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A nuclear holocaust or nuclear apocalypse would be widespread destruction, possibly causing the collapse of civilization, through the use of nuclear weapons. Under such a scenario, some of the Earth is made uninhabitable by nuclear warfare in future world wars.

Nuclear physicists and theorists have speculated that a thermonuclear war could result in the end of modern civilization on Earth due to the widespread devastation from nuclear blasts, firestorms, and radiation sickness; the temporary loss of much modern technology due to electromagnetic pulses; and the hypothetical long-term effects of a nuclear winter.

Early Cold War-era studies suggested that billions of humans would nonetheless survive the immediate effects of nuclear blasts and radiation following a global thermonuclear war. However, there is much debate about whether nuclear war could indirectly contribute to human extinction if environmental consequences, societal breakdown, and economic collapse rendered humanity vulnerable to a subsequent existential risk. For example, it has been argued that even a relatively small-scale nuclear exchange between India and Pakistan, involving on the order of hundreds of weapons, could cause a nuclear winter and kill more than a billion people.

Since 1947, the Doomsday Clock of the Bulletin of the Atomic Scientists has visualized how far the world is from a nuclear war.

The threat of a nuclear holocaust plays an important role in the popular perception of nuclear weapons. It features in the security concept of mutually assured destruction (MAD) and is a common scenario in survivalism. Nuclear holocaust is a common feature in literature and film, especially in speculative genres such as science fiction, dystopian and post-apocalyptic fiction.

Etymology and usage
The English word "holocaust", derived from the Greek term "holokaustos" meaning "completely burnt", refers to great destruction and loss of life, especially by fire.

Possibly the first printed use of the word "holocaust" to describe an imagined nuclear destruction appears in Reginald Glossop's 1926 novel The Orphan of Space: "Moscow ... beneath them ... a crash like a crack of Doom! The echoes of this Holocaust rumbled and rolled ... a distinct smell of sulphur ... atomic destruction." In the novel, an atomic weapon is planted in the office of the Soviet dictator who, with German help and Chinese mercenaries, is preparing the takeover of Western Europe.

In the 1960s, the word principally referred to nuclear destruction. After the mid-1970s, when the word "holocaust" became closely associated with the Nazi Holocaust, references to nuclear destruction have usually spoken of "atomic holocaust" or "nuclear holocaust".

Likelihood of nuclear war
As of 2016, humanity has about 15,000 nuclear weapons, thousands of which are on hair-trigger alert. While stockpiles have been on the decline following the end of the cold war, every nuclear country is currently undergoing modernization of its nuclear arsenal. Some experts believe this modernization may increase the risk of nuclear proliferation, nuclear terrorism, and accidental nuclear war.

While it is difficult to assess the actual probability of a nuclear conflict, since the beginning of the atomic age, there have been many near-misses suggesting that the probability that the anthropic principle may be at work.

John F. Kennedy estimated the probability of the Cuban Missile Crisis escalating to nuclear conflict as between 33% and 50%.

In a poll of experts at the Global Catastrophic Risk Conference in Oxford (17‐20 July 2008), the Future of Humanity Institute estimated the probability of complete human extinction by nuclear weapons at 1% within the century, the probability of 1 billion dead at 10% and the probability of 1 million dead at 30%. These results reflect the median opinions of a group of experts, rather than a probabilistic model; the actual values may be much lower or higher.

It has been argued that a small-scale nuclear war even between two countries could have devastating global consequences and, by definition, is more likely than complete human extinction.

Moral importance of human extinction risk
Some scholars, such as utilitarian philosopher Derek Parfit, argue that there is an overwhelming moral imperative to reduce even small risks of human extinction, given the magnitude of the calamity were the human race to become extinct.

Likelihood of complete human extinction
Many scholars have posited that a global thermonuclear war is likely to lead to human extinction, but this position is not supported by science. A nuclear war could lead to a massive loss of human life, but if the last few, most resilient humans settlements are unlikely to also die off, then human extinction is unlikely. However, the risk of extinction from nuclear war is not zero, as existing climate models that predict parts of the world will remain habitable could prove incorrect. In addition, a nuclear war could indirectly contribute to human extinction, if societal collapse rendered humanity vulnerable to a subsequent existential risk.

As a result of the extensive nuclear fallout of the 1954 Castle Bravo nuclear detonation, author Nevil Shute wrote the popular novel On the Beach which was released in 1957, in this novel so much fallout is generated in a nuclear war that all human life is extinguished. However the premise that all of humanity would die following a nuclear war and only the "cockroaches would survive" is critically dealt with in the 1988 book Would the Insects Inherit the Earth and Other Subjects of Concern to Those Who Worry About Nuclear War by nuclear weapons expert Philip J. Dolan.

In 1982 nuclear disarmament activist Jonathan Schell published The Fate of the Earth, which is regarded by many to be the first carefully argued presentation that concluded that extinction is a significant possibility from nuclear war. However, the assumptions made in this book have been thoroughly analyzed and determined to be "quite dubious". The impetus for Schell's work, according to physicist Brian Martin, was to argue that "if the thought of 500 million people dying in a nuclear war is not enough to stimulate action, then the thought of extinction will. Indeed, Schell explicitly advocates use of the fear of extinction as the basis for inspiring the "complete rearrangement of world politics".

The belief in "overkill" is also commonly encountered, with an example being the following statement made by nuclear disarmament activist Philip Noel-Baker in 1971 – "Both the US and the Soviet Union now possess nuclear stockpiles large enough to exterminate mankind three or four – some say ten – times over". Brian Martin suggested that the origin of this belief was from "crude linear extrapolations", and when analyzed it has no basis in reality. Similarly, it is common to see stated that the combined explosive energy released in the entirety of World War II was about 3 megatons, while a nuclear war with warhead stockpiles at Cold War highs would release 6000 WWII's of explosive energy. An estimate for the necessary amount of fallout to begin to have the potential of causing human extinction is regarded by physicist and disarmament activist Joseph Rotblat to be 10 to 100 times the megatonnage in nuclear arsenals as they stood in 1976; however, with the world megatonnage decreasing since the Cold War ended this possibility remains hypothetical.

According to the 1980 United Nations report General and Complete Disarmament: Comprehensive Study on Nuclear Weapons: Report of the Secretary-General, it was estimated that there were a total of about 40,000 nuclear warheads in existence at that time, with a potential combined explosive yield of approximately 13,000 megatons.

By comparison, in the Timeline of volcanism on Earth when the volcano Mount Tambora erupted in 1815 – turning 1816 into the Year Without A Summer due to the levels of global dimming sulfate aerosols and ash expelled – it exploded with a force of roughly 800 to 1,000 megatons, and ejected 160 km3 of mostly rock/tephra, which included 120 million tonnes of sulfur dioxide as an upper estimate. A larger eruption, approximately 74,000 years ago, in Mount Toba produced 2800 km3 of tephra, forming lake Toba, and produced an estimated 6000 e6t of sulfur dioxide. The explosive energy of the eruption may have been as high as equivalent to 20,000,000 megatons (Mt) of TNT, while the Chicxulub impact, connected with the extinction of the dinosaurs, corresponds to at least 70,000,000 Mt of energy, which is roughly 7000 times the maximum arsenal of the US and Soviet Union.

However, it must be noted that comparisons with supervolcanos are more misleading than helpful due to the different aerosols released, the likely air burst fuzing height of nuclear weapons and the globally scattered location of these potential nuclear detonations all being in contrast to the singular and subterranean nature of a supervolcanic eruption. Moreover, assuming the entire world stockpile of weapons were grouped together, it would be difficult due to the nuclear fratricide effect to ensure the individual weapons would go off all at once. Nonetheless, many people believe that a full-scale nuclear war would result, through the nuclear winter effect, in the extinction of the human species, though not all analysts agree on the assumptions inputted into these nuclear winter models.

Self-assured destruction
In a Bulletin of the Atomic Scientists feature, Alan Robock and Owen Toon argued that our growing understanding of the effects of nuclear winter necessitates that the doctrine of mutually assured destruction (MAD) be replaced with self-assured destruction (SAD). That is, regardless of whose cities burned, the effects of the resultant nuclear winter on the food supply would be felt worldwide. Even should the aggressor execute a successful first strike attack, they could not guarantee that most of their citizens would survive.

Importantly, Robock and Toon argued that the SAD concept applies not only to the U.S. and Russia, even after their proposed arsenal reductions, but to countries with smaller arsenals too. Britain, France, China, Israel, India, and Pakistan all have sufficient arsenals to threaten the global food supply via effects of mini nuclear winter, impaired precipitation, and ozone depletion.

Effects of nuclear war
Historically, it has been difficult to estimate the total number of casualties resulting from a global nuclear exchange because scientists are continually discovering new effects of nuclear weapons, and also revising existing models.

Early reports considered direct effects from nuclear blast and radiation and indirect effects from economic, social, and political disruption. In a 1979 report for the U.S. Senate, the Office of Technology Assessment estimated casualties under different scenarios. For a full-scale countervalue/counterforce nuclear exchange between the U.S. and the Soviet Union, they predicted U.S. deaths from 35 to 77 percent (70 million to 160 million dead at the time), and Soviet deaths from 20 to 40 percent of the population.

Although this report was made when nuclear stockpiles were at much higher levels than they are today, it also was made before the risk of nuclear winter was discovered in the early 1980s. Additionally, it did not consider other secondary effects, such electromagnetic pulses (EMP), and the ramifications they would have on modern technology and industry.

Nuclear winter
In the early 1980s, scientists began to consider the effects of smoke and soot arising from burning wood, plastics, and petroleum fuels in nuclear-devastated cities. It was speculated that the intense heat would carry these particulates to extremely high altitudes where they could drift for weeks and block out all but a fraction of the sun's light. A landmark 1983 study by the so-called TTAPS team (Richard P. Turco, Owen Toon, Thomas P. Ackerman, James B. Pollack and Carl Sagan) was the first to model these effects and coined the term "nuclear winter."

More recent studies make use of modern global circulation models and far greater computer power than was available for the 1980s studies. A 2007 study examined consequences of a global nuclear war involving moderate to large portions of the current global arsenal. The study found cooling by about 12-20 °C in much of the core farming regions of the US, Europe, Russia and China and as much as 35 °C in parts of Russia for the first two summer growing seasons. The changes they found were also much longer lasting than previously thought, because their new model better represented entry of soot aerosols in the upper stratosphere, where precipitation does not occur, and therefore clearance was on the order of 10 years. In addition, they found that global cooling caused a weakening of the global hydrological cycle, reducing global precipitation by about 45%.

The authors 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."

In contrast to the above investigations of global nuclear conflicts, studies have shown that even small-scale, regional nuclear conflicts could disrupt the global climate for a decade or more. In a regional nuclear conflict scenario where two opposing nations in the subtropics would each use 50 Hiroshima-sized nuclear weapons (about 15 kiloton each) on major populated centres, the researchers estimated as much as five million tons of soot would be released, which would produce a cooling of several degrees over large areas of North America and Eurasia, including most of the grain-growing regions. The cooling would last for years, and according to the research, could be "catastrophic". Additionally, the analysis showed a 10% drop in average global precipitation, with the largest losses in the low latitudes due to failure of the monsoons.

Regional nuclear conflicts could also inflict significant damage to the ozone. A 2008 study found that a regional nuclear weapons exchange could create a near-global ozone hole, triggering human health problems and impacting agriculture for at least a decade. This effect on the ozone would result from heat absorption by soot in the upper stratosphere, which would modify wind currents and draw in ozone-destroying nitrogen oxides. These high temperatures and nitrogen oxides would reduce ozone to the same dangerous levels we now experience below the ozone hole above Antarctica every spring.

Nuclear famine
It is difficult to estimate the number of casualties that would result from nuclear winter, but it is likely that the primary effect would be mass starvation due to disrupted agricultural production and distribution.

A 2012 study examined agricultural impacts of a hypothetical regional nuclear conflict between India and Pakistan (using climate predictions from Robock ) and showed very significant declines in both corn and soybean production in the U.S. midwest. Averaged over 10 years, corn production declined by 10% and soybean by 6-12%, depending on location. Year-to-year variability was high, with greatest declines occurring 5 years following the event.

A separate study examined rice production in China using the same climate model, but a different agricultural model. They predicted that rice production would decline by an average of 21% the first 4 years after soot injection and by 10% for the following 6 years. A followup study examined effects on Chinese rice, maize and wheat production, and found wheat was particularly affected, dropping more than 50% in the first year and averaging 39% in the first 5 years.

In a 2013 report, the International Physicians for the Prevention of Nuclear War (IPPNW) concluded that more than two billion people would be at risk of starvation in the event of a limited nuclear exchange, such as one that could occur between India and Pakistan, or by the use of even a small number of the nuclear weapons held by the US and Russia.

This report argued that the world is in a state in which it particularly vulnerable to even modest declines in food production. In turn, modest changes in average global temperature can have disproportionately large effects on crops. The agriculture studies mentioned above showed substantial declines in U.S. and Chinese crop production, and yet were probably conservative because they did not take into account ozone depletion or, more importantly, daily temperature extremes. They cite the example of the Mount Tambora volcanic eruption in 1815, which produced an average annual temperature deviation of only -0.7 °C, but which brought mid-summer killing frosts to the mid-Atlantic states and caused up to 75% crop losses in northern Europe.

In turn, they argue that small perturbations in the food supply are highly amplified for malnourished populations. In particular, about 800 million people are chronically malnourished and even a 10% decline in their food consumption would put them at risk. World reserves of grain stocks could serve as a buffer to this and are estimated to last approximately 68–77 days.

Lastly, they note that famines are often associated with epidemics, citing that the Tambora famine of 1816 triggered an epidemic of typhus in Ireland that spread to much of Europe and the Bengal famine of 1943, which was associated with major local epidemics of cholera, malaria, smallpox, and dysentery. Similarly, they propose that the vast and crowded megacities of the developing world would see major outbreaks of infectious diseases and illnesses.

Electromagnetic pulse
An electromagnetic pulse (commonly abbreviated as EMP, pronounced ) is a burst of electromagnetic radiation. Nuclear explosions create a pulse of electromagnetic radiation called a nuclear EMP or NEMP. Such EMP interference is known to be generally disruptive or damaging to electronic equipment. If a single nuclear weapon "designed to emit EMP were detonated 250 to 300 miles up over the middle of the country it would disable the electronics in the entire United States."

Given that all of the comforts and necessities we enjoy in the 21st century are predicated on electronics and their functioning, an EMP would disable hospitals, water treatment facilities, food storage facilities, and all electronic forms of communication. An EMP blast threatens the foundation which supports the existence of the modern human condition. Certain EMP attacks could lead to large loss of power for months or years. Currently, failures of the power grid are dealt with using support from the outside. In the event of an EMP attack, such support would not exist and all damaged components, devices, and electronics would need to be completely replaced.

In 2013, the US House of Representatives considered the "Secure High-voltage Infrastructure for Electricity from Lethal Damage Act" that would provide surge protection for some 300 large transformers around the country. The problem of protecting civilian infrastructure from electromagnetic pulse has also been intensively studied throughout the European Union, and in particular by the United Kingdom. While precautions have been taken, the EMP Commission estimated that, within 12 months of a nationwide blackout, up to 90% of the U.S. population would die from starvation, disease, and societal breakdown. The greatest threat to human survival in the aftermath of an EMP blast would be the inability to access clean drinking water. For comparison, in the aftermath of the 2010 Haitian earthquake, the water infrastructure had been devastated and led to at least 3,333 deaths from cholera in the first few months after the earthquake. Other countries would similarly see the resurgence of previously non-existent diseases as clean water becomes increasingly scarce.

Nuclear fallout
Nuclear fallout is the residual radioactive material propelled into the upper atmosphere following a nuclear blast or a nuclear reaction conducted in an unshielded facility, so called because it "falls out" of the sky after the explosion and the shock wave have passed.

The main radiation hazard from fallout is due to short-lived radionuclides external to the body. While most of the particles carried by nuclear fallout decay rapidly, some radioactive particles will have half-lives of seconds to a few months. Some radioactive isotopes, like strontium 90 and cesium 137, are very long lived and will create radioactive hot spots for up to 5 years after the initial explosion.

Fallout and black rain may contaminate waterways, agriculture, and soil depending on the factors described above. Contact with radioactive materials can lead to radiation poisoning through external exposure or accidental consumption. In acute doses over a short amount of time radiation will lead to prodromal syndrome, bone marrow death, central nervous system death and gastrointestinal death.

The following is a quote from a survivor of the Hiroshima bombing which describes black rain fallout:

Over longer periods of exposure to radiation cancer becomes the main health risk. Approximately 1 out of every 80 people exposed to 1 Gray will die from cancer, in addition to the normal rate of 20 out of 80. About 1 in 40 people will get cancer, in addition to the typical rates of 16-20 out of 40. Furthermore, long term radiation exposure can also lead to in utero effects on human development and transgenerational genetic damage.

Nuclear power plant breakdown
Nuclear reactors can fail in a variety of ways. Should the instability of the nuclear material generate unexpected behavior, it may result in an uncontrolled power excursion. Normally, the cooling system in a reactor is designed to be able to handle the excess heat this causes; however, should the reactor also experience a loss-of-coolant accident, then the fuel may melt or cause the vessel in which it is contained to overheat and melt. This event is called a nuclear meltdown.

After shutting down for some time the reactor still needs external energy to power its cooling systems. Normally this energy is provided by the power grid to which that plant is connected, or by emergency diesel generators. Failure to provide power for the cooling systems, as happened in Fukushima, can cause serious accidents.

In the aftermath of a nuclear holocaust, there would be an inability for personnel to maintain the necessary coolant protocol. Furthermore, the destruction left by nuclear blasts would disrupt the production and delivery of the coolant material to nuclear power plants. These blasts would also destroy the electric grid that is necessary for maintaining and regulating the cooling of the nuclear power plants. In light of the failure of these essential systems nuclear power plants would face a risk of meltdown and failure in the aftermath of a nuclear war.

Nuclear reactors also become preferred targets during military conflict and, over the past three decades, have been repeatedly attacked during military air strikes, occupations, invasions and campaigns. Their essential role in powering large populations and the increased destruction ensured by great radioactive nuclear fallout makes them high risk targets during military conflict.