would accompany the coldest average surface temperatures in the last 1000 years. We calculate summer enhancements in UV indices of 30–80% over Mid-Latitudes, suggesting widespread damage to human health, agriculture, and terrestrial and aquatic ecosystems. Killing frosts would reduce growing seasons by 10–40 days per year for 5 years. Surface temperatures would be reduced for more than 25 years, due to thermal inertia and albedo effects in the ocean and expanded sea ice. The combined cooling and enhanced UV would put significant pressures on global food supplies and could trigger a global nuclear famine.</blockquote>

2018

Researchers at Los Alamos National Laboratory published the results of a multi-scale study of the climate impact of a regional nuclear exchange, the same scenario considered by Robock et al. and by Toon et al. in 2007. Unlike previous studies, this study simulated the processes whereby black carbon would be lofted into the atmosphere and found that very little would be lofted into the stratosphere and, as a result, the long-term climate impacts were much lower than those studies had concluded. In particular, "none of the simulations produced a nuclear winter effect", and "the probability of significant global cooling from a limited exchange scenario as envisioned in previous studies is highly unlikely". This study has been contradicted by results in several subsequent studies claiming the 2018 study to be flawed.

Research published in the peer-reviewed journal Safety suggested that no nation should possess more than 100 nuclear warheads because of the blowback effect on the aggressor nation's own population because of "nuclear autumn".

2019

2019 saw the publication of two studies on nuclear winter that build on previous modeling and describe new scenarios of nuclear winter from smaller exchanges of nuclear weapons than have been previously simulated.

As in the 2007 study by Robock et al., This amount of black carbon far exceeds that which has been emitted in the atmosphere by all volcanic eruptions in the past 1,200 years but is less than the asteroid impact which caused a mass extinction event 66 million years ago.

2022

thumb|Percent of the world's population dead from a nuclear war per simulations by Xia et al. (2022, see esp. their Table 1) The bottom axis is the total megatonnage (number of nuclear weapons used times average yield) simulated to produce the quantity of soot plotted on the top axis. "IND-PAK" marks a range of hypothetical nuclear wars between [[India and weapons of mass destruction|India (IND) and Pakistan (PAK). "USA-RUS" marks a simulated nuclear war between the US (USA) and Russia (RUS). "PRK" = a simulated nuclear war in which North Korea (the People's Republic of Korea, PRK) used their existing nuclear arsenal estimated at 30 weapons with an average yield of 17 kt.]]

According to a peer-reviewed study published in the journal Nature Food in August 2022,

Another paper published that year, from the Tohoku University Earth science scholar Kunio Kaiho, compared the impact of nuclear winter scenarios on marine and terrestrial animal life with that of historical extinction events. Kaiho estimated that a minor nuclear war (which he defined as a nuclear exchange between India and Pakistan or an event of equivalent magnitude) would cause extinctions of 10–20% of species on its own, while a major nuclear war (defined as a nuclear exchange between United States and Russia) would cause the extinctions of 40–50% of animal species, which is comparable to some of the "Big Five" mass extinction events. For comparison, what he considered the most likely scenario of anthropogenic climate change, with of warming by 2100 and by 2500, would send around 12–14% of animal species extinct under the same methodology.

2023

Since 2023, the U.S. National Academies of Science, Engineering, and Medicine has established an Independent Study on Potential Environmental Effects of Nuclear War. The aim is to evaluate all research on nuclear winter, and the final report was planned for a 2024 release date.

the committee was still working on the report.

2025 study - Impact on global agriculture

In 2025, researchers at Pennsylvania State University used the Cycles agroecosystem model to simulate how a nuclear winter could impact global corn yields (Zea mays), treating corn as a proxy for global staple crops. The study modeled production across 38,572 locations under six scenarios of soot injection into the upper atmosphere, ranging from about 5 million to 165 million tonnes.

A regional nuclear war (~5.5 Mt soot) could reduce worldwide corn production by about 7%, while a full-scale global conflict (~165 Mt soot) might cut yields by around 80%.

The researchers also estimated that ozone depletion following a large-scale conflict would increase ultraviolet-B radiation, peaking six to eight years later, causing an additional ~7 % decline in corn yields. In the worst-case scenario, this would bring the total reduction to roughly 87%.

Global agricultural recovery was projected to take between seven and twelve years, depending on the severity of the conflict and location, with longer delays at higher latitudes.

  • Would cities readily firestorm, and if so how much soot would be generated?
  • Atmospheric longevity: would the quantities of soot assumed in the models remain in the atmosphere for as long as projected or would far more soot precipitate as black rain much sooner?
  • Timing of events: how reasonable is it for the modeling of firestorms or war to commence in late spring or summer (this is done in almost all US-Soviet nuclear winter papers, thereby giving rise to the largest possible degree of modeled cooling)?
  • Darkness and opacity: how much light-blocking effect the assumed quality of the soot reaching the atmosphere would have? most models continue to suggest that some deleterious global cooling would still result, under the assumption that a large number of fires occurred in the spring or summer. Starley L. Thompson's less primitive mid-1980s 3-dimensional model, which notably contained the very same general assumptions, led him to coin the term "nuclear autumn" to more accurately describe the climate results of the soot in this model, in an on camera interview in which he dismisses the earlier "apocalyptic" models.

A major criticism of the assumptions that continue to make these model results possible appeared in the 1987 book Nuclear War Survival Skills (NWSS), a civil defense manual by Cresson Kearny for the Oak Ridge National Laboratory. According to the 1988 publication An assessment of global atmospheric effects of a major nuclear war, Kearny's criticisms were directed at the excessive amount of soot that the modelers assumed would reach the stratosphere. Kearny cited a Soviet study that modern cities would not burn as firestorms, as most flammable city items would be buried under non-combustible rubble and that the TTAPS study included a massive overestimate on the size and extent of non-urban wildfires that would result from a nuclear war. This was done in an effort to convey to his readers that contrary to the popular opinion at the time, in the conclusion of these two climate scientists, "on scientific grounds the global apocalyptic conclusions of the initial nuclear winter hypothesis can now be relegated to a vanishing low level of probability". show that they, "resisted the interpretation that this means a rejection of the basic points made about nuclear winter". In the Alan Robock et al. 2007 paper, they write 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]."

The contribution of smoke from the ignition of live non-desert vegetation, living forests, grasses and so on, nearby to many missile silos is a source of smoke originally assumed to be very large in the initial "Twilight at Noon" paper, and also found in the popular TTAPS publication. However, this assumption was examined by Bush and Small in 1987 and they found that the burning of live vegetation could only conceivably contribute very slightly to the estimated total "nonurban smoke production". This reduction in the estimate of the non-urban smoke hazard is supported by the earlier preliminary Estimating Nuclear Forest Fires publication of 1984, and Operation Redwing test series.

thumb|During the [[Bombing of Tokyo|Operation Meeting House firebombing of Tokyo on 9–10 March 1945, 1,665 tons (1.66 kilotons) of incendiary and high-explosive bombs in the form of bomblets were dropped on the city, causing the destruction of over 10,000 acres of buildings – , the most destructive and deadliest bombing operation in history.]]

thumb|The first nuclear bombing in history used a [[little boy|16-kiloton nuclear bomb, approximately 10 times as much energy as delivered onto Tokyo, yet due in part to the comparative inefficiency of larger bombs, a much smaller area of building destruction occurred when contrasted with the results from Tokyo. Only of Hiroshima was destroyed by blast, fire, and firestorm effects. Similarly, Major Cortez F. Enloe, a surgeon in the USAAF who worked with the United States Strategic Bombing Survey (USSBS), noted that the even more energetic 22-kiloton nuclear bomb dropped on Nagasaki did not result in a firestorm and thus did not do as much fire damage as the conventional airstrikes on Hamburg which did generate a firestorm. Thus, whether a city will firestorm depends primarily not on the size or type of bomb dropped, but rather on the density of fuel present in the city. Moreover, it has been observed that firestorms are not likely in areas where modern buildings (constructed of bricks and concrete) have totally collapsed. By comparison, Hiroshima, and Japanese cities in general in 1945, had consisted of mostly densely-packed wooden houses along with the common use of shoji paper sliding walls. The fire hazard construction practices present in cities that have historically firestormed are now illegal in most countries for general safety reasons, and therefore cities with firestorm potential are far rarer than was common at the time of World War II.]]

A paper by the United States Department of Homeland Security, finalized in 2010, states that after a nuclear detonation targeting a city "If fires are able to grow and coalesce, a firestorm could develop that would be beyond the abilities of firefighters to control. However experts suggest in the nature of modern US city design and construction may make a raging firestorm unlikely". The nuclear bombing of Nagasaki for example, did not produce a firestorm. This was similarly noted as early as 1986–1988, when the assumed quantity of fuel "mass loading" (the amount of fuel per square meter) in cities underpinning the winter models was found to be too high and intentionally creates heat fluxes that loft smoke into the lower stratosphere, yet assessments "more characteristic of conditions" to be found in real-world modern cities, had found that the fuel loading, and hence the heat flux that would result from efficient burning, would rarely loft smoke much higher than 4&nbsp;km. Following his investigation into the Siberian fire of 1915, Seitz criticized the "nuclear winter" model results for being based on successive worst-case events: