The exact date of the eruption is unknown, but the pattern of ash
deposits suggests that it occurred during the northern summer because
only the summer monsoon could have deposited Toba ashfall in the South China Sea. The eruption lasted perhaps 9 to 14 days. The most recent two high-precision argon–argon datings dated the eruption to 73,880 ± 320 and 73,700 ± 300 years ago. Five distinct magma bodies were activated within a few centuries before the eruption. The eruption commenced with small and limited air-fall and was directly followed by the main phase of ignimbrite flows. The ignimbrite phase is characterized by low eruption fountain, but co-ignimbrite column developed on top of pyroclastic flows reached a height of 32 km (20 mi). Petrological constraints on sulfur emission yielded a wide range from 1×1013 to 1×1015 g, depending on the existence of separate sulfur gas in the Toba magma chamber. The lower end of the estimate is due to the low solubility of sulfur in the magma. Ice core records estimate the sulfur emission on the order of 1×1014 g.
Effects of the eruption
Bill Rose and Craig Chesner of Michigan Technological University have estimated that the total amount of material released in the eruption was at least 2,800 km3 (670 cu mi)—about 2,000 km3 (480 cu mi) of ignimbrite that flowed over the ground, and approximately 800 km3
(190 cu mi) that fell as ash mostly to the west. However, as more
outcrops become available, the most recent estimate of eruptive volume
is 3,800 km3 (910 cu mi) dense-rock equivalent (DRE), of which 1,800 km3 (430 cu mi) was deposited as ash fall and 2,000 km3 (480 cu mi) as ignimbrite, making this eruption the largest during the Quaternary period. Previous volume estimates have ranged from 2,000 km3 (480 cu mi) to 6,000 km3 (1,400 cu mi). Inside the caldera, the maximum thickness of pyroclastic flows is over 600 m (2,000 ft). The outflow sheet originally covered an area of 20,000–30,000 km2 (7,700–11,600 sq mi) with thickness nearly 100 m (330 ft), likely reaching into the Indian Ocean and the Straits of Malacca. The air-fall of this eruption blanketed the Indian subcontinent in a layer of 5 cm (2.0 in) ash, the Arabian Sea in 1 mm (0.039 in), the South China Sea in 3.5 cm (1.4 in), and Central Indian Ocean Basin in 10 cm (3.9 in). Its horizon of ashfall covered an area of more than 38,000,000 km2 (15,000,000 sq mi) in 1 cm (0.39 in) or more thickness (~7.5% of the Earth's surface). In Sub-Saharan Africa, microscopic glass shards from this eruption are also discovered on the south coast of South Africa, in the lowlands of northwest Ethiopia, in Lake Malawi, and in Lake Chala. In South China, Toba tephras is found in Huguangyan Maar Lake.
The subsequent collapse formed a caldera that filled with water,
creating Lake Toba. The island in the center of the lake is formed by a resurgent dome.
Climatic effects
Climate at the time of the eruption
Greenland stadial 20 (GS20) is a millennium-long cold event in the north Atlantic ocean that started around the time of the Toba eruption. The timing of the initiation of GS20 is dated to 74.0–74.2 kyr, and the entire event lasted about 1,500 years. It is the stadial part of Dansgaard–Oeschger event 20 (DO20), commonly explained by an abrupt reduction in the strength of the Atlantic meridional overturning circulation (AMOC). Weaker AMOC caused warming in the Southern Ocean and Antarctica, and this asynchrony is known as bipolar seesaw. The start of the GS20 cooling event corresponds to the start of the Antarctic Isotope Maxima 19 (AIM19) warming event. GS20 was associated with iceberg discharges into the North Atlantic, thus it was also named Heinrich stadial 7a. Heinrich events tend to be longer, colder and with weaker AMOC in the Atlantic ocean than other DO stadials. From 74 to 58 kyr, Earth transitioned from interglacial marine isotope stage (MIS) 5 to glacial MIS 4, experiencing cooling and glacial expansion. This transition is a part of the Pleistocene interglacial-glacial cycle driven by variations in the Earth's orbit. Ocean temperatures cooled by 0.9 °C (1.6 °F). Sea level fell 60 m (200 ft). Northern Hemisphere ice sheets embarked on significant expansion and surpassed the extent of the Last Glacial Maximum in eastern Europe, Northeast Asia and the North American Cordillera. Southern Hemisphere glaciation grew to its maximum extent during MIS 4. Australasia, Africa and Europe were characterized by increasingly cold and arid environments.
Possible climate records of the eruption
While the Toba eruption occurred in the backdrop of the rapid climate
transitions of GS20 and MIS 4, triggered by changes in ocean currents
and insolation, whether the eruption played any role in accelerating these events is
much more heavily debated. South China Sea marine records of climate,
sampled at every centennial interval, shows 1 °C (1.8 °F) cooling above
the Toba ash layer for a thousand years but the authors concede that it
may just be GS20. Arabian Sea marine records confirm that Toba ash occurred after the
onset of GS20 but also that GS20 is not colder than GS21 in the records,
from which authors conclude that the eruption did not intensify GS20
cooling. Dense sampling of environmental records, at every 6–9-year interval, in Lake Malawi, show no cooling-induced change in lake ecology and in grassy woodlands after the deposition of Toba ash, but cooling-forced aridity killed high-elevation afromontane forests. The Lake Malawi studies concluded that the environmental effects of the
eruption were mild and limited to less than a decade in East Africa, but these studies are questioned due to sediment mixing which would have diminished the cooling signal. Environmental records from a Middle Stone Age site in Ethiopia, however, show that a severe drought occurred concurrently with the Toba ash layer, which altered early human foraging behaviours.
Toba ash records have not been identified in ice core samples.
However, four sulfate events in the ice strata are proposed to represent
the deposition of aerosols from the Toba eruption. One sulfate event at 73.75–74.16 kyr, which has all the characteristics
of the Toba eruption, is among the largest sulfate loadings that have
ever been identified. In the ice core records, GS20 cooling was already underway by the time
of sulfate deposition; however, a 110-year period of accelerated cooling
followed this sulfate event. The authors interpret this acceleration as
AMOC weakened by the Toba eruption.
Climate modeling
The modeled climate effects of the Toba eruption hinges on the mass
of sulfurous gases and aerosol microphysical processes. Modeling on an
emission of 8.5×1014 g of sulfur, which is 100 times the 1991 Pinatubo sulphur,
volcanic winter has a maximum global mean cooling of 3.5 °C (6.3 °F)
and returns gradually within the range of natural variability 5 years
after the eruption. An initiation of a 1,000-year cold period or ice age
is not supported by the model. Two other emission scenarios, 1×1014 g and 1×1015 g, were investigated using state-of-art simulations provided by the Community Earth System Model.
Maximum global mean cooling was 2.3 °C (4.1 °F) for the lower emission
and 4.1 °C (7.4 °F) for the higher emission scenarios. A strong decrease
in precipitation occurs in the high emission scenario. Negative
temperature anomalies return to less than 1 °C (1.8 °F) within 3 and 6
years for each emission scenario after the eruption. But so far no model can simulate aerosol microphysical processes with
sufficient accuracy, empirical constraints from historical eruptions
suggest that aerosol size may substantially reduce the magnitude of
cooling to less than 1.5 °C (2.7 °F), no matter how much sulfur is
emitted.
Toba catastrophe theory
The Toba catastrophe theory holds that the eruption caused a severe global volcanic winter of six to ten years and contributed to a 1,000-year-long cooling episode, resulting in a genetic bottleneck in humans. However, some physical evidence disputes the association with the
millennium-long cold event and genetic bottleneck, and some consider the
theory disproven.
History
In 1972, an analysis of human hemoglobins
found very few variants, and to account for this low frequency of
variation, the human population must have been as low as a few thousand
until very recently. More genetic studies confirmed an effective population on the order of 10,000 for much of human history. Subsequent research on the differences in human mitochondrial DNA sequences dated a rapid growth from a small effective population size of 1,000 to 10,000, sometime between 35 and 65 kyr ago. Recent research shows the extent of climate change was much smaller than believed by proponents of the theory.
In 1993, science journalist Ann Gibbons posited that population
growth was suppressed by the cold climate of the last Pleistocene Ice
Age, possibly exacerbated by the Toba super-eruption which at the time
was dated to between 73 and 75 kyr near the beginning of glacial period
MIS 4. The subsequent explosive human expansion was believed to be the result of the end of the ice age. Geologist Michael R. Rampino of New York University and volcanologist Stephen Self of the University of Hawaiʻi at Mānoa supported her theory. In 1998, anthropologist Stanley H. Ambrose of the University of Illinois Urbana-Champaign used coalescence evidence of some genes to hypothesize that the Toba eruption caused a human population crash
to only a few thousand surviving individuals, and the subsequent
recovery was suppressed by the global glacial condition of MIS 4 until
the climate eventually transitioned to the warmer condition of MIS 3
about 60,000 years ago, during which rapid human population expansion
occurred.
Possible effects on Homo
At least two other Homo lineages, H. neanderthalensis and Denisovans, survived the Toba eruption and subsequent MIS 4 ice age, as their latest presence is dated to ca. 40 kyr, and ca. 55 kyr. Other lineages, including H. floresiensis and H. luzonensis, may have also survived through the eruption. More recently, reconstructions of human demographic history using whole-genome sequencing and discoveries of archaeological cultures within the Toba ash layer add further light to how humans had fared during the eruption and the following GS20 and MIS 4 ice age.
Human demographic history
Recent analyses apply Markov models to the complete set of genetic material to infer human population history. In non-African populations, studies recover a long-term steep decline
in numbers starting 200 kyr and reaching the lowest point around 40–60
kyr. During this bottleneck non-African populations experienced 5- to 15-fold reduction, with an effective population size of only 1,000–3,000 individuals by 50 kyr, consistent with the earliest mtDNA studies. This severe non-African contraction is consistent with a founder effect caused by Out-of-Africa
dispersal. As a small group with a size of a few thousand people
migrated from the African continent into the Near East, the drastic
reduction in numbers imprinted on non-African genomic diversity. Genetic analysis identified 56 selective sweeps
related to cold adaptations in non-African populations, of which 31
sweeps occurred during 72–97 kyr. This event of closely timed selections
is named the "Arabian Standstill" and may have been caused by the
severe cold arid conditions from the onset of MIS 4 and exacerbated by
the Toba super-eruption.
African populations experienced a slightly earlier, milder bottleneck and recovered earlier. Estimated effective population size based on samples from the Luhya and Maasai people attained their lowest numbers around 70–80 kyr, while those from the Yoruba people reached a nadir around 50 kyr, though the long-term declining trend already started before 200 kyr. The estimated remaining effective population sizes are around 10,000
individuals, larger than the estimated non-African size during their
bottleneck. Unlike the non-African populations, there is no consensus as to the
cause of the African bottleneck. Proposed causes include climatic
deterioration (from MIS 5, Toba eruption, GS20 and/or MIS 4), reduction in substructure across African populations, and founder effects from the dispersal within Africa.
Earlier genetic analysis of Alu sequences across the entire human genome
has shown that the effective human population size was less than 26,000
at 1.2 million years ago; possible explanations for the low population
size of human ancestors may include repeated population crashes or
periodic replacement events from competing Homo subspecies. Whole-genome analysis similarly recovers very low African population sizes around 1 million years ago. This 1 million year old bottleneck is thought to have been caused by
severe ice age MIS 22 which marked the mid-Pleistocene climate
transition with widespread aridity across Africa
Archaeological studies
Other research has cast doubt on an association between the Toba Caldera Complex and a genetic bottleneck. For example, ancient stone tools at the Jurreru Valley in southern India (Andhra Pradesh)
were found above and below a thick layer of ash from the Toba eruption
and were very similar across these layers, suggesting that the dust
clouds from the eruption did not wipe out this local population. However, another site in India, the Middle Son Valley (in Madhya Pradesh),
exhibits evidence of a major population decline and it has been
suggested that the abundant springs of the Jurreru Valley may have
offered its inhabitants unique protection. At the Jurreru Valley in southern India, Middle Paleolithic stone tools below the Toba ash layer are dated by OSL to 77±4 kyr,
while the age of stone tools above the ash layer is constrained to be
no older than 55 kyr. This age gap is suspected to be due to the removal
of post-eruption sediments or decimation of the local population until
re-occupation at 55 kyr. Additional archaeological evidence from southern and northern India
also suggests a lack of evidence for effects of the eruption on local
populations, causing the authors of the study to conclude, "many forms
of life survived the supereruption, contrary to other research which has
suggested significant animal extinctions and genetic bottlenecks". However, some researchers have questioned the techniques utilized to
date artifacts to the period subsequent to the Toba supervolcano. The Toba Catastrophe also coincides with the disappearance of the Skhul and Qafzeh hominins. Evidence from pollen analysis has suggested prolonged deforestation in
South Asia, and some researchers have suggested that the Toba eruption
may have forced humans to adopt new adaptive strategies, which may have
permitted them to replace Neanderthals and "other archaic human species".
Genetic bottlenecks in other mammals
Some evidence indicates population crashes of other animals after the Toba eruption. The populations of the Eastern African chimpanzee, Bornean orangutan, central Indian macaque, gorillascheetah and tiger, all expanded from very small populations around 70,000–55,000 years ago.
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.
Nuclear winter is a severe and prolonged global climatic cooling effect that is hypothesized 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 animalmass extinction event.
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. It was within this context that the climatic effects of soot from fires
became the new focus of the climatic effects of nuclear war. 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.
As nuclear devices need not be detonated to ignite a firestorm, the term "nuclear winter" is something of a misnomer. The majority of papers published on the subject state that without
qualitative justification, nuclear explosions are the cause of the
modeled firestorm effects. The only phenomenon that is modeled by
computer in the nuclear winter papers is the climate forcing agent of firestorm-soot, a product which can be ignited and formed by myriad means. Although rarely discussed, the proponents of the hypothesis state that
the same "nuclear winter" effect would occur if 100 large scale
conventional firestorms were ignited.
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 airburstingnuclear weapon use during an American-Soviet total war. This larger number of firestorms, which are not in themselves modeled, are presented as causing nuclear winter conditions as a result of the
smoke inputted into various climate models, with the depths of severe
cooling lasting for as long as a decade. During this period, summer
drops in average temperature could be up to 20 °C (36 °F) in core
agricultural regions of the US, Europe, and China, and as much as 35 °C
(63 °F) in Russia. 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.
Since the advent of photography that captured evidence of tall clouds, 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. The existence of a tipping point into a new stratospheric condition where the aerosols would not be removed within this time frame remains to be determined.
Picture of a pyrocumulonimbus cloud
taken from a commercial airliner cruising at about 10 km. In 2002,
various sensing instruments detected 17 distinct pyrocumulonimbus cloud
events in North America alone.
The nuclear winter scenario assumes that 100 or more city firestorms are ignited by nuclear explosions, and that the firestorms lift large amounts of sooty smoke into the upper troposphere
and lower stratosphere by the movement offered by the pyrocumulonimbus
clouds that form during a firestorm. At 10–15 kilometres (6–9 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 prevent a
portion of the sun's light from reaching the surface, causing surface
temperatures to drop drastically. In this scenario it is predicted that surface air temperatures would be the same as, 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 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. 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.
As the incendiary effects of a nuclear explosion do not present any especially characteristic features, 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. While it is believed that the modeled climate-cooling-effects from the
mass of soot injected into the stratosphere by 100 firestorms (one to
five million metric tons)
would have been detectable with technical instruments in WWII, five
percent of that would not have been possible to observe at that time.
Aerosol removal timescale
Smoke rising in Lochcarron, Scotland, is stopped by an overlying natural low-level inversion layer of warmer air (2006).
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" phase, which produces "black rain" near the fire site, 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 affecting the timescale of the soot particles' residence are how quickly the aerosol of soot collides and coagulates with other particles via Brownian motion, and falls out of the atmosphere via gravity-driven dry deposition, and the time it takes for the "phoretic effect" to move coagulated particles to a lower level in the atmosphere. Whether by coagulation or the phoretic effect, 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.
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
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.
Ozone depletion
Nuclear detonations produce large amounts of nitrogen oxides
by combining elements in the surrounding air. These are then lifted
upwards by thermal convection. As they reach the stratosphere, these
nitrogen oxides are capable of catalytically breaking down the ozone present in this part of the atmosphere. Ozone depletion would allow a much greater intensity of harmful ultraviolet radiation from the sun to reach the ground.
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 50 miles (80 km) 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 CO2 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.
Other more straightforward hypothetical versions exist of the
hypothesis that nuclear winter might give way to a nuclear summer. The
high temperatures of the nuclear fireballs could destroy the ozone gas
of the middle stratosphere.
History
Early work
The mushroom cloud height as a function of explosive yield detonated as surface bursts. 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. Another means of stratospheric entry is from high altitude nuclear detonations, one example of which includes the 10.5 kiloton Soviet test no.#88 of 1961, detonated at 22.7 km. US high-yield upper atmospheric tests, Teak and Orange were also assessed for their ozone destruction potential. 0 = Approx altitude commercial aircraft operate 1 = Fat Man 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."[77]
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 1010 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 °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" 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 the section of this 1975 NRC book pertaining to the issue of
fireball generated NOx and ozone layer loss therefrom, the NRC presented
model calculations from the early-to-mid 1970s on the effects of a
nuclear war with the use of large numbers of multi-megaton yield
detonations, which returned conclusions that this could reduce ozone
levels by 50 percent or more in the northern hemisphere.
However, independent of the computer models presented in the 1975 NRC works, a paper in 1973 in the journal Nature
depicts the stratospheric ozone levels worldwide overlaid upon the
number of nuclear detonations during the era of atmospheric testing. The
authors conclude that neither the data nor their models show any
correlation between the approximate 500 Mt in historical atmospheric
testing and an increase or decrease of ozone concentration. 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 × 1034 additional molecules of nitric oxide (about 5,000 tons per Mt, 5 × 109 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.
In 1982 journalist Jonathan Schell in his popular and influential book The Fate of the Earth,
introduced the public to the belief that fireball generated NOx would
destroy the ozone layer to such an extent that crops would fail from
solar UV radiation and then similarly painted the fate of the Earth, as
plant and aquatic life going extinct. In the same year, 1982, Australian
physicist Brian Martin, who frequently corresponded with John Hampson who had been greatly responsible for much of the examination of NOx generation, penned a short historical synopsis on the history of interest in the
effects of the direct NOx generated by nuclear fireballs, and in doing
so, also outlined Hampson's other non-mainstream viewpoints,
particularly those relating to greater ozone destruction from
upper-atmospheric detonations as a result of any widely used anti-ballistic missile (ABM-1 Galosh) system. 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.
More recent accounts on the specific ozone layer destruction
potential of NOx species are much less than earlier assumed from
simplistic calculations, as "about 1.2 million tons" of natural and anthropogenic generated stratospheric NOx is believed to be formed each year according to Robert P. Parson in the 1990s.
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. Similarly in 1985 it was noted by T. G. Parsons that the story "Torch" by C. Anvil, which also appeared in Astounding Science Fiction
magazine, but in the April 1957 edition, contains the essence of the
"Twilight at Noon"/"nuclear winter" hypothesis. In the story, a nuclear
warhead ignites an oil field, and the soot produced "screens out part of
the sun's radiation", resulting in Arctic temperatures for much of the
population of North America and the Soviet Union.
1980s
The 1988 Air Force Geophysics Laboratory publication, An assessment of global atmospheric effects of a major nuclear war
by H. S. Muench, et al., contains a chronology and review of the major
reports on the nuclear winter hypothesis from 1983 to 1986. In general,
these reports arrive at similar conclusions as they are based on "the
same assumptions, the same basic data", with only minor model-code
differences. They skip the modeling steps of assessing the possibility
of fire and the initial fire plumes and instead start the modeling
process with a "spatially uniform soot cloud" which has found its way
into the atmosphere.
Although never openly acknowledged by the multi-disciplinary team who authored the most popular 1980s TTAPS model, in 2011 the American Institute of Physics
states that the TTAPS team (named for its participants, who had all
previously worked on the phenomenon of dust storms on Mars, or in the
area of asteroid impact events: Richard P. Turco, Owen Toon, Thomas P. Ackerman, James B. Pollack and Carl Sagan) announcement of their results in 1983 "was with the explicit aim of promoting international arms control". 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".
In 1981, William J. Moran began discussions and research in the National Research Council
(NRC) on the airborne soil/dust effects of a large exchange of nuclear
warheads, having seen a possible parallel in the dust effects of a war
with that of the asteroid-created K-T boundary and its popular analysis a year earlier by Luis Alvarez in 1980. An NRC study panel on the topic met in December 1981 and April 1982 in preparation for the release of the NRC's The Effects on the Atmosphere of a Major Nuclear Exchange, published 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 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".
It was after being confronted with these results that they "chanced" upon the notion, as "an afterthought" of nuclear detonations igniting massive fires everywhere and,
crucially, the smoke from these conventional fires then going on to
absorb sunlight, causing surface temperatures to plummet. In early 1982, the two circulated a draft paper with the first
suggestions of alterations in short-term climate from fires presumed to
occur following a nuclear war. Later in the same year, the special issue of Ambio
devoted to the possible environmental consequences of nuclear war by
Crutzen and Birks was titled "The Atmosphere after a Nuclear War:
Twilight at Noon", and largely anticipated the nuclear winter
hypothesis. The paper looked into fires and their climatic effect and 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 percent of sunlight from reaching the Earth's surface.
This darkness, they said, could exist "for as long as the fires burned",
which was assumed to be many weeks, with effects such as: "The normal
dynamic and temperature structure of the atmosphere would...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 implication of their work was that a successful nuclear decapitation strike could have severe climatic consequences for the perpetrator.
After reading a paper by N. P. Bochkov and E. I. Chazov, 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."
In the same year Alexander Ginzburg, 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. Golitsyn would use Ginzburg's largely unmodified dust-cloud model with
soot assumed as the aerosol in the model instead of soil dust and in an
identical fashion to the results returned, when computing dust-cloud
cooling in the Martian atmosphere, the cloud high above the planet would
be heated while the planet below would cool drastically. Golitsyn
presented his intent to publish this Martian-derived Earth-analog model
to the Andropov instigated Committee of Soviet Scientists in Defence of Peace Against the Nuclear Threat
in May 1983, an organization that Golitsyn would later be appointed
vice-chairman. The establishment of this committee was done with the
expressed approval of the Soviet leadership with the intent "to expand
controlled contacts with Western "nuclear freeze" activists". 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.
Both Golitsyn and Sagan 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. (Two years later Alexandrov disappeared under mysterious
circumstances). Richard Turco and Starley L. Thompson were both critical
of the Soviet research. Turco called it "primitive" and Thompson said
it used obsolete US computer models. Later they were to rescind these criticisms and instead applauded
Alexandrov's pioneering work, saying that the Soviet model shared the
weaknesses of all the others.
In 1984, the World Meteorological Organization
(WMO) commissioned Golitsyn and N. A. Phillips to review the state of
the science. They found that studies generally assumed a scenario where
half of the world's nuclear weapons would be used, ~5000 Mt, destroying
approximately 1,000 cities, and creating large quantities of
carbonaceous smoke – 1–2×1014 g being most likely, with a range of 0.2–6.4×1014 g (NAS; TTAPS assumed 2.25×1014).
The smoke resulting would be largely opaque to solar radiation but
transparent to infrared, thus cooling the Earth by blocking sunlight,
but not creating warming by 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 megaton-equivalent explosion could release up to five
million tons of dust, but most would quickly fall out; high altitude
dust is estimated at 0.1–1 million tons per megaton-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 cooling up to 15–42 °C
between 14 and 35 days after the war, with a "baseline" of about 20 °C.
Somewhat more sophisticated calculations using 3-D GCMs produced similar results: temperature drops of about 20 °C, though with regional variations.
All[which?]
calculations show large heating (up to 80 °C) at the top of the smoke
layer at about 10 km (6.2 mi); 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 °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 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 °C
Ozone depletion of 50% leading to 200% increase in UV radiation incident on surface.
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.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.
One of the major results of TTAPS' 1990 paper was the re-iteration of the team's 1983 model that 100 oil refinery fires would be sufficient to bring about a small scale, but still globally deleterious nuclear winter.
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 48,000 feet (15,000 m) 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 3,000 feet (910 m) 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, lofting to about 10,000 feet (3,000 m) and a few as high as 20,000 feet (6,100 m).
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 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 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 °C within 200 km of
the source.
This satellite photo of the south of 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 °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. 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 were still "too high". Following the results of the Kuwaiti oil fires being in disagreement
with the core nuclear winter promoting scientists, 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 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 recent evidence suggests that this
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 or wet deposition would occur.
Recent modeling
Between 1990 and 2003, commentators noted that no peer-reviewed papers on "nuclear winter" were published.
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, primarily the assessment that as few as 100 firestorms would
result in a nuclear winter. However, far from the hypothesis being "new", it drew the same
conclusion as earlier 1980s models, which similarly regarded 100 or so
city firestorms as a threat.
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.
2007 study on global nuclear war
A study published in the Journal of Geophysical Research
in July 2007, titled "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 similar to the size of the
world's arsenals twenty years earlier). The authors used a global
circulation model, ModelE from the NASAGoddard Institute for Space Studies,
which they noted "has been tested extensively in global warming
experiments and to examine the effects of volcanic eruptions on
climate". The model was used to investigate the effects of a war
involving the entire current global nuclear arsenal, projected to
release about 150 Tg of smoke into the atmosphere, as well as a war
involving about one third of the current nuclear arsenal, projected to
release about 50 Tg of smoke. In the 150 Tg case they found that:
A global average surface cooling of
−7 °C to −8 °C persists for years, and after a decade the cooling is
still −4 °C (Fig. 2). Considering that the global average cooling at the
depth of the last ice age 18,000 yr ago was about −5 °C, this would be a
climate change unprecedented in speed and amplitude in the history of
the human race. The temperature changes are largest over land ....
Cooling of more than −20 °C occurs over large areas of North America and
of more than −30 °C over much of Eurasia, including all agricultural
regions.
In addition, they found that this cooling caused a weakening of the
global hydrological cycle, reducing global precipitation by about 45%.
As for the 50 Tg case involving one third of current nuclear arsenals,
they said that the simulation "produced climate responses very similar
to those for the 150 Tg case, but with about half the amplitude," but
that "the time scale of response is about the same". They did not
discuss the implications for agriculture in depth, but noted that a 1986
study which assumed no food production for a year projected that "most
of the people on the planet would run out of food and starve to death by
then" and commented that their own results show that, "This period of
no food production needs to be extended by many years, making the
impacts of nuclear winter even worse than previously thought."
2014
In 2014, Michael J. Mills (at the US National Center for Atmospheric Research,
NCAR), et al., 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 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:
...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.
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., a 2019 study by Coupe et al.
models a scenario in which 150 Tg of black carbon is released into the
atmosphere following an exchange of nuclear weapons between the United
States and Russia where both countries use all of the nuclear weapons treaties permit them to. 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. Coupe et al. used the "whole atmosphere community climate model version 4" (WACCM4), which has a higher resolution and is more effective at simulating aerosols and stratospheric chemistry than the ModelE simulation used by Robock et al.
The WACCM4 model simulates that black carbon molecules increase
to ten times their normal size when they reach the stratosphere. ModelE
did not account for this effect. This difference in black carbon
particle size results in a greater optical depth in the WACCM4 model across the world for the first two years after the initial injection due to greater absorption of sunlight in the stratosphere. This will have the effect of increasing stratospheric temperatures by
100K and result in ozone depletion that is slightly greater than ModelE
predicted. Another consequence of the larger particle size is accelerating the
rate at which black carbon molecules fall out of the atmosphere; after
ten years from the injection of black carbon into the atmosphere, WACCM4
predicts 2 Tg will remain, while ModelE predicted 19 Tg.
The 2019 model and the 2007 model both predict significant temperature decreases across the globe, however the increased resolution
and particle simulation in 2019 predict a greater temperature anomaly
in the first six years after injection but a faster return to normal
temperatures. Between a few months after the injection to the sixth year
of anomaly, the WACCM4 predicts cooler global temperatures than ModelE,
with temperatures more than 20K below normal leading to freezing
temperatures during the summer months over much of the northern
hemisphere leading to a 90% reduction in agricultural growing seasons in
the midlatitudes, including the midwestern United States. WACCM4 simulations also predict a 58% reduction in global annual
precipitation from normal levels in years three and four after
injection, a 10% higher reduction than predicted in ModelE.
Toon et al. simulated a nuclear scenario in 2025 where India and Pakistan
engage in a nuclear exchange in which 100 urban areas in Pakistan and
150 urban areas in India are attacked with nuclear weapons ranging from
15 kt to 100 kt and examined the effects of black carbon released into
the atmosphere from airburst-only detonations. The researchers modeled the atmospheric effects if all weapons were 15
kt, 50 kt, and 100 kt, providing a range where a nuclear exchange would
likely fall into given the recent nuclear tests performed by both
nations. The ranges provided are large because neither India nor
Pakistan is obligated to provide information on their nuclear arsenals,
so their extent remains largely unknown.
Toon et al. assume that either a firestorm or conflagration
will occur after each detonation of the weapons, and the amount of
black carbon inserted into the atmosphere from the two outcomes will be
equivalent and of a profound extent; in Hiroshima in 1945, it is predicted that the firestorm released 1,000
times more energy than was released during the nuclear explosion. Such a large area being burned would release large amounts of black
carbon into the atmosphere. The amount released ranges from 16.1 Tg if
all weapons were 15 kt or less to 36.6 Tg for all 100 kt weapons. For the 15 kt and 100kt range of weapons, the researchers modeled
global precipitation reductions of 15% to 30%, temperature reductions
between 4K and 8K, and ocean temperature decreases of 1K to 3K. If all weapons used were 50 kt or more, Hadley cell circulation would be disrupted and cause a 50% decrease in precipitation in the American midwest. Net primary productivity
(NPP) for oceans decreases from 10% to 20% for the 15 kt and 100 kt
scenarios, respectively, while land NPP decreases between 15% and 30%;
particularly affected are midlatitude agricultural regions in the United
States and Europe, experiencing 25-50% reductions in NPP. As predicted by other literature, once the black carbon is removed from
the atmosphere after ten years, temperatures and NPP will return to
normal.
2021
Coupe et al. report the simulation of an El Niño
effect lasting several years after six nuclear scenarios ranging from 5
to 150 Tg soot under the CESM-WACCM4 model. They term the change a
"Nuclear Niño" and describe various changes in the ocean currents.
2022
Percent of the world's population dead from a nuclear war per simulations by Xia et al. (2022, see esp. their Table 1) with models fit thereto. The vertical axis is the percent of the
world's population expected to die within a few years after a one-week
long nuclear war that injects between 1.5 and 150 Tg (teragrams =
million metric tons) of smoke (soot) into the stratosphere, shown on the
top axis. 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 (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.[146]
According to a peer-reviewed study published in the journal Nature Food in August 2022, a full-scale nuclear war between the United States and Russia,
which together hold more than 90% of the world's nuclear weapons, would
kill 360 million people directly and more than 5 billion indirectly by
starvation during a nuclear winter.
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 minornuclear 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 3 °C (5.4 °F) of warming by 2100 and 3.8 °C (6.8 °F) 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.
As of 12 March 2025, the committee was still working on the report.
2025 study - Impact on global agriculture
In 2025, researchers at Pennsylvania State University used the Cycles agroecosystemmodel 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. To help mitigate such impacts, the authors suggested "agricultural
resilience kits," containing seeds of fast-growing, cold-tolerant crops
suited to different regions.
Criticism and debate
The five major and largely independent underpinnings that the nuclear
winter concept has and continues to receive criticism over are regarded
as:
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?
Lofting: how much soot would be lofted into the stratosphere?
While the highly popularized initial 1983 TTAPS 1-dimensional model
forecasts were widely reported and criticized in the media, in part
because every later model predicts far less of its "apocalyptic" level
of cooling, 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. The TTAPS authors responded that, amongst other things, they did not
believe target planners would intentionally blast cities into rubble,
but instead argued fires would begin in relatively undamaged suburbs
when nearby sites were hit, and partially conceded his point about
non-urban wildfires. Dr. Richard D. Small, director of thermal sciences at the
Pacific-Sierra Research Corporation similarly disagreed strongly with
the model assumptions, in particular the 1990 update by TTAPS that
argues that some 5,075 Tg of material would burn in a total US-Soviet
nuclear war, as analysis by Small of blueprints and real buildings
returned a maximum of 1,475 Tg of material that could be burned,
"assuming that all the available combustible material was actually
ignited".
Although Kearny was of the opinion that future more accurate
models would, "indicate there will be even smaller reductions in
temperature", including future potential models that did not so readily
accept that firestorms would occur as dependably as nuclear winter
modellers assume, in NWSS Kearny summarized the comparatively moderate cooling estimate of no more than a few days, from the 1986 Nuclear Winter Reappraised model by Starley Thompson and Stephen Schneider. 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".
However, a 1988 article by Brian Martin in Science and Public Policy states that—although Nuclear Winter Reappraised
concluded the US-Soviet "nuclear winter" would be much less severe than
originally thought, with the authors describing the effects more 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 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]." In 2007 Schneider expressed his tentative support for the cooling
results of the limited nuclear war (Pakistan and India) analyzed in the
2006 model, 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", and "anything that you can do to discourage people from
thinking that there is any way to win anything with a nuclear exchange
is a good idea".
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". With the vegetation's potential to sustain burning only probable 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 reduction in the estimate of the non-urban smoke hazard is supported by the earlier preliminary Estimating Nuclear Forest Fires publication of 1984, and by the 1950–1960s in-field examination of surface-scorched, mangled but never burnt-down tropical forests on the surrounding islands from the shot points in the Operation Castle and Operation Redwing test series.
During the 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 – 16 square miles (41 km2), the most destructive and deadliest bombing operation in history.The first nuclear bombing in history used a 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 4.5 square miles (12 km2) 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 km.
Russell Seitz, Associate of the Harvard University Center for
International Affairs, argues that the winter models' assumptions give
results which the researchers want to achieve and is a case of
"worst-case analysis run amok". In September 1986, Seitz published "Siberian fire as 'nuclear winter' guide" in the journal Nature,
in which he investigated the 1915 Siberian fire, which started in the
early summer months and was caused by the worst drought in the region's
recorded history. The fire ultimately devastated the region, burning the
world's largest boreal forest,
the size of Germany. While approximately 8˚C of daytime summer cooling
occurred under the smoke clouds during the weeks of burning, no increase
in potentially devastating agricultural night frosts occurred. Following his investigation into the Siberian fire of 1915, Seitz
criticized the "nuclear winter" model results for being based on
successive worst-case events:
The improbability of a string of 40 such coin tosses coming up heads approaches that of a pat royal flush.
Yet it was represented as a "sophisticated one-dimensional model" – a
usage that is oxymoronic, unless applied to [the British model Lesley
Lawson] Twiggy.
Seitz cited Carl Sagan, adding an emphasis: "In almost any realistic case
involving nuclear exchanges between the superpowers, global
environmental changes sufficient to cause an extinction event equal to
or more severe than that of the close of the Cretaceous when the dinosaurs and many other species died out are likely." Seitz comments: "The ominous rhetoric
italicized in this passage puts even the 100 megaton [the original 100
city firestorm] scenario ... on a par with the 100 million megaton blast
of an asteroid striking the Earth. This [is] astronomical mega-hype
..." Seitz concludes:
As the science progressed and more
authentic sophistication was achieved in newer and more elegant models,
the postulated effects headed downhill. By 1986, these worst-case
effects had melted down from a year of arctic darkness to warmer
temperatures than the cool months in Palm Beach! A new paradigm of broken clouds and cool spots had emerged. The once global hard frost had retreated back to the northern tundra. Mr. Sagan's elaborate conjecture had fallen prey to Murphy's lesser-known Second Law: If everything MUST go wrong, don't bet on it.
Seitz's opposition caused the proponents of nuclear winter to issue
responses in the media. 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 US-USSR war models. Most models choose the summer in the Northern
Hemisphere as the start point to produce the maximum soot lofting and
therefore eventual winter effect. However, it has been pointed out that
if the same number of firestorms occurred in the autumn or 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
would be negligible, according to a January model run by Covey et al. Schneider conceded the issue in 1990, saying "a war in late fall or winter would have no appreciable [cooling] effect".
Anspaugh also expressed frustration that although a managed
forest fire in Canada on 3 August 1985 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. Peter V. Hobbs,
who would later successfully attain funding to fly into and sample the
smoke clouds from the Kuwait oil fires in 1991, also expressed
frustration that he was denied funding to sample the Canadian, and other
forest fires in this way. Turco wrote a 10-page memorandum with information derived from his
notes and some satellite images, claiming that the smoke plume reached
6 km in altitude.
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. She 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 in 2006 are still "based on those assumed in earlier
nuclear winter simulations".
John Maddox, editor of the journal Nature, issued a series of skeptical comments about nuclear winter studies during his tenure. Similarly S. Fred Singer was a long term vocal critic of the hypothesis
in the journal and in televised debates with Carl Sagan.
Critical response to the more modern papers
In a 2011 response to the more modern papers on the hypothesis, Russell Seitz published a comment in Nature challenging Alan Robock's claim that there has been no real scientific debate about the "nuclear winter" concept. In 1986 Seitz also contends that many others are reluctant to speak out for fear of being stigmatized as "closet Dr. Strangeloves"; physicist Freeman Dyson
of Princeton for example stated "It's an absolutely atrocious piece of
science, but I quite despair of setting the public record straight." According to the Rocky Mountain News, Stephen Schneider had been called
a fascist by some disarmament supporters for having written his 1986
article "Nuclear Winter Reappraised." MIT meteorologist Kerry Emanuel similarly wrote in a review in Nature
that the winter concept is "notorious for its lack of scientific
integrity" due to the unrealistic estimates selected for the quantity of
fuel likely to burn, the imprecise global circulation models used.
Emanuel ends by stating that the evidence of other models point to
substantial scavenging of the smoke by rain. Emanuel also made an "interesting point" about questioning proponents'
objectivity when it came to strong emotional or political views that
they hold.
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 worked on soot rain-out models and supported the predictions made by his own and other nuclear winter models. However, he 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: "We must wait for
a new generation of GCMs
to be implemented to examine potential consequences quantitatively". He
also states that, in his view, "nuclear winter was largely politically
motivated from the beginning".
Policy implications
During the Cuban Missile Crisis, Fidel Castro and Che Guevara called on the USSR to launch a nuclear first strike
against the US in the event of a US invasion of Cuba. In the 1980s,
Castro was pressuring the Kremlin to adopt a harder line against the US
under President Ronald Reagan,
even arguing for the potential use of nuclear weapons. As a direct
result of this, a Soviet official was dispatched to Cuba in 1985 with an
entourage of "experts", who detailed the ecological effect on Cuba in
the event of nuclear strikes on the United States. Soon after, the
Soviet official recounts, Castro lost his prior "nuclear fever". In 2010, Alan Robock 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 the nationwide state-controlled
television station in the country.
However, according to Robock, insofar 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.
United
States and Soviet Union nuclear stockpiles. The effects of trying to
make others believe the results of the models on nuclear winter, does
not appear to have decreased either country's nuclear stockpiles in the
1980s, only the failing Soviet economy and the dissolution of the country between 1989 and 1991 which marks the end of the Cold War and with it the relaxation of the "arms race", appears to have had an effect. The effects of the electricity generating Megatons to Megawatts
program can also be seen in the mid-1990s, continuing the trend in
Russian reductions. A similar chart focusing solely on quantity of
warheads in the multi-megaton range is also available. Moreover, total deployed US and Russian strategic weapons increased steadily from 1983 until the Cold War ended.
In a 2012 "Bulletin of the Atomic Scientists" feature, Robock and
Toon, who had routinely mixed their disarmament advocacy into the
conclusions of their "nuclear winter" papers, argue in the political realm that the hypothetical effects of nuclear
winter necessitates that the doctrine they assume is active in Russia
and US, "mutually assured destruction" (MAD), should instead be replaced with their own "self-assured destruction" (SAD) concept, because, regardless of whose cities burned, the effects of the
resultant nuclear winter that they advocate would be, in their view,
catastrophic. In a similar vein, in 1989 Carl Sagan and Richard Turco wrote a policy implications paper that appeared in Ambio
that suggested 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 [extreme] nuclear
winter."
An originally classified 1984 US interagency intelligence assessment states that in both the preceding 1970s and 1980s, the Soviet and US military were already following the "existing trends" in warhead miniaturization, of higher accuracy and lower yield nuclear warheads. This is seen when assessing the most numerous physics packages in the US arsenal, which in the 1960s were the B28 and W31, however, both quickly became less prominent with the 1970s mass production runs of the 50 Kt W68, the 100 Kt W76 and in the 1980s, with the B61. This trend towards miniaturization, enabled by advances in inertial guidance and accurate GPS
navigation etc., was motivated by a multitude of factors, namely the
desire to leverage the physics of equivalent megatonnage that
miniaturization offered; of freeing up space to fit more MIRV warheads and decoys on each missile. Alongside the desire to still destroy hardened targets 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 range of potential thermal radiation
ignited fires was already reduced with miniaturization. For example,
the most popular nuclear winter paper, the 1983 TTAPS paper, had
described a 3000 Mt counterforce attack on ICBM
sites with each individual warhead having approximately one Mt of
energy; however not long after publication, Michael Altfeld of Michigan State University and political scientist Stephen Cimbala of Pennsylvania State University argued that the then already developed and deployed smaller, more accurate warheads (e.g. W76), together with lower detonation heights, could produce the same counterforce strike with a total of only 3 Mt of energy being expended. They continue that, if
the nuclear winter models prove to be representative of reality, then
far less climatic-cooling would occur, even if firestorm prone areas
existed in the target list,
as lower fusing heights such as surface bursts would also limit the
range of the burning thermal rays due to terrain masking and shadows
cast by buildings, while also temporarily lofting far more localized fallout when compared to airburst fuzing – the standard mode of employment against un-hardened targets.
The 1951 Shot Uncle of Operation Buster-Jangle, had a yield about a tenth of the 13 to 16 Kt Hiroshima bomb, 1.2 Kt, and was detonated 5.2 m (17 ft) beneath ground level.[202] No thermal flash of heat energy was emitted to the surroundings in this shallow buried test. The explosion resulted in a cloud that rose to 3.5 km (11,500 ft). The resulting crater was 260 feet (79 m) wide and 53 feet (16 m) deep. The yield is similar to that of an Atomic Demolition Munition.
Altfeld and Cimbala argue that true belief in nuclear winter might lead
nations towards building greater arsenals of weapons of this type. However, despite being complicated due to the advent of Dial-a-yield
technology, data on these low yield nuclear weapons suggests that they,
as of 2012, make up about a tenth of the arsenal of the US and Russia,
and the fraction of the stockpile that they occupy has diminished since
the 1970–1990s, not grown. A factor in this is that very thin devices with yields of approximately
one kiloton of energy are nuclear weapons that make very inefficient
use of their nuclear materials, e.g. two-point implosion. Thus a more psychologically detering higher efficiency/higher yield device, can instead be constructed from the same mass of fissile material.
This logic is similarly reflected in the originally classified 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 serve yet further
motivation to follow the existing trends, towards the development of more accurate, and even lower explosive yield, nuclear weapons. As the winter hypothesis 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
(RNEP), would safeguard against the nuclear winter potential. With the
latter capabilities of the then, largely still conceptual RNEP,
specifically cited by the influential nuclear warfare analyst Albert Wohlstetter.[208] 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.
In an interview in 2000 with Mikhail Gorbachev
(the leader of the Soviet Union from 1985 to 1991), the following
statement was posed to him: "In the 1980s, you warned about the
unprecedented dangers of nuclear weapons and took very daring steps to
reverse the arms race", with Gorbachev replying "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, stating that the hypothesis
is not scientifically convincing. The report predicted that Soviet nuclear policy would be to maintain their strategic nuclear posture, such as their fielding of the high throw-weightSS-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 doctrine – 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 1985, the United States Senate met to discuss the science and politics of nuclear winter. During the congressional hearing, the influential analyst Leon Gouré
presented evidence that perhaps the Soviets have simply echoed Western
reports rather than producing unique findings. Gouré hypothesized that
Soviet research and discussions of nuclear war may serve only Soviet
political agendas, rather than to reflect actual opinions of Soviet
leadership.
In 1986, the Defense Nuclear Agency document An update of Soviet research on and exploitation of Nuclear winter 1984–1986 charted the minimal [public domain] research contribution on, and Soviet propaganda usage of, the nuclear winter phenomenon.
There is some doubt as to when the Soviet Union began modelling
fires and the atmospheric effects of nuclear war. Former Soviet
intelligence officer Sergei Tretyakov claimed that, under the directions of Yuri Andropov, the KGB invented the concept of "nuclear winter" in order to stop the deployment of NATO Pershing II missiles. They are said to have distributed to peace groups, the environmental movement and the journal Ambio disinformation based on a faked "doomsday report" by the Soviet Academy of Sciences by Georgii Golitsyn, Nikita Moiseyev and Vladimir Alexandrov concerning the climatic effects of nuclear war. Although it is accepted that the Soviet Union exploited the nuclear winter hypothesis for propaganda purposes, Tretyakov's inherent claim that the KGB funnelled disinformation to Ambio,
the journal in which Paul Crutzen and John Birks published the 1982
paper "Twilight at Noon", has not been corroborated as of 2009. In an interview in 2009 conducted by the National Security Archive, Vitalii Nikolaevich Tsygichko (a Senior Analyst at the Soviet Academy of Sciences
and military mathematical modeler) stated that Soviet military analysts
were discussing the idea of "nuclear winter" years before U.S.
scientists, although they did not use that exact term.
A number of solutions have been proposed to mitigate the potential
harm of a nuclear winter if one appears inevitable. The problem has been
attacked at both ends; some solutions focus on preventing the growth of
fires and therefore limiting the amount of smoke that reaches the
stratosphere in the first place, and others focus on food production
with reduced sunlight, 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 using 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 stable, firestorm.
In the book Feeding Everyone No Matter What,
under the worst-case scenario predictions of nuclear winter, the
authors present various unconventional food possibilities. These include
natural-gas-digesting bacteria, the most well known being Methylococcus capsulatus, that is presently used as a feed in fish farming; bark bread, a long-standing famine food using the edible inner bark of trees, and part of Scandinavian history during the Little Ice Age; increased fungiculture or mushrooms such as the honey fungi that grow directly on moist wood without sunlight; and variations of wood or cellulosic biofuel production, which typically already creates edible sugars/xylitol from inedible cellulose, as an intermediate product before the final step of alcohol generation. One of the book's authors, mechanical engineer David Denkenberger,
states that mushrooms could theoretically feed everyone for three years.
Seaweed, like mushrooms, can also grow in low-light conditions.
Dandelions and tree needles could provide Vitamin C, and bacteria could
provide Vitamin E. More conventional cold-weather crops such as potatoes
might get sufficient sunlight at the equator to remain feasible.
Large-scale food stockpiling
To feed portions of civilization through a nuclear winter, large
stockpiles of food storage prior to the event would have to be
accomplished. Such stockpiles should be placed underground, at higher
elevations and near the equator to mitigate high altitude UV and
radioactive isotopes. Stockpiles should also be placed near populations
most likely to survive the initial catastrophe. One consideration is who
would sponsor the stockpiling. "There may be a mismatch between those
most able to sponsor the stockpiles (i.e., the pre-catastrophe wealthy)
and those most able to use the stockpiles (the pre-catastrophe rural
poor)." The minimum annual global wheat storage is approximately 2 months.
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 2 ˚C of surface warming as a result of the
doubling in CO2 levels within 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 been proposed
by Paul Crutzen and others. According to the threshold "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; and
after 10 years, average global temperatures would still be 0.5 °C lower
than before the soot injection.
An animation depicting a massive asteroid–Earth impact and subsequent impact crater formation. The asteroid connected with the extinction of the Cretaceous–Paleogene extinction event released an estimated energy of 100 teratonnes of TNT (420 ZJ). corresponding to 100,000,000 Mt of energy, roughly 10,000 times the
maximum combined arsenals of the US and Soviet Union in the Cold War. This is hypothesized to have produced sufficient ground-energy coupling to have caused severe mantle plume (volcanism) at the antipodal point (the opposite side of the world).
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. The effects of smoke in the atmosphere (short wave absorption) are
sometimes termed an "antigreenhouse" effect, and a strong analog is the
hazy atmosphere of Titan.
Pollack, Toon and others were involved in developing models of Titan's
climate in the late 1980s, at the same time as their early nuclear
winter studies.
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 Wendy Wolbach, H. Jay Melosh and Owen Toon, suggests that as a result
of massive impact events, the small sand-grain-sized ejecta fragments created can meteoricallyre-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 Cretaceous–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 firestorm winter, however, has been questioned in more recent years (2003–2013) by Claire Belcher, Tamara Goldinand Melosh, who had initially supported the hypothesis, with this re-evaluation being dubbed the "Cretaceous-Palaeogene firestorm debate" by Belcher.
Depending
on the size of the meteor, it will either burn up high in the
atmosphere or reach lower levels and explode in an air burst akin to the
Chelyabinsk meteor of 2013, which approximated the thermal effects of a nuclear explosion.
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, Owen Toon et al. in
2013 were critical of the re-evaluations the hypothesis is undergoing.
It is difficult to successfully ascertain 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 directly by the meteor impact is difficult to determine.
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