Nuclear winter

Nuclear winter (also known as atomic winter) is a hypothetical climatic effect, most often considered a potential threat following a countervalue, or city-targeted, nuclear war. Climate models suggest that the ignition of a hundred or more firestorms that are comparable in intensity to that observed in Hiroshima in 1945 would produce a small nuclear winter. The burning of these firestorms would result in the injection of soot into the Earth's stratosphere, producing an anti-greenhouse effect. The models conclude that the magnitude of this effect from the cumulative products of 100 firestorms would reach sufficient extent to unmistakably alter the global climate, resulting in agricultural losses from the colder weather, and lasting for a period of years, whereas an all-out US-Russia war would cause catastrophic summer cooling by about 20 °C in core agricultural regions of the US, Europe and China, and by as much as 35 °C in Russia.

On the fundamental level, it is known that firestorms can inject sooty smoke into the stratosphere, as each natural occurrence of a wildfire firestorm has been found to "surprisingly frequently" generate minor "nuclear winter" effects. This is somewhat analogous to the frequent volcanic eruptions that inject sulfates into the stratosphere and thereby produce minor 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 ascertain both the length and depth of the cooling effect of firestorms.

Mechanism

The nuclear winter scenario assumes that 100 or more city firestorms are ignited by the nuclear explosions of a nuclear war, and the firestorms lift large enough amounts of sooty smoke into the upper troposphere and lower stratosphere, soot lifted by the movement offered by the pyrocumulonimbus clouds that form during a firestorm. At 10â€"15 kilometres (6â€"9 miles) above the Earth's surface, the absorption of sunlight could further heat the soot in 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 heat the stratosphere and block out a portion of the sun's light from reaching the surface, 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 months to years on end.

The modeled stable inversion layer of hot soot between the troposphere and high stratosphere that produces the anti-greenhouse effect was dubbed the "Smokeosphere" by Stephen Schneider et al. in their 1988 paper.

Although it is common in the climate models for the city firestorms to be ignited by nuclear explosions, they need not be ignited by nuclear devices; more conventional ignition sources can instead be the spark of the firestorms. As 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, or lack thereof, of an initial nuclear explosion. For example, the mushroom cloud from the bomb dropped on Hiroshima reached a height of six kilometers (middle troposphere) within a few minutes and then dissipated due to winds, while the individual fires within the city took almost three hours to form into a firestorm and produce a "pyrocumulus" cloud, a cloud that is assumed to have reached upper tropospheric heights, as over its multiple hours of burning, the firestorm released an estimated 1000 times the energy of the bomb.

While the firestorm of Dresden and Hiroshima and the mass fires of Tokyo and Nagasaki occurred with mere months separating them in 1945, the more intense and conventionally lit Hamburg firestorm occurred in 1943. Despite this, these five fires potentially placed five percent as much smoke into the stratosphere as the hypothetical 100 nuclear-ignited fires of modern models. While it is believed that the effects of the mass of soot emitted by 100 firestorms (one to five teragrams) would have been detectable with technical instruments in WWII, only five percent of that would not have been possible to observe at that time.

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 most important physical removal mechanism is "rainout", both during the "fire-driven convective column" phaseâ€"which produces "black rain" near the fire siteâ€"and rainout after the convective plume's dispersal, where the smoke is no longer concentrated and thus "wet removal" is believed to be "very efficient." However these efficient removal mechanisms in the troposphere are avoided in the Robock 2007 study, where solar heating is modeled to quickly "loft" the soot into the stratosphere, "detraining" or separating the darker soot particles from the fire clouds' whiter water condensation.

Once in the stratosphere, the physical removal mechanisms having an impact on the timescale of the soot particles' residence are how quickly the aerosol of soot coagulates with other particles, and falls out of the atmosphere via gravity-driven dry deposition, and, to a slower degree, the time it takes for solar radiation pressure to force 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 carbonaceous component of the smoke, via reactions with oxidative species such as ozone and nitrogen oxides, both of which are found at all levels of the atmosphere, and which also occur at greater concentrations when air is heated to high temperatures, which will be discussed later.

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.

The satellite tracking of wildfire smoke from pyrocumulonimbus-cloud-injected aerosols indicates that the aerosols are removed in a time span under approximately two months.

Aerosolâ€"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 optical properties. 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 carbon black particles. However, as the soot of greatest importance is that which is injected to the highest altitudes by the pyroconvection of the firestormâ€"a fire being fed with storm-force winds of airâ€"it is estimated that the majority of the soot under these conditions is of the more oxidized carbon black nature.

Consequences

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 miles (80 km) 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.

Nuclear summer

A "nuclear summer" is a hypothesized scenario in which, after a nuclear winter has abated, a greenhouse effect then occurs due to CO₂ released by combustion and methane released from decay of dead organic matter.

History

Early work

In 1952, a few weeks prior to the Ivy Mike(10.4 megaton) test on Elugelab island, there was a concern that the "small particles"/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. According to a document by the Defense Threat Reduction Agency, this report was the initial study of the "nuclear winter" concept that was popularized by others decades later. It indicated no appreciable chance of explosion-induced climate change.

Following numerous surface bursts of high yield "Hydrogen bomb" explosions on Pacific Proving Ground islands such as those of Ivy Mike in the year 1952 and Castle Bravo(15 megaton) in 1954, The Effects of Nuclear Weapons by Samuel Glasstone was published in 1957 which contained a section entitled "Nuclear Bombs and the Weather" (pages 69â€"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."

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 10¹⁰ kg 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 CO₂, that is, to produce ~ 2 degree celsius of cooling.

In 1969, Paul Crutzen discovered that NOx (oxides of nitrogen) could be an efficient catalyst for the destruction of the ozone layer/stratospheric ozone. With studies on the potential effects of NOx generated by engine heat in stratosphere flying Supersonic Transport(SST) airplanes in the 1970s serving as a backdrop, John Hampson in 1974 suggested in the journal Nature that due to the nuclear fireballs creation of atmospheric NOx, 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. Hampson's hypothesis "led directly", in 1975, to the United States National Research Council (NRC) reporting on the models of ozone depletion following nuclear war in the book Long-Term Worldwide Effects of Multiple Nuclear-Weapons Detonations. In this 1975 book it states that a nuclear war involving 4000Mt (megaton) 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".

A study published in 1976 on the experimental measurements of an earlier atmospheric nuclear test as it affected the ozone layer found that nuclear detonations are tentatively exonerated in depleting ozone, after initially discouraging model calculations. In total about 500 megatons were atmospherically detonated between 1945 and 1971, with a peak occurring in 1961-62, when 340 megatons were detonated in the atmosphere by the United States and Soviet Union. During this 1-2 year peak, counting only the multi-megaton range detonations in the two nations nuclear test series, a total yield estimated at 300 megatons of energy was released, due to this, 3 x 10^34 additional molecules of nitric oxide(about 5000 tons 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, thus the 1985 book The Effects on the Atmosphere of a Major Nuclear Exchange states: "one can not draw definite conclusions about the effects of nuclear explosions on stratospheric ozone".

Science Fiction

The first published suggestion that a cooling of climate or winter could be an effect of a nuclear war, appears to have been originally put forth by Poul Anderson and F.N Waldrop in their post war story "Tomorrow's Children", which appeared in the March 1947 issue of the Astounding Science Fiction magazine, the story which is primarily about a team of scientists hunting down mutants, warns of a "Fimbulwinter" caused by dust that blocked sunlight after the recent fictitious nuclear war and speculates that this 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.

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 in preparation of the release of The Effects on the Atmosphere of a Major Nuclear Exchange in 1985.

As part of a study on the creation of oxidizing species such as NOx and ozone in the troposphere after a nuclear war, 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 by Crutzen and Birks titled "Twilight at Noon" anticipating the nuclear winter scenario. The paper which looked into fires and their climatic effect as "an afterthought" discussed particulate matter from large fires, nitrogen oxide, ozone depletion and the effect of nuclear twilight on agriculture. Crutzen and Birks' calculations suggested that smoke particulates 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

Interest in nuclear war environmental effects also arose in the USSR. After becoming aware of the 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 large nuclear war. 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.

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.

Upon learning of the TTAPS scenarios, Vladimir Alexandrov and G. I. Stenchikov also published a report in 1983 on the climatic consequences of nuclear war based on simulations with a three-dimensional global circulation model. Two years later Vladimir Alexandrov disappeared under mysterious circumstances. Richard Turco and Starley L. Thompson were critical of the Soviet model, Turco claimed it was "a primitive rendition of an obsolete US model".

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 - 1â€"7011200000000000000â™ 2×10¹⁴ g being mostly likely, with a range of 0.2â€"7011640000000000000â™ 6.4×10¹⁴ g (NAS; TTAPS assumed 7014225000000000000â™ 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 15â€"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.

Kuwait wells in the first Gulf War

Following Iraq's invasion of Kuwait and Iraqi threats of igniting the country's 800 or so oil wells were made, 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. As threatened, the wells were set ablaze by the retreating Iraqis by March 1991 and the 600 or so successfully set Kuwaiti 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 daily at their peak intensity.

In articles printed in the Wilmington morning star and the Baltimore Sun newspapers of January 1991, prominent authors of nuclear winter papers - Richard P. Turco, John W. Birks, Carl Sagan, Alan Robock and Paul Crutzen together collectively stated that they expected catastrophic nuclear winter like effects with continental sized impacts of "sub-freezing" temperatures as a result of if the Iraqis went through with their threats of igniting 300 to 500 pressurized oil wells and they burned for a few months.

Later when Operation Desert Storm had begun in late January 1991, coinciding with the first few oil fires being lit, Dr. S. Fred Singer and Carl Sagan discussed the possible environmental impacts 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, a region of the atmosphere beginning around 48,000 feet (15,000 m) above sea level at Kuwait, resulting in global effects and that he believed the net effects would be very similar to the explosion of the Indonesian volcano Tambora in 1815, which resulted in the year 1816 being known as the Year Without a Summer.

He reported on initial modeling estimates that forecast impacts 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, said that his calculations showed that the smoke would go to an altitude of about 3,000 feet (910 m) and then be rained out after about three to five days and thus the lifetime of the smoke would be limited. Both height estimates made by Singer and Sagan turned out to be wrong, albeit with Singers narrative being closer to what transpired, with the comparatively minimal atmospheric effects remaining limited to the Persian Gulf region, with smoke plumes, in general, lofting to about 10,000 feet (3,000 m) and a few times as high as 20,000 feet (6,100 m).

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 4â€"6 °C over the Persian Gulf, but not much smoke reached stratospheric altitudes and Asia was spared."

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 impacts in Asia and possibly the Northern Hemisphere as a whole.

The Atmospheric scientist tasked with studying the atmospheric impact of the Kuwaiti fires by the National Science Foundation, Peter 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 20,000 feet (6,100 m), 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 impacts 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 °C within ~200 km of the source.

The idea of oil well and oil reserve smoke pluming to the stratosphere serving as a main contributor to the soot of a nuclear winter was a central tenet 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. Hobbs compared the papers' assumed "emission factor" or soot generating efficiency from ignited oil pools and found, upon comparing to measured values from oil pools at Kuwait, which were the greatest soot producers, the emissions of soot assumed in the nuclear winter calculations are still "too high". Following the results of the Kuwaiti oil fires being in disagreement with the core nuclear winter promoting scientists, the 1990s nuclear winter papers generally attempted to distance themselves from suggesting oil well and reserve smoke will reach the stratosphere.

In 2007, a nuclear winter study, which will be discussed later, 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].

However the above simulation notably contained the assumption that no dry and wet deposition/rain would occur.

Eruption of Mt. Pinatubo and agriculture

The eruption of the Philippines volcano Mount Pinatubo in June 1991 ejected roughly 10 km³ (2.4 cu mi) of magma and 17,000,000 tonnes (19,000,000 short tons) of sulfur dioxide SO₂ into the air, introducing ten times as much total SO₂ as the Kuwaiti fires, mostly during the explosive Plinian/Ultra-Plinian event of June 15, 1991, creating a global stratospheric SO₂ haze layer which persisted for years. This resulted in the global average temperature dropping by about 0.5 °C (0.9 °F). As volcanic ash falls out of the atmosphere rapidly, the negative agricultural effects of the eruption were largely immediate and localized to a relatively small area in close proximity to the eruption, as they were caused by the resulting thick ash cover that resulted. Globally however, despite a several-month 5% drop in overall solar irradiation, and a reduction in direct sunlight by 30%, there was no negative impact to global agriculture. Surprisingly, a 3-4 year increase in global Agricultural productivity and forestry growth was observed, excepting boreal forest regions.

The means by which this was discovered, is that initially at the time, a mysterious drop in the rate at which carbon dioxide (CO₂) was filling the atmosphere was observed, and numerous scientists assumed this to be due to the lowering of the Earth's temperature, and with that, a slow down in plant and soil respiration, indicating a deleterious impact to global agriculture from the volcanic haze layer. However upon actual investigation, the reduction of the rate at which carbon dioxide filled the atmosphere did not match up with the hypothesis that plant respiration rates had declined. Instead the advantagous anomaly was relatively firmly linked to an unprecedented increase in the growth/net primary production, of global plant life. The mechanism by which the increase in plant growth was possible, was that the 30% reduction of direct sunlight can also be expressed as an increase or "enhancement" in the amount of diffuse sunlight.

With, owing to its intrinsic nature, can illuminate under-canopy leaves permitting more efficient total whole-plant photosynthesis than would otherwise be the case. In stark contrast to the effect of totally clear skies and the direct sunlight that results from it, which casts shadows onto understorey leaves, strickly limiting plant photosynthesis to the top canopy layer. This increase in global agriculture from the volcanic haze layer also naturally results as a product of other aerosols that are not emitted by volcanoes, such as man-made "moderately thick smoke loading" pollution, as the same mechanism, the "aerosol direct radiative effect" is behind both.

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. However far from being "new", the very same beginning to "significant" nuclear winter effects, was in the mid 1980s models, similarly regarded to have been a threat from a total of 100 or so city firestorms.

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 10¹²Â 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 â€"7 °C to â€"8 °C persists for years, and after a decade the cooling is still â€"4 °C (Fig. 2). Considering that the global average cooling at the depth of the last ice age 18,000 yr ago was about â€"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 â€"20 °C occurs over large areas of North America and of more than â€"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."

2014

In 2014, M. J. Mills (at the US National Center for Atmospheric Research, NCAR), O. B. Toon (of the original TTAPS team), J. Lee-Taylor, and A. Robock published "Multi-decadal global cooling and unprecedented ozone loss following a regional nuclear conflict" in the journal Earth's Future. The authors used computational models developed by NCAR to simulate the climatic effects of a regional nuclear war in which 100 "small" (15 kt) weapons are detonated over cities. They concluded, in part, that

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.

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 amount of cooling would last only a few days. 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."

John Maddox, editor of the journal Nature, issued a series of skeptical comments about nuclear winter studies during his tenure, being a long-time critic of environmental doomsdayism. Similarly S. Fred Singer was a long term vocal critic of the hypothesis in the journal and in televised debates with Carl Sagan.

Russell Seitz, Associate of the Harvard University Center for International Affairs, argues that the models' assumptions give results which the researchers want to achieve and is a case of "worst-case analysis run amok". Seitz's opposition caused the proponents of nuclear winter to issue responses in the media, and while both sides made important points, they were largely incapable of collaborating as the proponents believed it was simply necessary to show only the possibility of climatic catastrophe, often a worst-case scenario, while opponents insisted that to be taken seriously, nuclear winter should be shown as likely under “reasonable” scenarios. One of these areas of contention, as elucidated by Lynn R. Anspaugh, is upon the question of which season should be used as the backdrop for the models, most models choose the summer in the Northern Hemisphere as the start point to produce the maximum cooling effect, whereas it has been pointed out that if the firestorms occurred in the winter months, when there is much less intense sunlight to loft soot into a stable region of the stratosphere, the magnitude of the cooling effect from the same number of firestorms as ignited in the summer models, would be zero according to a January model run by Covey et al.

Lynn R. Anspaugh also expressed frustration that although a managed forest fire in Canada is said to have been lit by proponents of nuclear winter, with the fire potentially serving as an opportunity to do some basic measurements of the optical properties of the smoke and smoke-to-fuel ratio, which would have helped refine the estimates of these critical model inputs, the proponents did not indicate that any such measurements were made.

In 1986, atmospheric scientist Joyce Penner from the Lawrence Livermore National Laboratory published an article in Nature in which she focused on the specific variables of the smoke's optical properties and the quantity of smoke remaining airborne after the city fires and found that the published estimates of these variables varied so widely that depending on which estimates were chosen the climate effect could be negligible, minor or massive. The assumed optical properties for black carbon in more recent nuclear winter papers(2006) are still "based on those assumed in earlier nuclear winter simulations".

William R. Cotton Professor of Atmospheric Science at Colorado State University, specialist in cloud physics modeling and co-creator of the highly influential, and previously mentioned RAMS atmosphere model, had in the 1980s modeled and supported the predictions made by earlier nuclear winter papers, but has since reversed this position according to a book co-authored by him in 2007, stating that, amongst other systematically examined assumptions; far more rain out/wet deposition of soot will occur than is assumed in modern papers on the subject and that "We must wait for a new generation of GCMs to be implemented to examine potential consequences quantitatively".

The contribution of smoke from the ignition of live non-desert vegetation, living forests and so on near to many missile silos, a source of smoke originally brought up in the initial Twilight at Noon paper, was found after examination by Bush and Small in 1987, that the burning of live vegetation would contribute only slightly to the estimated total "nonurban smoke production". With the vegetation likely to only sustain burning if it is within a radius or two from the surface of the nuclear fireball, which is at a distance that would also experience extreme blast winds that would influence any such fires. This conclusion is supported by the 1950-60s in-field examination of tropical forests after Operation Castle, and Operation Redwing.

In a homeland security paper finalized in 2010, fire experts stated that due to the nature of modern city design and construction, with the U.S.  serving as an example, a firestorm is unlikely after a nuclear detonation in a modern city. This is not to say that fires won't occur over a large area after a detonation, but that the fires would not coalesce and form the all important stratosphere punching firestorm plume that the nuclear winter papers require as a prerequisite assumption in their climate computer models. The nuclear bombing of Nagasaki for example, did not produce a firestorm. However, even a conflagration that is not a firestorm can inject smoke into the stratosphere, as happened in a wildfire in Alberta in 2001.

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 2010 Alan Robock, a co-author of nuclear winter papers was summoned to Cuba to help Castro promote his new view that nuclear war would bring about Armageddon, Robock's 90 minute lecture was later aired on nationwide television in the country. However according to Robock, in so far as getting US government attention and affecting nuclear policy, he has failed. In 2009, together with Owen Toon, he gave a talk to the United States Congress but nothing transpired from it and the then presidential science adviser, John Holdren, did not respond to their requests in 2009 or at the time of writing in 2011.

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."

However a 1984 US Interagency Intelligence Assessment expresses a far more skeptical and cautious approach by stating that as the hypothesis is not convincing scientifically, they predicted that Soviet nuclear policy would be to maintain their strategic nuclear posture, such as their fielding of the high throw-weight SS-18 missile and they would merely attempt to exploit the hypothesis for propaganda purposes, such as directing scrutiny on the US portion of the nuclear arms race. Moreover it goes on to express the belief that if Soviet officials did begin to take nuclear winter seriously, it would probably make them demand exceptionally high standards of scientific proof for the hypothesis, as the implications of it would undermine their military doctrineâ€"a level of scientific proof which perhaps could not be met without field experimentation. The un-redacted portion of the document ends with the suggestion that substantial increases in Soviet Civil defense food stockpiles might be an early indicator that Nuclear Winter was beginning to influence Soviet upper echelon thinking.

In 1985 Time magazine noted "the suspicions of some Western scientists that the nuclear winter hypothesis was promoted by Moscow to give anti-nuclear groups in the U.S. and Europe some fresh ammunition against America's arms buildup."

In 1986, the Defense Nuclear Agency document An update of Soviet research on and exploitation of Nuclear winter 1984-1986 charted the minimal research contribution on, and Soviet propaganda usage of, the nuclear winter phenomenon.

Dr. Vitalii Nikolaevich Tsygichko, a Senior Analyst at the Soviet Academy of Sciences, author of the study, Mathematical Model of Soviet Strategic Operations on the Continental Theater, and a former member of the General Staff, has said that Soviet 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. Starley L. Thompson, of the National Center for Atmospheric Research, Boulder, Colorado, says that Soviet research into nuclear winter in 1983 used US computer models that had been developed in the early 1970s. Soviet intelligence officer Sergei Tretyakov, who defected in 1990, maintained that "the KGB was responsible for creating the entire nuclear winter story to stop the Pershing missiles".

In 1989 Carl Sagan and colleague Richard Turco wrote a policy implications paper that appeared in Ambio that suggests that as nuclear winter is a "well-established prospect", both superpowers should jointly reduce their nuclear arsenals to "Canonical Deterrent Force" levels of 100-300 individual warheads each, such that in "the event of nuclear war [this] would minimize the likelihood of nuclear winter."

As the implications of nuclear winter began to be taken seriously in the late 1980s, military analysts turned to reinforce "existing trends" in warhead miniaturization, of higher accuracy and lower yield nuclear warheads. This trend, enabled by GPS navigation etc., was motivated by the desire to still destroy the target but while reducing the severity of fallout collateral damage depositing on neighboring, and potentially friendly, countries. As it relates to the likelihood of nuclear winter, the hazard from thermal radiation ignited fires would also be reduced. While 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 shadows cast by buildings, while also temporarily lofting far more radioactive soil into the atmosphere. This logic is similarly reflected in the 1984 Interagency Intelligence assessment, which suggests that targeting planners would simply have to consider target combustibility along with yield, height of burst, timing and other factors to reduce the amount of smoke to safeguard against the potentiality of a nuclear winter. Therefore as a consequence of attempting to limit the target fire hazard by reducing the range of thermal radiation with fuzing for surface and sub-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 argued that belief in the possibility of nuclear winter would actually make nuclear war more likely, contrary to the views of Sagan and others, because it would inspire the development of more accurate, and lower explosive yield, nuclear weapons. As it suggests that the replacement of the then Cold War viewed strategic nuclear weapons in the multi-megaton yield range, with weapons of explosive yields closer to tactical nuclear weapons, such as the Robust Nuclear Earth Penetrator, would safeguard against the nuclear winter potential. Tactical nuclear weapons, on the low end of the scale have yields that overlap with large conventional weapons, and are therefore often viewed "as blurring the distinction between conventional and nuclear weapons", making the prospect of using them "easier" in a conflict.

Mitigation techniques

A number of solutions have been proposed to mitigate the potential harm of a nuclear winter if one appears inevitable; with the problem being attacked at both ends, from those focusing on preventing the growth of fires and therefore limiting the amount of smoke that reaches the stratosphere in the first place, to food production under dimmed skies with the assumption that the very worst-case analysis results of the nuclear winter models prove accurate and no other mitigation strategies are fielded.

Fire control

In a report from 1967, techniques included various methods of applying liquid nitrogen, dry ice, and water to nuclear-caused fires. The report considered attempting to stop the spread of fires by creating firebreaks by blasting combustible material out of an area, possibly even with nuclear weapons, along with the use of preventative Hazard reduction burns. According to the report, one of the most promising techniques investigated was initiation of rain from seeding of mass-fire thunderheads and other clouds passing over the developing, and then steady-state, firestorm.

Producing food without sunlight

David Denkenberger and Joshua Pearce have proposed in Feeding Everyone No Matter What a variety of alternate foods which convert fossil fuels or biomass into food without sunlight to address nuclear winter. The solution using a fossil fuel energy source is natural-gas-digesting bacteria. One example of a biomass alternate food is that mushrooms can grow directly on wood without sunlight. Another example is that cellulosic biofuel production typically already creates sugar as an intermediate product.

Large-scale food stockpiling

The minimum annual global wheat storage is approximately 2 months. To feed everyone despite nuclear winter, years of food storage prior to the event has been proposed. While the suggested masses of preserved food would likely never get used as a nuclear winter is comparatively unlikely to occur, the stockpiling of food would have the positive result of ameliorating the impact of the far more frequent distruptions to regional food supplies caused by lower-level conflicts and droughts. There is however the danger that if a sudden rush to food stockpiling occurs without the buffering effect offered by Victory gardens etc., it may exacerbate current food security problems by elevating present food prices.

Climate engineering

Despite the name "nuclear winter", nuclear events are not necessary to produce the modeled climatic effect. In an effort to find a quick and cheap solution to the global warming projection of at least two degrees of surface warming as a result of doubling of CO2 levels in the atmosphere, through solar radiation management, a form of climate engineering, the underlying nuclear winter effect has been looked at as perhaps holding potential. Besides the more common suggestion to inject sulfur compounds into the stratosphere to approximate the effects of a volcanic winter, the injection of other chemical species such as the release of a particular type of soot particle, to create minor "nuclear winter" conditions, has also been proposed by Paul Crutzen and others. According to the threshold/minor "nuclear winter" computer models, if one to five teragrams of firestorm-generated soot is injected into the low stratosphere, it is modeled, through the anti-greenhouse effect, to heat the stratosphere but cool the lower troposphere and produce 1.25 °C cooling for two to three years; after 10 years, average global temperatures would still be 0.5 °C lower than before the soot injection.

Potential climatic precedence

Similar climatic effects to "nuclear winter" followed historical supervolcano eruptions, which plumed sulfate aerosols high into the stratosphere, with this being known as a volcanic winter.

Similarly, extinction-level comet and asteroid impacts are also believed to have generated impact winters by the pulverization of massive amounts of fine rock dust. This pulverized rock can also produce "volcanic winter" effects, if sulfate-bearing rock is hit in the impact and lofted high into the air, and "nuclear winter" effects, with the heat of the heavier rock ejecta igniting regional and possibly even global forest firestorms.

This global "impact firestorms" hypothesis, initially supported by Wolbach, Melosh and veteran nuclear winter modeler Owen Toon, suggests that as a result of massive impact events, the small sand-grain-sized ejecta fragments created can meteorically re-enter the atmosphere forming a hot blanket of global debris high in the air, potentially turning the entire sky red-hot for minutes to hours, and with that, burning the complete global inventory of above-ground carbonaceous material, including rain forests. This hypothesis is suggested as a means to explain the severity of the Cretaceousâ€"Paleogene extinction event, as the earth impact of an asteroid about 10 km wide which precipitated the extinction is not regarded as sufficiently energetic to have caused the level of extinction from the initial impact's energy release alone.

The global "impact firestorms"/firestorm winter, however, has been questioned in more recent years (2003-2013) by Claire Belcher, Tamara Goldin and H. Jay Melosh, with this re-evaluation being dubbed the "Cretaceous-Palaeogene firestorm debate" by Belcher.

The issues raised by these scientists in the debate are the perceived low quantity of soot in the sediment beside the fine-grained iridium-rich asteroid dust layer, if the quantity of re-entering ejecta was perfectly global in blanketing the atmosphere, and if so, the duration and profile of the re-entry heating, whether it was a high thermal pulse of heat or the more prolonged and therefore more incendiary "oven" heating, and finally, how much the "self-shielding effect" from the first wave of now-cooled meteors in dark flight contributed to diminishing the total heat experienced on the ground from later waves of meteors, in part due to the Cretaceous period being a high-atmospheric-oxygen era, with concentrations above that of the present day. In 2013, Owen Toon et al. were critical of the re-evaluations the hypothesis is undergoing. It will be difficult to successfully tease out the percentage contribution of the soot in this period's geological sediment record from living plants and fossil fuels present at the time, in much the same manner that the fraction of the material ignited by the meteor's heating effects will be difficult to determine, as other ignition sources that were also present at, or soon after, the impact such as mantle lava flows complicate the matter.

See also

• Doomsday device
• Younger Dryas impact hypothesis, a similarly controversial hypothesis that an impact event & fires triggered the last ice age.
• Younger Dryas, most recent ice age, ended approximately 10,000 BC, the event may be linked with the birth of agriculture.
• Impact event
• Impact winter
• Volcanic winter
• Krakatoa, 1883 eruption, which caused approximately 1 kelvin of global cooling for 2 years due to sulfate emissions.
• Year Without a Summer, 1815, created by a volcanic eruption in Tambora.
• Laki, 1783 eruption of an Icelandic volcano which produced continentally localized cooling for 1â€"2 years.
• Toba catastrophe theory, a controversial hypothesis that a volcanic winter produced by the eruption of a volcano in Toba, Indonesia, created a human population bottleneck approx 80,000 years ago.
• List of states with nuclear weapons
• Global dimming, global reduction in ground insolation, due to the atmospheric injection of aerosols from various sources.
• Nuclear darkness, hypothetical global nuclear war induced deep, and prolonged, dimming of daylight during the nuclear winter.
• Nuclear summer, hypothetical prediction of a prolonged increase in global temperatures following the end of a nuclear war.

• List of wildfires
• List of historic fires

• Dalton minimum, 1790 to 1830, a period of prolonged solar minimum activity, resulting in Earth receiving lower insolation values.
• Maunder Minimum, 1645 to 1715, a period of deeper solar minimum activity resulting in reduced insolation and coinciding with the little ice age.

Documentaries

• On the 8th Day - Nuclear winter documentary (1984) filmed by the BBC and available on Internet video hosting websites; chronicles the rise of the hypothesis, with lengthy interviews of the prominent scientists who published the nascent papers on the subject.

Media

• 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.
• Threads: A 1984 docu-drama that Carl Sagan assisted in an advisory capacity.
• A Path Where No Man Thought: Nuclear Winter and the End of the Arms Race: A book authored by Richard P. Turco and Carl Sagan, published in 1990; it explains the nuclear winter hypothesis and, with that, advocates nuclear disarmament.

External links

• The Encyclopedia of Earth, Nuclear Winter Lead Author: Alan Robock. Last Updated: July 31, 2008
• Nuclear Winter Simulation Animation
• New studies of climatic consequences of regional nuclear conflict from Alan Robock, including links to new studies published in 2007.

References

Footnotes