Nuclear holocaust

A nuclear holocaust, also known as a nuclear apocalypse, nuclear annihilation, nuclear armageddon, or atomic holocaust, is a theoretical scenario where the mass detonation of nuclear weapons causes widespread destruction and radioactive fallout, with global consequences. Such a scenario envisages large parts of the Earth becoming uninhabitable due to the effects of nuclear warfare, potentially causing the collapse of civilization, the extinction of humanity, or the termination of most biological life on Earth.
Besides the immediate destruction of cities by nuclear blasts, the potential aftermath of a nuclear war could involve firestorms, a nuclear winter, widespread radiation sickness from fallout, and/or the temporary loss of much modern technology due to electromagnetic pulses. Some scientists, such as Alan Robock, have speculated that a thermonuclear war could result in the end of modern civilization on Earth, in part due to a long-lasting nuclear winter. In one model, the average temperature of Earth following a full thermonuclear war falls for several years by 7 to 8 °C on average.
Early Cold War-era studies suggested that billions of humans would survive the immediate effects of nuclear blasts and radiation following a global thermonuclear war. The International Physicians for the Prevention of Nuclear War believe that nuclear war could indirectly contribute to human extinction via secondary effects, including environmental consequences, societal breakdown, and economic collapse.
The threat of a nuclear holocaust plays an important role in the anti-nuclear movement and the development of popular perception of nuclear weapons. It features in the security concept of mutually assured destruction 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.
One early 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.
More broadly, the use of nuclear weapons, particularly their testing, has been discussed as genocide, ecocide, environmental racism, nuclear imperialism, and colonialism by anti-nuclear activists.

Likelihood of nuclear war

As of 2021, humanity has about 13,410 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. The Bulletin advanced their symbolic Doomsday Clock in 2015, citing among other factors "a nuclear arms race resulting from modernization of huge arsenals". In January 2020, it was moved forward to 100 seconds before midnight. In 2023, it was moved forward to 90 seconds before midnight. In 2025, it was moved to 89 seconds before midnight. In January 2026, the clock moved to 85 seconds befor midnight.
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, 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.
Scientists have argued that even a small-scale nuclear war between two countries, such as India and Pakistan, could have devastating global consequences and such local conflicts are more likely than full-scale nuclear war.

Moral importance of human extinction risk

In his book Reasons and Persons, philosopher Derek Parfit posed the following question:

Compare three outcomes:
Peace.
A nuclear war that kills 99% of the world's existing population.
A nuclear war that kills 100%.
would be worse than, and would be worse than. Which is the greater of these two differences?

He continues that "Most people believe that the greater difference is between and. I believe that the difference between and is very much greater." Thus, he argues, even if it would be bad if massive numbers of humans died, human extinction would itself be much worse because it prevents the existence of all future generations. And given the magnitude of the calamity were the human race to become extinct, Nick Bostrom argues that there is an overwhelming moral imperative to reduce even small risks of human extinction.

Likelihood of complete human extinction

Many scholars have posited that a global thermonuclear war with Cold War-era stockpiles, or even with the current smaller stockpiles, may lead to human extinction. This position was bolstered when nuclear winter was first conceptualized and modelled in 1983. However, models from the past decade consider total extinction very unlikely, and suggest parts of the world would remain habitable. Technically the risk may not be zero, as the climatic effects of nuclear war are uncertain and could theoretically be larger, but also smaller, than current models suggest. There could also be indirect risks, such as a societal collapse following nuclear war that can make humanity much more vulnerable to other existential threats.
A related area of inquiry is: if a future nuclear arms race someday leads to larger stockpiles or more dangerous nuclear weapons than existed at the height of the Cold War, at what point could war with such weapons result in human extinction? Physicist Leo Szilard warned in the 1950s that a deliberate doomsday device could be constructed by surrounding powerful hydrogen bombs with a massive amount of cobalt. Cobalt has a half-life of five years, and its global fallout might, some physicists have posited, be able to clear out all human life via lethal radiation intensity. The main motivation for building a cobalt bomb in this scenario is its reduced expense compared with the arsenals possessed by superpowers; such a doomsday device does not need to be launched before detonation and thus does not require expensive missile delivery systems, and the hydrogen bombs do not need to be miniaturized for delivery via missile. The system for triggering it might have to be completely automated, in order for the deterrent to be effective. A modern twist might be to also lace the bombs with aerosols designed to exacerbate nuclear winter. A major caveat is that nuclear fallout transfer between the northern and southern hemispheres is expected to be small; unless a bomb detonates in each hemisphere, the effect of a bomb detonated in one hemisphere on the other is diminished.

Effects of nuclear war

Historically, it has been difficult to estimate the total number of deaths 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, 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 first theorized in the early 1980s. Additionally, it did not consider other secondary effects, such as electromagnetic pulses, 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 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 the 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, Canada, 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 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 layer. 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 that are experienced below the ozone hole above Antarctica every spring.