Human extinction needs to be differentiated from the extinction of all life on Earth and from the extinction of major components of human culture (e.g. through a global catastrophe leaving only small, scattered human populations, which might survive in isolation).
"Existential risks" are risks that threaten the entire future of humanity, whether by causing human extinction or by otherwise permanently crippling human progress. Many scholars argue 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 anthropocentricutilitarian 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.:23–4
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.
A pandemic involving one or more viruses, prions, or antibiotic-resistant bacteria. Past examples were the Spanish flu outbreak in 1918, 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, e.g. when using animals as "carriers". 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.
Climate change; for example, global warming caused by human emission of carbon dioxide may render the planet uninhabitable. According to CDIAC, the Carbon Dioxide Information Analysis Center, increases in carbon emissions per head are closely followed by the growth in human population (a nearly four-fold increase in the past 100 years). On a longer time scale, Milankovitch cycles, also known as Quaternary Climatic Oscillations, affect the climate in various ways.
There is no other population of a large vertebrate animal like humans in the history of the planet that has grown so much, so fast, or with such devastating consequences to fellow earthlings. The population grew rapidly from 1 billion in 1800 to 2 billion in 1930, and has reached over 7 billion today. Thus people will annually absorb more primary productivity of the Earth's terrestrial net[clarification needed], and increase the use of land. The Center for Biological Diversity hypothesises an eventual crisis when the population grows beyond the capacity of its environmental sustainment and reduces that capacity to below the original level. Evidence suggests birth rates may be rising in the 21st century in the developed world. The work of Hans Rosling, a Swedish medical doctor, academic, statistician and public speaker, projects the global population peaking at less than 12 billion. A 2014 study published in Science asserts that the human population will grow to around 11 billion by 2100 and growth will continue into the next century.
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.
Without regulation, scientific advancement has a potential to risk human extinction as a result of the effects or use of totally new technologies. Some scenarios include:
Normal biological evolution of humanity will continue and change humans over geological time scales. Although this could, in a non-phylogenetic taxonomy, be considered to give rise to a new species, such an ongoing evolution would biologically not be considered a species extinction. Given the likelihood that significant genetic exchange between human populations will continue, it is highly unlikely that humans will split into multiple species through natural evolution.
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 do harbor a risk of the extinction of the "old" human species by means of the new, posthuman entity.
Perception of and reactions to human extinction risk
Because human extinction is unprecedented, speculation about the probability of different scenarios is highly subjective. Nick Bostrom 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.
Some scholars believe 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. Any number of events could lead to a massive loss of human life; but if the last few (but see minimum viable population), most resilient, humans are unlikely to also die off, then that particular human extinction scenario is not credible.
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.
Even though the importance and potential impact of research on existential risks is often highlighted, relatively few research efforts are being made in this field. In 2001 Bostrom stated:
There is more scholarly work on the life-habits of the dung fly than on existential risks [to humanity].
Although existential risks are less manageable by individuals than, e.g., 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."
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.
^ For research on this, see Psychological science volume 15 (2004): Decisions From Experience and the Effect of Rare Events in Risky Choice. The under-perception of rare events mentioned above is actually the opposite of the phenomenon originally described by Kahneman in "prospect theory" (in their original experiments the likelihood of rare events is overestimated). However, further analysis of the bias has shown that both forms occur: When judging from description people tend to overestimate the described probability, so this effect taken alone would indicate that reading the extinction scenarios described here should make the reader overestimate the likelihood of any probabilities given. However, the effect that is more relevant to common consideration of human extinction is the bias that occurs with estimates from experience, and these are in the opposite direction: When judging from personal experience people who have never heard of or experienced their species become extinct would be expected to dramatically underestimate its likelihood. SociobiologistE. 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." (Is Humanity Suicidal? The New York Times Magazine 30 May 1993).
^ Leslie (1996) discusses the survivorship bias (which he calls an "observational selection" effect on page 139) he says that the a priori certainty of observing an "undisasterous 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." (From: Random dynamics and relations between the number of fermion generations and the fine structure constants, Acta Pysica Polonica B, May 1989).
^ 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 lengthy rebuttal to SETI enthusiast's optimism about alien intentions concludes: "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." (See full text at SETIleague.org.)
^Adams, Robert Merrihew (October 1989). "Should Ethics be More Impersonal? a Critical Notice of Derek Parfit, Reasons and Persons". The Philosophical Review. 98 (4): 439–484. doi:10.2307/2185115. JSTOR2185115.
^Vitousek, P. M., H. A. Mooney, J. Lubchenco, and J. M. Melillo. 1997. Human Domination of Earth's Ecosystems. Science 277 (5325): 494–499; Pimm, S. L. 2001. The World According to Pimm: a Scientist Audits the Earth. McGraw-Hill, NY; The Guardian. 2005. Earth is All Out of New Farmland. 7 December 2005.
^Rose Somerville; John Somerville, introduction (1981). Soviet Marxism and nuclear war : an international debate : from the proceedings of the special colloquium of the XVth World Congress of Philosophy. Greenwood Press. p. 151. ISBN978-0-313-22531-4.
^Benatar, David (2008). Better Never to Have Been: The Harm of Coming into Existence. Oxford University Press. p. 224. ISBN978-0199549269. Although there are many non-human species - especially carnivores - that also cause a lot of suffering, humans have the unfortunate distinction of being the most destructive and harmful species on earth. The amount of suffering in the world could be radically reduced if there were no more humans.
^Best, Steven (2014). The Politics of Total Liberation: Revolution for the 21st Century. Palgrave Macmillan. p. 165. ISBN978-1137471116. But considered from the standpoint of animals and the earth, the demise of humanity would be the best imaginable event possible, and the sooner the better. The extinction of Homo sapiens would remove the malignancy ravaging the planet, destroy a parasite consuming its host, shut down the killing machines, and allow the earth to regenerate while permitting new species to evolve.