Human extinction is the hypothetical complete end of the human species. This may result either from natural causes or due to anthropogenic (human) causes, but the risks of extinction through natural disaster, such as a meteorite impact or large-scale volcanism, are generally considered to be comparatively low. Anthropogenic human extinction is sometimes called omnicide.
Many possible scenarios of anthropogenic extinction have been proposed, such as climate change, global nuclear annihilation, biological warfare and ecological collapse. Some scenarios center on emerging technologies, such as advanced artificial intelligence, biotechnology, or self-replicating nanobots. The probability of anthropogenic human extinction within the next hundred years is the topic of an active debate.
The study of human extinction arose relatively recently in human history. Ancient Western philosophers such as Plato, Aristotle, and Lucretius wrote of the end of humankind only as part of a cycle of renewal. Later philosophers such as Al-Ghazali, William of Ockham, and Gerolamo Cardano expanded the study of logic and probability and began discussing abstract possible worlds, including a world without humans. The notion that species can go extinct gained scientific acceptance during the Age of Enlightenment in the 17th and 18th centuries, and by 1800, Georges Cuvier had identified 23 extinct prehistoric species.
In the 19th century, human extinction became a popular topic in science (e.g., Thomas Robert Malthus's An Essay on the Principle of Population) and fiction (e.g., Mary Shelley's The Last Man). In 1863, a few years after Charles Darwin published On the Origin of Species, William King proposed that Neanderthals were an extinct species of the genus Homo. At the turn of the 20th century, Russian cosmism, a precursor to modern transhumanism, advocated avoiding humanity's extinction by colonizing space.
The large scale destruction of World War I and the development of nuclear weapons at the end of World War II demonstrated that omnicide (human extinction caused by human actions) was not only possible, but plausible. In 1950, Leo Szilard suggested it was technologically feasible to build a cobalt bomb that could render the planet unlivable. Rachel Carson's 1962 Silent Spring raised awareness of environmental catastrophe. In 1983, Brandon Carter proposed the Doomsday argument, which used Bayesian probability to predict the total number of humans that will ever exist. By the beginning of the 21st century, the study of "existential risks" that threaten humankind became "a growing field of rigorous scientific inquiry".
Many experts who study these issues estimate that the total chance of human extinction in the 21st century is between 1 and 20%. In 2008, an informal survey of experts on different global catastrophic risks at the Global Catastrophic Risk Conference at the University of Oxford suggested a 19% chance of human extinction by the year 2100. The conference report cautions that the results should be taken "with a grain of salt"; the results were not meant to capture all large risks and did not include, for example, climate change, and the results likely reflect many cognitive biases of the conference participants.
In his book The Precipice: Existential Risk and the Future of Humanity, Toby Ord gives his overall estimate at the likelihood of human extinction risk this century as 1 in 6.
In a 2010 interview with The Australian, Australian scientist Frank Fenner predicted the extinction of the human race within a century, primarily as the result of human overpopulation, environmental degradation and climate change.
- A common belief is that climate change could result in human extinction. In November 2017, a statement by 15,364 scientists from 184 countries indicated that increasing levels of greenhouse gases from use of fossil fuels, human population growth, deforestation, and overuse of land for agricultural production, particularly by farming ruminants for meat consumption, are trending in ways that forecast an increase in human misery over coming decades. An October 2017 report published in The Lancet stated that toxic air, water, soils, and workplaces were collectively responsible for nine million deaths worldwide in 2015, particularly from air pollution which was linked to deaths by increasing susceptibility to non-infectious diseases, such as heart disease, stroke, and lung cancer. The report warned that the pollution crisis was exceeding "the envelope on the amount of pollution the Earth can carry" and "threatens the continuing survival of human societies". Carl Sagan and others have raised the prospect of extreme runaway global warming turning Earth into an uninhabitable Venus-like planet. Some scholars argue that much of the world would become uninhabitable under severe global warming, but even these scholars do not tend to argue that it would lead to complete human extinction, according to Kelsey Piper of Vox. All the IPCC scenarios, including the most pessimistic ones, predict temperatures compatible with human survival. The question of human extinction under "unlikely" outlier models is not generally addressed by the scientific literature. Factcheck.org judges that climate change fails to pose an established 'existential risk', stating: "Scientists agree climate change does pose a threat to humans and ecosystems, but they do not envision that climate change will obliterate all people from the planet." On a much longer time scale, natural shifts such as Milankovitch cycles (quaternary climatic oscillations) could create unknown climate variability and change.
- A pandemic involving one or more viruses, prions, or antibiotic-resistant bacteria. Past pandemics include the 1918 Spanish flu outbreak estimated to have killed 3-5% of the global population, the 14th century Eurasian Black Death pandemic, and the various European viruses that decimated indigenous American populations. A deadly pandemic restricted to humans alone would be self-limiting as its mortality would reduce the density of its target population. A pathogen with a broad host range in multiple species, however, could eventually reach even isolated human populations, U.S. officials assess that an engineered pathogen capable of "wiping out all of humanity", if left unchecked, is technically feasible and that the technical obstacles are "trivial". However, they are confident that in practice, countries would be able to "recognize and intervene effectively" to halt the spread of such a microbe and prevent human extinction.
- Human activity has triggered an extinction event often referred to as the sixth "mass extinction", which scientists consider a major threat to the continued existence of human civilization. The 2019 Global Assessment Report on Biodiversity and Ecosystem Services, published by the United Nations' Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services, asserts that roughly one million species of plants and animals face extinction from human impacts such as expanding land use for industrial agriculture and livestock rearing, along with overfishing. A 1997 assessment states that over a third of Earth's land has been modified by humans, that atmospheric carbon dioxide has increased around 30 percent, that humans are the dominant source of nitrogen fixation, that humans control most of the Earth's accessible surface fresh water, and that species extinction rates may be over a hundred times faster than normal.
- Overpopulation: The Global Footprint Network estimates that current activity uses resources twice as fast as they can be naturally replenished, and that growing human population and increased consumption pose the risk of resource depletion and a concomitant population crash. Evidence suggests birth rates may be rising in the 21st century in the developed world. Projections vary; researcher Hans Rosling has projected population growth to start to plateau around 11 billion, and then to slowly grow or possibly even shrink thereafter. A 2014 study published in Science asserts that the human population will grow to around 11 billion by 2100 and that growth will continue into the next century.
- Population decline through a preference for fewer children. If developing world demographics are assumed to become developed world demographics, and if the latter are extrapolated, some projections suggest an extinction before the year 3000. John A. Leslie estimates that if the reproduction rate drops to the German or Japanese level the extinction date will be 2400.[a] However, some models suggest the demographic transition may reverse itself due to evolutionary biology.
- A supervolcanic eruption.
Some scenarios involve extinction as a result of the effects or use of totally new technologies. Scenarios include:
- Nuclear and biological weapons, whether used in war or terrorism, could result in human extinction. Some fear a hypothetical World War III could cause the annihilation of humankind, perhaps by a resulting nuclear winter as has been hypothesized by experts. Noun and Chyba propose three categories of measures to reduce risks from biotechnology and natural pandemics: Regulation or prevention of potentially dangerous research, improved recognition of outbreaks and developing facilities to mitigate disease outbreaks (e.g. better and/or more widely distributed vaccines).
- The creators of a superintelligent entity could inadvertently give it goals that lead it to annihilate the human race. A survey of AI experts estimated that the chance of human-level machine learning having an "extremely bad (e.g., human extinction)" long-term effect on humanity is 5%.
- Uncontrolled nanotechnology (grey goo) incidents resulting in the destruction of the Earth's ecosystem (ecophagy). Chris Phoenix and Treder classify catastrophic risks posed by nanotechnology into three categories: (1) From augmenting the development of other technologies such as AI and biotechnology. (2) By enabling mass-production of potentially dangerous products that cause risk dynamics (such as arms races) depending on how they are used. (3) From uncontrolled self-perpetuating processes with destructive effects. Several researchers say the bulk of risk from nanotechnology comes from the potential to lead to war, arms races and destructive global government.
- Creation of a micro black hole on Earth during the course of a scientific experiment, or other unlikely scientific accidents in high-energy physics research, such as vacuum phase transition or strangelet incidents. There were worries concerning the Large Hadron Collider at CERN as it is feared that collision of protons at near the speed of light will result in the creation of a black hole, but it has been pointed out that much more energetic collisions take place currently in Earth's atmosphere.
- Some scenarios envision that humans could use genetic engineering or technological modifications to split into normal humans and a new species – posthumans. Such a species could be fundamentally different from any previous life form on Earth, e.g. by merging humans with technological systems. Such scenarios assess the risk that the "old" human species will be outcompeted and driven to extinction by the new, posthuman entity.
- A geological or cosmological disaster such as an impact event of a near-Earth object (NEOs), which serve as an absolute threat to the survival of living species. A single extraterrestrial event (asteroid or comet impact) can lead to widespread species extinctions. However, none of the large "dinosaur-killer" asteroids known to Spaceguard pose a near-term threat of collision with Earth.
- Supernovae, gamma-ray bursts, solar flares, and cosmic rays, if strong enough, could be lethal to humans on Earth.
- The Earth will naturally become uninhabitable due to the Sun's stellar evolution, within about a billion years. In around 1 billion years from now, the Sun's brightness may increase as a result of a shortage of hydrogen, and the heating of its outer layers may cause the Earth's oceans to evaporate, leaving only minor forms of life. Well before this time, the level of carbon dioxide in the atmosphere will be too low to support plant life, destroying the foundation of the food chains. See Future of the Earth. About 7–8 billion years from now, if and after the Sun has become a red giant, the Earth will probably be engulfed by an expanding Sun and destroyed. According to standard physics, the entire universe over much, much larger timescales will become gradually uninhabitable, resulting eventually in unavoidable human extinction associated with the heat death of the universe.
- Invasion by militarily superior extraterrestrials (see alien invasion) – often considered to be a scenario purely from the realm of science fiction, professional SETI researchers have given serious consideration to this possibility, but conclude that it is unlikely.[b]
Nick Bostrom, a philosopher at the University of Oxford known for his work on existential risk, argues that it would be "misguided" to assume that the probability of near-term extinction is less than 25% and that it will be "a tall order" for the human race to "get our precautions sufficiently right the first time", given that an existential risk provides no opportunity to learn from failure. A little more optimistically, philosopher John Leslie assigns a 70% chance of humanity surviving the next five centuries, based partly on the controversial philosophical doomsday argument that Leslie champions. Leslie's argument is somewhat frequentist, based on the observation that human extinction has never observed, but requires subjective anthropic arguments. Leslie also discusses the anthropic survivorship bias (which he calls an "observational selection" effect on page 139) and states that the a priori certainty of observing an "undisastrous past" could make it difficult to argue that we must be safe because nothing terrible has yet occurred. He quotes Holger Bech Nielsen's formulation: "We do not even know if there should exist some extremely dangerous decay of say the proton which caused eradication of the earth, because if it happens we would no longer be there to observe it and if it does not happen there is nothing to observe."
Some scholars argue that certain scenarios such as global thermonuclear war would have difficulty eradicating every last settlement on Earth. Physicist Willard Wells points out that any credible extinction scenario would have to reach into a diverse set of areas, including the underground subways of major cities, the mountains of Tibet, the remotest islands of the South Pacific, and even to McMurdo Station in Antarctica, which has contingency plans and supplies for a long isolation. In addition, elaborate bunkers exist for government leaders to occupy during a nuclear war. The existence of nuclear submarines, which can stay hundreds of meters deep in the ocean for potentially years at a time, should also be considered. Any number of events could lead to a massive loss of human life; but if the last few (see minimum viable population) most resilient humans are unlikely to also die off, then that particular human extinction scenario may not seem credible.
Stephen Hawking advocated colonizing other planets within the solar system once technology progresses sufficiently, in order to improve the chance of human survival from planet-wide events such as global thermonuclear war.
More economically, some scholars propose the establishment on Earth of one or more self-sufficient, remote, permanently occupied settlements specifically created for the purpose of surviving global disaster. Economist Robin Hanson argues that a refuge permanently housing as few as 100 people would significantly improve the chances of human survival during a range of global catastrophes.
Substantially larger numbers, such as 500 million deaths, and especially qualitatively different scenarios such as the extinction of the entire human species, seem to trigger a different mode of thinking... People who would never dream of hurting a child hear of an existential risk, and say, "Well, maybe the human species doesn't really deserve to survive".
All past predictions of human extinction have proven to be false. To some, this makes future warnings seem less credible. Nick Bostrom argues that the lack of human extinction in the past is weak evidence that there will be no human extinction in the future, due to survivor bias and other anthropic effects.
Sociobiologist E. O. Wilson argued that: "The reason for this myopic fog, evolutionary biologists contend, is that it was actually advantageous during all but the last few millennia of the two million years of existence of the genus Homo... A premium was placed on close attention to the near future and early reproduction, and little else. Disasters of a magnitude that occur only once every few centuries were forgotten or transmuted into myth."
"Existential risks" are risks that threaten the entire future of humanity, whether by causing human extinction or by otherwise permanently crippling human progress. Multiple scholars have argued based on the size of the "cosmic endowment" that because of the inconceivably large number of potential future lives that are at stake, even small reductions of existential risk have great value. Some of the arguments run as follows:
- Carl Sagan wrote in 1983: "If we are required to calibrate extinction in numerical terms, I would be sure to include the number of people in future generations who would not be born.... (By one calculation), the stakes are one million times greater for extinction than for the more modest nuclear wars that kill "only" hundreds of millions of people. There are many other possible measures of the potential loss – including culture and science, the evolutionary history of the planet, and the significance of the lives of all of our ancestors who contributed to the future of their descendants. Extinction is the undoing of the human enterprise."
- Philosopher Derek Parfit in 1984 makes an anthropocentric utilitarian argument that, because all human lives have roughly equal intrinsic value no matter where in time or space they are born, the large number of lives potentially saved in the future should be multiplied by the percentage chance that an action will save them, yielding a large net benefit for even tiny reductions in existential risk.
- Humanity has a 95% probability of being extinct in 7,800,000 years, according to J. Richard Gott's formulation of the controversial Doomsday argument, which argues that we have probably already lived through half the duration of human history.
- Philosopher Robert Adams in 1989 rejects Parfit's "impersonal" views, but speaks instead of a moral imperative for loyalty and commitment to "the future of humanity as a vast project... The aspiration for a better society – more just, more rewarding, and more peaceful... our interest in the lives of our children and grandchildren, and the hopes that they will be able, in turn, to have the lives of their children and grandchildren as projects."
- Philosopher Nick Bostrom argues in 2013 that preference-satisfactionist, democratic, custodial, and intuitionist arguments all converge on the common-sense view that preventing existential risk is a high moral priority, even if the exact "degree of badness" of human extinction varies between these philosophies.
Parfit argues that the size of the "cosmic endowment" can be calculated from the following argument: If Earth remains habitable for a billion more years and can sustainably support a population of more than a billion humans, then there is a potential for 1016 (or 10,000,000,000,000,000) human lives of normal duration.:453–4 Bostrom goes further, stating that if the universe is empty, then the accessible universe can support at least 1034 biological human life-years; and, if some humans were uploaded onto computers, could even support the equivalent of 1054 cybernetic human life-years.
Some philosophers posit that human extinction would not be a bad thing, but a good thing. Antinatalist David Benatar argues that coming into existence is always a serious harm, therefore it's better that people don't come into existence in the future. Further, David Benatar, animal rights activist Steven Best, and anarchist Todd May, posit that human extinction would be a positive thing for the other organisms on the planet, and the planet itself, citing for example the omnicidal nature of human civilization. The environmental view in favor of human extinction is shared by the members of Voluntary Human Extinction Movement who call for refraining from reproduction and allowing the human species to go peacefully extinct, thus stopping further environmental degradation.
Psychologist Steven Pinker calls existential risk a "useless category" that can distract real threats such as climate change and nuclear war. In contrast, other researchers argue that both research and other initiatives relating to existential risk are underfunded. Nick Bostrom states that more research has been done on Star Trek, snowboarding, or dung beetles than on existential risks. Bostrom's comparisons have been criticized as "high-handed". As of 2020, the Biological Weapons Convention organization has an annual budget of US$1.4 million.
Although existential risks are less manageable by individuals than – for example – health risks, according to Ken Olum, Joshua Knobe, and Alexander Vilenkin, the possibility of human extinction does have practical implications. For instance, if the "universal" Doomsday argument is accepted it changes the most likely source of disasters, and hence the most efficient means of preventing them. They write: "... you should be more concerned that a large number of asteroids have not yet been detected than about the particular orbit of each one. You should not worry especially about the chance that some specific nearby star will become a supernova, but more about the chance that supernovas are more deadly to nearby life than we believe."
Multiple organizations with the goal of helping prevent human extinction exist. Some examples are the Future of Humanity Institute, the Centre for the Study of Existential Risk, the Future of Life Institute, and the Machine Intelligence Research Institute.
Jean-Baptiste Cousin de Grainville's 1805 Le dernier homme (The Last Man), which depicts human extinction due to infertility, is considered the first modern apocalyptic novel and credited with launching the genre. Other notable early works include Mary Shelley's 1826 The Last Man, depicting human extinction caused by a pandemic, and Olaf Stapledon's 1937 Star Maker, "a comparative study of omnicide".
Some 21st century pop-science works, including The World Without Us by Alan Weisman, pose an artistic thought experiment: wondering what would happen to the rest of the planet if humans suddenly disappeared. A threat of human extinction, such as through a technological singularity (also called an intelligence explosion), drives the plot of innumerable science fiction stories; an influential early example is the 1951 film adaption of When Worlds Collide. Usually the extinction threat is narrowly avoided, but some exceptions exist, such as R.U.R. and Steven Spielberg's A.I.
- Deep ecology
- Doomsday argument
- Doomsday Clock
- End time
- Extinction event
- Extinction Rebellion
- Global catastrophic risk
- Great Filter
- Holocene extinction
- Human overpopulation
- Life After People
- List of dates predicted for apocalyptic events
- Mutual assured destruction
- Nuclear holocaust
- Societal collapse
- Speculative evolution
- Space and survival
- Toba catastrophe theory
- Voluntary Human Extinction Movement
- For the "West Germany" extrapolation see: Leslie, 1996 (The End of the World) in the "War, Pollution, and disease" chapter (page 74). In this section the author also mentions the success (in lowering the birth rate) of programs such as the sterilization-for-rupees programs in India, and surveys other infertility or falling birth-rate extinction scenarios. He says that the voluntary small family behaviour may be counter-evolutionary, but that the meme for small, rich families appears to be spreading rapidly throughout the world. In 2150 the world population is expected to start falling.
- Former NASA consultant David Brin's criticizes SETI optimism about alien intentions, stating "This is an area in which discussion is called for" and arguing: "The worst mistake of first contact, made throughout history by individuals on both sides of every new encounter, has been the unfortunate habit of making assumptions. It often proved fatal."
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