A global catastrophic risk is a hypothetical future event which could damage human well-being on a global scale,[2] even crippling or destroying modern civilization.[3] An event that could cause human extinction or permanently and drastically curtail humanity's potential is known as an existential risk.[4]

Potential global catastrophic risks include anthropogenic risks (technology, governance) and natural or external risks.[3] Examples of technology risks are hostile artificial intelligence and destructive biotechnology or nanotechnology. Insufficient or malign global governance creates risks in the social and political domain, such as a global war, including nuclear holocaust, bioterrorism using genetically modified organisms, cyberterrorism destroying critical infrastructure like the electrical grid; or the failure to manage a natural pandemic. Problems and risks in the domain of earth system governance include global warming, environmental degradation, including extinction of species, famine as a result of non-equitable resource distribution, human overpopulation, crop failures and non-sustainable agriculture. Examples of non-anthropogenic risks are an asteroid impact event, a supervolcanic eruption, a lethal gamma-ray burst, a geomagnetic storm destroying electronic equipment, natural long-term climate change, or hostile extraterrestrial life.

Philosopher Nick Bostrom classifies risks according to their scope and intensity.[5] A "global catastrophic risk" is any risk that is at least "global" in scope, and is not subjectively "imperceptible" in intensity. Those that are at least "trans-generational" (affecting all future generations) in scope and "terminal"[clarification needed] in intensity are classified as existential risks. While a global catastrophic risk may kill the vast majority of life on earth, humanity could still potentially recover. An existential risk, on the other hand, is one that either destroys humanity (and, presumably, all but the most rudimentary species of non-human lifeforms and/or plant life) entirely or at least prevents any chance of civilization recovering. Bostrom considers existential risks to be far more significant.[6]

Similarly, in Catastrophe: Risk and Response, Richard Posner singles out and groups together events that bring about "utter overthrow or ruin" on a global, rather than a "local or regional" scale. Posner singles out such events as worthy of special attention on cost-benefit grounds because they could directly or indirectly jeopardize the survival of the human race as a whole.[7] Posner's events include meteor impacts, runaway global warming, grey goo, bioterrorism, and particle accelerator accidents.

Researchers experience difficulty in studying near human extinction directly, since humanity has never been destroyed before.[8] While this does not mean that it will not be in the future, it does make modelling existential risks difficult, due in part to survivorship bias.

Bostrom identifies four types of existential risk. "Bangs" are sudden catastrophes, which may be accidental or deliberate. He thinks the most likely sources of bangs are malicious use of nanotechnology, nuclear war, and the possibility that the universe is a simulation that will end. "Crunches" are scenarios in which humanity survives but civilization is irreversibly destroyed. The most likely causes of this, he believes, are exhaustion of natural resources, a stable global government that prevents technological progress, or dysgenic pressures that lower average intelligence. "Shrieks" are undesirable futures. For example, if a single mind enhances its powers by merging with a computer, it could dominate human civilization. Bostrom believes that this scenario is most likely, followed by flawed superintelligence and a repressive totalitarian regime. "Whimpers" are the gradual decline of human civilization or current values. He thinks the most likely cause would be evolution changing moral preference, followed by extraterrestrial invasion.[4]

Some risks, such as that from asteroid impact, with a one-in-a-million chance of causing humanity's extinction in the next century,[9] have had their probabilities predicted using straightforward, well-understood, and (in principle) precise methods (although even in cases like these, the exact rate of large impacts is contested).[10] Similarly, the frequency of volcanic eruptions of sufficient magnitude to cause catastrophic climate change, similar to the Toba Eruption, which may have almost caused the extinction of the human race,[11] has been estimated at about 1 in every 50,000 years.[12]

The relative danger posed by other threats is much more difficult to calculate. In 2008, an informal survey of small but illustrious group 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".[13] 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.[3]

The 2016 annual report by the Global Challenges Foundation estimates that an average American is more than five times more likely to die during a human-extinction event than in a car crash.[14][15]

There are significant methodological challenges in estimating these risks with precision. Most attention has been given to risks to human civilization over the next 100 years, but forecasting for this length of time is difficult. The types of threats posed by nature may prove relatively constant, though new risks could be discovered. Anthropogenic threats, however, are likely to change dramatically with the development of new technology; while volcanoes have been a threat throughout history, nuclear weapons have only been an issue since the 20th century. Historically, the ability of experts to predict the future over these timescales has proved very limited. Man-made threats such as nuclear war or nanotechnology are harder to predict than natural threats, due to the inherent methodological difficulties in the social sciences. In general, it is hard to estimate the magnitude of the risk from this or other dangers, especially as both international relations and technology can change rapidly.

Existential risks pose unique challenges to prediction, even more than other long-term events, because of observation selection effects. Unlike with most events, the failure of a complete extinction event to occur in the past is not evidence against their likelihood in the future, because every world that has experienced such an extinction event has no observers, so regardless of their frequency, no civilization observes existential risks in its history.[8] These anthropic issues can be avoided by looking at evidence that does not have such selection effects, such as asteroid impact craters on the Moon, or directly evaluating the likely impact of new technology.[5]

Some scholars have strongly favored reducing existential risk on the grounds that it greatly benefits future generations. Derek Parfit argues that extinction would be a great loss because our descendants could potentially survive for four billion years before the expansion of the Sun makes the Earth uninhabitable.[16][17] Nick Bostrom argues that there is even greater potential in colonizing space. If future humans colonize space, they may be able to support a very large number of people on other planets, potentially lasting for trillions of years.[6] Therefore, reducing existential risk by even a small amount would have a very significant impact on the expected number of people who will exist in the future.

Exponential discounting might make these future benefits much less significant. However, Gaverick Matheny has argued that such discounting is inappropriate when assessing the value of existential risk reduction.[9]

Some economists have discussed the importance of global catastrophic risks, though not existential risks. Martin Weitzman argues that most of the expected economic damage from climate change may come from the small chance that warming greatly exceeds the mid-range expectations, resulting in catastrophic damage.[18] Richard Posner has argued that we are doing far too little, in general, about small, hard-to-estimate risks of large-scale catastrophes.[19]

Numerous cognitive biases can influence people's judgment of the importance of existential risks, including scope insensitivity, hyperbolic discounting, availability heuristic, the conjunction fallacy, the affect heuristic, and the overconfidence effect.[20]

Scope insensitivity influences how bad people consider the extinction of the human race to be. For example, when people are motivated to donate money to altruistic causes, the quantity they are willing to give does not increase linearly with the magnitude of the issue: people are roughly as concerned about 200,000 birds getting stuck in oil as they are about 2,000.[21] Similarly, people are often more concerned about threats to individuals than to larger groups.[20]

There are economic reasons that can explain why so little effort is going into existential risk reduction. It is a global good, so even if a large nation decreases it, that nation will only enjoy a small fraction of the benefit of doing so. Furthermore, the vast majority of the benefits may be enjoyed by far future generations, and though these quadrillions of future people would in theory perhaps be willing to pay massive sums for existential risk reduction, no mechanism for such a transaction exists.[5]

Some sources of catastrophic risk are natural, such as meteor impacts or supervolcanos. Some of these have caused mass extinctions in the past.

On the other hand, some risks are man-made, such as global warming,[22] environmental degradation, engineered pandemics and nuclear war. According to the Future of Humanity Institute, human extinction is more likely to result from anthropogenic causes than natural causes.[5][23]

In 2012, Cambridge University created The Cambridge Project for Existential Risk which examines threats to humankind caused by developing technologies.[24] The stated aim is to establish within the University a multidisciplinary research centre, Centre for the Study of Existential Risk, dedicated to the scientific study and mitigation of existential risks of this kind.[24]

The Cambridge Project states that the "greatest threats" to the human species are man-made; they are artificial intelligence, global warming, nuclear war, and rogue biotechnology.[25]

It has been suggested that learning computers that rapidly become superintelligent may take unforeseen actions or that robots would out-compete humanity (one technological singularity scenario).[26] Because of its exceptional scheduling and organizational capability and the range of novel technologies it could develop, it is possible that the first Earth superintelligence to emerge could rapidly become matchless and unrivaled: conceivably it would be able to bring about almost any possible outcome, and be able to foil virtually any attempt that threatened to prevent it achieving its objectives.[27] It could eliminate, wiping out if it chose, any other challenging rival intellects; alternatively it might manipulate or persuade them to change their behavior towards its own interests, or it may merely obstruct their attempts at interference.[27] In Bostrom's book, Superintelligence: Paths, Dangers, Strategies, he defines this as the control problem.[28]

Vernor Vinge has suggested that a moment may come when computers and robots are smarter than humans. He calls this "the Singularity."[29] He suggests that it may be somewhat or possibly very dangerous for humans.[30] This is discussed by a philosophy called Singularitarianism.

Physicist Stephen Hawking, Microsoft founder Bill Gates and SpaceX founder Elon Musk have expressed concerns about the possibility that AI could evolve to the point that humans could not control it, with Hawking theorizing that this could "spell the end of the human race".[31] In 2009, experts attended a conference hosted by the Association for the Advancement of Artificial Intelligence (AAAI) to discuss whether computers and robots might be able to acquire any sort of autonomy, and how much these abilities might pose a threat or hazard. They noted that some robots have acquired various forms of semi-autonomy, including being able to find power sources on their own and being able to independently choose targets to attack with weapons. They also noted that some computer viruses can evade elimination and have achieved "cockroach intelligence." They noted that self-awareness as depicted in science-fiction is probably unlikely, but that there were other potential hazards and pitfalls.[29] Various media sources and scientific groups have noted separate trends in differing areas which might together result in greater robotic functionalities and autonomy, and which pose some inherent concerns.[32][33] Eliezer Yudkowsky believes that risks from artificial intelligence are harder to predict than any other known risks. He also argues that research into artificial intelligence is biased by anthropomorphism. Since people base their judgments of artificial intelligence on their own experience, he claims that they underestimate the potential power of AI. He distinguishes between risks due to technical failure of AI, which means that flawed algorithms prevent the AI from carrying out its intended goals, and philosophical failure, which means that the AI is programmed to realize a flawed ideology.[34]

Biotechnology can pose a global catastrophic risk in the form of bioengineered organisms (viruses, bacteria, fungi, plants or animals). In many cases the organism will be a pathogen of humans, livestock, crops or other organisms we depend upon (e.g. pollinators or gut bacteria). However, any organism able to catastrophically disrupt ecosystem functions, e.g. highly competitive weeds, outcompeting essential crops, poses a biotechnology risk.

A biotechnology catastrophe may be caused by accidentally releasing a genetically engineered organism escaping from controlled environments, by the planned release of such an organism which then turns out to have unforeseen and catastrophic interactions with essential natural or agro-ecosystems, or by intentional usage of biological agents in biological warfare, bioterrorism attacks.[35] Terrorist applications of biotechnology have historically been infrequent.[35] To what extent this is due to a lack of capabilities or motivation is not resolved.[35]

Exponential growth has been observed in the biotechnology sector and Noun and Chyba predict that this will lead to major increases in biotechnological capabilities in the coming decades.[35] They argue that risks from biological warfare and bioterrorism are distinct from nuclear and chemical threats because biological pathogens are easier to mass-produce and their production is hard to control (especially as the technological capabilities are becoming available even to individual users).[35]

Given current development, more risk from novel, engineered pathogens is to be expected in the future.[35] Pathogens may be intentionally or unintentionally genetically modified to change virulence and other characteristics.[35] For example, a group of Australian researchers unintentionally changed characteristics of the mousepox virus while trying to develop a virus to sterilize rodents.[35] The modified virus became highly lethal even in vaccinated and naturally resistant mice.[36][37] The technological means to genetically modify virus characteristics are likely to become more widely available in the future if not properly regulated.[35]

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).[35]

(See also Natural pathogens below.)

Cyberattacks have the potential to destroy everything from personal data to electric grids. Christine Peterson, co-founder and past president of the Foresight Institute, believes a cyberattack on electric grids has the potential to be a catastrophic risk.[38] Peterson also identifies attacks on Internet of Things devices as potentially catastrophic.

Global warming refers to the warming caused by human technology since the 19th century or earlier. Global warming reflects abnormal variations to the expected climate within the Earth's atmosphere and subsequent effects on other parts of the Earth. Projections of future climate change suggest further global warming, sea level rise, and an increase in the frequency and severity of some extreme weather events and weather-related disasters. Effects of global warming include loss of biodiversity, stresses to existing food-producing systems, increased spread of known infectious diseases such as malaria, and rapid mutation of microorganisms.

It has been suggested that runaway global warming (runaway climate change) might cause Earth to become searingly hot like Venus. In less extreme scenarios, it could cause the end of civilization as we know it.[39]

An environmental or ecological disaster, such as world crop failure and collapse of ecosystem services, could be induced by the present trends of overpopulation, economic development,[40] and non-sustainable agriculture. An October 2017 report published in The Lancet stated that toxic air, water, soils, and workplaces were collectively responsible for 9 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.[41] 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.[41]

Most environmental scenarios involve one or more of the following: Holocene extinction event,[42] scarcity of water that could lead to approximately one half of the Earth's population being without safe drinking water, pollinator decline, overfishing, massive deforestation, desertification, climate change, or massive water pollution episodes. Detected in the early 21st century, a threat in this direction is colony collapse disorder,[43] a phenomenon that might foreshadow the imminent extinction[44] of the Western honeybee. As the bee plays a vital role in pollination, its extinction would severely disrupt the food chain.

Romanian American economist Nicholas Georgescu-Roegen, a progenitor in economics and the paradigm founder of ecological economics, has argued that the carrying capacity of Earth that is, Earth's capacity to sustain human populations and consumption levels is bound to decrease sometime in the future as Earth's finite stock of mineral resources is presently being extracted and put to use; and consequently, that the world economy as a whole is heading towards an inevitable future collapse, leading to the demise of human civilization itself.[45]:303f Ecological economist and steady-state theorist Herman Daly, a student of Georgescu-Roegen, has propounded the same argument by asserting that "... all we can do is to avoid wasting the limited capacity of creation to support present and future life [on Earth]."[46]:370

Ever since Georgescu-Roegen and Daly published these views, various scholars in the field have been discussing the existential impossibility of distributing Earth's finite stock of mineral resources evenly among an unknown number of present and future generations. This number of generations is likely to remain unknown to us, as there is little way of knowing in advance if or when mankind will eventually face extinction. In effect, any conceivable intertemporal distribution of the stock will inevitably end up with universal economic decline at some future point.[47]:253256 [48]:165 [49]:168171 [50]:150153 [51]:106109 [52]:546549 [53]:142145

Nick Bostrom suggested that in the pursuit of knowledge, humanity might inadvertently create a device that could destroy Earth and the Solar System.[54] Investigations in nuclear and high-energy physics could create unusual conditions with catastrophic consequences. For example, scientists worried that the first nuclear test might ignite the atmosphere.[55][56] More recently, others worried that the RHIC[57] or the Large Hadron Collider might start a chain-reaction global disaster involving black holes, strangelets, or false vacuum states. These particular concerns have been refuted,[58][59][60][61] but the general concern remains.

Biotechnology could lead to the creation of a pandemic, chemical warfare could be taken to an extreme, nanotechnology could lead to grey goo in which out-of-control self-replicating robots consume all living matter on earth while building more of themselvesin both cases, either deliberately or by accident.[62]

Many nanoscale technologies are in development or currently in use.[63] The only one that appears to pose a significant global catastrophic risk is molecular manufacturing, a technique that would make it possible to build complex structures at atomic precision.[64] Molecular manufacturing requires significant advances in nanotechnology, but once achieved could produce highly advanced products at low costs and in large quantities in nanofactories of desktop proportions.[63][64] When nanofactories gain the ability to produce other nanofactories, production may only be limited by relatively abundant factors such as input materials, energy and software.[63]

Molecular manufacturing could be used to cheaply produce, among many other products, highly advanced, durable weapons.[63] Being equipped with compact computers and motors these could be increasingly autonomous and have a large range of capabilities.[63]

Phoenix and Treder classify catastrophic risks posed by nanotechnology into three categories:

At the same time, nanotechnology may be used to alleviate several other global catastrophic risks.[63]

Several researchers state that the bulk of risk from nanotechnology comes from the potential to lead to war, arms races and destructive global government.[36][63][65] Several reasons have been suggested why the availability of nanotech weaponry may with significant likelihood lead to unstable arms races (compared to e.g. nuclear arms races):

Since self-regulation by all state and non-state actors seems hard to achieve,[67] measures to mitigate war-related risks have mainly been proposed in the area of international cooperation.[63][68] International infrastructure may be expanded giving more sovereignty to the international level. This could help coordinate efforts for arms control. International institutions dedicated specifically to nanotechnology (perhaps analogously to the International Atomic Energy Agency IAEA) or general arms control may also be designed.[68] One may also jointly make differential technological progress on defensive technologies, a policy that players should usually favour.[63] The Center for Responsible Nanotechnology also suggests some technical restrictions.[69] Improved transparency regarding technological capabilities may be another important facilitator for arms-control.

A grey goo is another catastrophic scenario, which was proposed by Eric Drexler in his 1986 book Engines of Creation[70] and has been a theme in mainstream media and fiction.[71][72] This scenario involves tiny self-replicating robots that consume the entire biosphere using it as a source of energy and building blocks. Nowadays, however, nanotech expertsincluding Drexlerdiscredit the scenario. According to Chris Phoenix a "so-called grey goo could only be the product of a deliberate and difficult engineering process, not an accident".[73]

The scenarios that have been explored most frequently are nuclear warfare and doomsday devices. Although the probability of a nuclear war per year is slim, Professor Martin Hellman has described it as inevitable in the long run; unless the probability approaches zero, inevitably there will come a day when civilization's luck runs out.[74] During the Cuban missile crisis, U.S. president John F. Kennedy estimated the odds of nuclear war at "somewhere between one out of three and even".[75] The United States and Russia have a combined arsenal of 14,700 nuclear weapons,[76] and there is an estimated total of 15,700 nuclear weapons in existence worldwide.[76]

While popular perception sometimes takes nuclear war as "the end of the world", experts assign low probability to human extinction from nuclear war.[77][78] In 1982, Brian Martin estimated that a USSoviet nuclear exchange might kill 400450 million directly, mostly in the United States, Europe and Russia, and maybe several hundred million more through follow-up consequences in those same areas.[77]

Nuclear war could yield unprecedented human death tolls and habitat destruction. Detonating large numbers of nuclear weapons would have an immediate, short term and long-term effects on the climate, causing cold weather and reduced sunlight and photosynthesis[79] that may generate significant upheaval in advanced civilizations.[80]

Beyond nuclear, other threats to humanity include biological warfare (BW) and bioterrorism. By contrast, chemical warfare, while able to create multiple local catastrophes, is unlikely to create a global one.

The 20th century saw a rapid increase in human population due to medical developments and massive increases in agricultural productivity[81] such as the Green Revolution.[82] Between 1950 and 1984, as the Green Revolution transformed agriculture around the globe, world grain production increased by 250%. The Green Revolution in agriculture helped food production to keep pace with worldwide population growth or actually enabled population growth. The energy for the Green Revolution was provided by fossil fuels in the form of fertilizers (natural gas), pesticides (oil), and hydrocarbon-fueled irrigation.[83] David Pimentel, professor of ecology and agriculture at Cornell University, and Mario Giampietro, senior researcher at the National Research Institute on Food and Nutrition (INRAN), place in their 1994 study Food, Land, Population and the U.S. Economy the maximum U.S. population for a sustainable economy at 200 million. To achieve a sustainable economy and avert disaster, the United States must reduce its population by at least one-third, and world population will have to be reduced by two-thirds, says the study.[84]

The authors of this study believe that the mentioned agricultural crisis will begin to have an effect on the world after 2020, and will become critical after 2050. Geologist Dale Allen Pfeiffer claims that coming decades could see spiraling food prices without relief and massive starvation on a global level such as never experienced before.[85][86]

Wheat is humanity's third-most-produced cereal. Extant fungal infections such as Ug99[87] (a kind of stem rust) can cause 100% crop losses in most modern varieties. Little or no treatment is possible and infection spreads on the wind. Should the world's large grain-producing areas become infected, the ensuing crisis in wheat availability would lead to price spikes and shortages in other food products.[88]

Several asteroids have collided with earth in recent geological history. The Chicxulub asteroid, for example, is theorized to have caused the extinction of the non-avian dinosaurs 66 million years ago at the end of the Cretaceous. No sufficiently large asteroid currently exists in an Earth-crossing orbit; however, a comet of sufficient size to cause human extinction could impact the Earth, though the annual probability may be less than 108.[89] Geoscientist Brian Toon estimates that a 60-mile meteorite would be large enough to "incinerate everybody".[90] Asteroids with around a 1km diameter have impacted the Earth on average once every 500,000 years; these are probably too small to pose an extinction risk, but might kill billions of people.[89][91] Larger asteroids are less common. Small near-Earth asteroids are regularly observed. As of 2013, Spaceguard estimates it has identified 95% of all NEOs over 1km in size.[92]

In 1.4 million years, the star Gliese 710 is expected to start causing an increase in the number of meteoroids in the vicinity of Earth when it passes within 1.1 light-years of the Sun, perturbing the Oort cloud. Dynamic models by Garca-Snchez predict a 5% increase in the rate of impact.[93] Objects perturbed from the Oort cloud take millions of years to reach the inner Solar System.

Extraterrestrial life could invade Earth[94] either to exterminate and supplant human life, enslave it under a colonial system, steal the planet's resources, or destroy the planet altogether.

Although evidence of alien life has never been documented, scientists such as Carl Sagan have postulated that the existence of extraterrestrial life is very likely. In 1969, the "Extra-Terrestrial Exposure Law" was added to the United States Code of Federal Regulations (Title 14, Section 1211) in response to the possibility of biological contamination resulting from the U.S. Apollo Space Program. It was removed in 1991.[95] Scientists consider such a scenario technically possible, but unlikely.[96]

An article in The New York Times discussed the possible threats for humanity of intentionally sending messages aimed at extraterrestrial life into the cosmos in the context of the SETI efforts. Several renowned public figures such as Stephen Hawking and Elon Musk have argued against sending such messages on the grounds that extraterrestrial civilizations with technology are probably far more advanced than humanity and could pose an existential threat to humanity.[97]

Climate change refers to a lasting change in the Earth's climate. The climate has ranged from ice ages to warmer periods when palm trees grew in Antarctica. It has been hypothesized that there was also a period called "snowball Earth" when all the oceans were covered in a layer of ice. These global climatic changes occurred slowly, prior to the rise of human civilization about 10 thousand years ago near the end of the last Major Ice Age when the climate became more stable.[citation needed] However, abrupt climate change on the decade time scale has occurred regionally.[citation needed] Since civilization originated during a period of stable climate, a natural variation into a new climate regime (colder or hotter) could pose a threat to civilization.[citation needed]

In the history of the Earth, many ice ages are known to have occurred.[citation needed] More ice ages will be possible at an interval of 40,000100,000 years.[citation needed] An ice age would have a serious impact on civilization because vast areas of land (mainly in North America, Europe, and Asia) could become uninhabitable. It would still be possible to live in the tropical regions, but with possible loss of humidity and water.[citation needed] Currently, the world is existing in an interglacial period within a much older glacial event.[citation needed] The last glacial expansion ended about 10,000 years ago, and all civilizations evolved later than this. Scientists do not predict that a natural ice age will occur anytime soon.[citation needed]

A number of astronomical threats have been identified. Massive objects, e.g. a star, large planet or black hole, could be catastrophic if a close encounter occurred in the Solar System. In April 2008, it was announced that two simulations of long-term planetary movement, one at the Paris Observatory and the other at the University of California, Santa Cruz, indicate a 1% chance that Mercury's orbit could be made unstable by Jupiter's gravitational pull sometime during the lifespan of the Sun. Were this to happen, the simulations suggest a collision with Earth could be one of four possible outcomes (the others being Mercury colliding with the Sun, colliding with Venus, or being ejected from the Solar System altogether). If Mercury were to collide with Earth, all life on Earth could be obliterated entirely: an asteroid 15km wide is believed to have caused the extinction of the non-avian dinosaurs, whereas Mercury is 4,879km in diameter.[98]

Another cosmic threat is a gamma-ray burst, typically produced by a supernova when a star collapses inward on itself and then "bounces" outward in a massive explosion. Under certain circumstances, these events are thought to produce massive bursts of gamma radiation emanating outward from the axis of rotation of the star. If such an event were to occur oriented towards the Earth, the massive amounts of gamma radiation could significantly affect the Earth's atmosphere and pose an existential threat to all life. Such a gamma-ray burst may have been the cause of the OrdovicianSilurian extinction events. Neither this scenario nor the destabilization of Mercury's orbit are likely in the foreseeable future.[99]

If the Solar System were to pass through a dark nebula, a cloud of cosmic dust, severe global climate change would occur.[100]

A powerful solar flare or solar superstorm, which is a drastic and unusual decrease or increase in the Sun's power output, could have severe consequences for life on Earth.

If our universe lies within a false vacuum, a bubble of lower-energy vacuum could come to exist by chance or otherwise in our universe, and catalyze the conversion of our universe to a lower energy state in a volume expanding at nearly the speed of light, destroying all that we know without forewarning.[101][further explanation needed] Such an occurrence is called vacuum decay.

The magnetic poles of the Earth shifted many times in geologic history. The duration of such a shift is still debated. Theories exist that during such times, the Earth's magnetic field would be substantially weakened, threatening civilization by allowing radiation from the Sun, especially solar wind, solar flares or cosmic radiation, to reach the surface. These theories have been somewhat discredited, as statistical analysis shows no evidence for a correlation between past reversals and past extinctions.[102][103]

Numerous historical examples of pandemics[104] had a devastating effect on a large number of people. The present, unprecedented scale and speed of human movement make it more difficult than ever to contain an epidemic through local quarantines. A global pandemic has become a realistic threat to human civilization.

Naturally evolving pathogens will ultimately develop an upper limit to their virulence.[105] Pathogens with the highest virulence, quickly killing their hosts reduce their chances of spread the infection to new hosts or carriers.[106] This simple model predicts that - if virulence and transmission are not genetically linked - pathogens will evolve towards low virulence and rapid transmission. However, this is not necessarily a safeguard against a global catastrophe, for the following reasons:

1. The fitness advantage of limited virulence is primarily a function of a limited number of hosts. Any pathogen with a high virulence, high transmission rate and long incubation time may have already caused a catastrophic pandemic before ultimately virulence is limited through natural selection. 2. In models where virulence level and rate of transmission are related, high levels of virulence can evolve.[107] Virulence is instead limited by the existence of complex populations of hosts with different susceptibilities to infection, or by some hosts being geographically isolated.[105] The size of the host population and competition between different strains of pathogens can also alter virulence.[108] 3. A pathogen that infects humans as a secondary host and primarily infects another species (a zoonosis) has no constraints on its virulence in people, since the accidental secondary infections do not affect its evolution.[109]

Naturally evolving organisms, like the products of biotechnology, can disrupt essential ecosystem functions.

An example of a pathogen able to threaten global food security is the wheat rust Ug99.

Other examples are neobiota (invasive species), i.e. organisms that become disruptive to ecosystems once transportedoften as a result of human activityto a new geographical region. Normally the risk is a local disruption. If it becomes coupled with serious crop failures and a global famine it may, however, pose a global catastrophic risk.

A remote possibility is a megatsunami. It has been suggested that a megatsunami caused by the collapse of a volcanic island could, for example, destroy the entire East Coast of the United States, but such predictions are based on incorrect assumptions and the likelihood of this happening has been greatly exaggerated in the media.[110] While none of these scenarios are likely to destroy humanity completely, they could regionally threaten civilization. There have been two recent high-fatality tsunamisafter the 2011 Thoku earthquake and the 2004 Indian Ocean earthquake. A megatsunami could have astronomical origins as well, such as an asteroid impact in an ocean.[111]

A geological event such as massive flood basalt, volcanism, or the eruption of a supervolcano[112] could lead to a so-called volcanic winter, similar to a nuclear winter. One such event, the Toba eruption,[113] occurred in Indonesia about 71,500 years ago. According to the Toba catastrophe theory,[114] the event may have reduced human populations to only a few tens of thousands of individuals. Yellowstone Caldera is another such supervolcano, having undergone 142 or more caldera-forming eruptions in the past 17 million years.[115] A massive volcano eruption would eject extraordinary volumes of volcanic dust, toxic and greenhouse gases into the atmosphere with serious effects on global climate (towards extreme global cooling: volcanic winter if short-term, and ice age if long-term) or global warming (if greenhouse gases were to prevail).

When the supervolcano at Yellowstone last erupted 640,000 years ago, the thinnest layers of the ash ejected from the caldera spread over most of the United States west of the Mississippi River and part of northeastern Mexico. The magma covered much of what is now Yellowstone National Park and extended beyond, covering much of the ground from Yellowstone River in the east to the Idaho falls in the west, with some of the flows extending north beyond Mammoth Springs.[116]

According to a recent study, if the Yellowstone caldera erupted again as a supervolcano, an ash layer one to three millimeters thick could be deposited as far away as New York, enough to "reduce traction on roads and runways, short out electrical transformers and cause respiratory problems". There would be centimeters of thickness over much of the U.S. Midwest, enough to disrupt crops and livestock, especially if it happened at a critical time in the growing season. The worst-affected city would likely be Billings, Montana, population 109,000, which the model predicted would be covered with ash estimated as 1.03 to 1.8 meters thick.[117]

The main long-term effect is through global climate change, which reduces the temperature globally by about 515 degrees C for a decade, together with the direct effects of the deposits of ash on their crops. A large supervolcano like Toba would deposit one or two meters thickness of ash over an area of several million square kilometers.(1000 cubic kilometers is equivalent to a one-meter thickness of ash spread over a million square kilometers). If that happened in some densely populated agricultural area, such as India, it could destroy one or two seasons of crops for two billion people.[118]

However, Yellowstone shows no signs of a supereruption at present, and it is not certain that a future supereruption will occur there.[119][120]

Research published in 2011 finds evidence that massive volcanic eruptions caused massive coal combustion, supporting models for significant generation of greenhouse gases. Researchers have suggested that massive volcanic eruptions through coal beds in Siberia would generate significant greenhouse gases and cause a runaway greenhouse effect.[121] Massive eruptions can also throw enough pyroclastic debris and other material into the atmosphere to partially block out the sun and cause a volcanic winter, as happened on a smaller scale in 1816 following the eruption of Mount Tambora, the so-called Year Without a Summer. Such an eruption might cause the immediate deaths of millions of people several hundred miles from the eruption, and perhaps billions of deaths[122] worldwide, due to the failure of the monsoon[citation needed], resulting in major crop failures causing starvation on a profound scale.[122]

A much more speculative concept is the verneshot: a hypothetical volcanic eruption caused by the buildup of gas deep underneath a craton. Such an event may be forceful enough to launch an extreme amount of material from the crust and mantle into a sub-orbital trajectory.

Planetary management and respecting planetary boundaries have been proposed as approaches to preventing ecological catastrophes. Within the scope of these approaches, the field of geoengineering encompasses the deliberate large-scale engineering and manipulation of the planetary environment to combat or counteract anthropogenic changes in atmospheric chemistry. Space colonization is a proposed alternative to improve the odds of surviving an extinction scenario.[123] Solutions of this scope may require megascale engineering. Food storage has been proposed globally, but the monetary cost would be high. Furthermore, it would likely contribute to the current millions of deaths per year due to malnutrition.[citation needed]

Some survivalists stock survival retreats with multiple-year food supplies.

The Svalbard Global Seed Vault is buried 400 feet (120m) inside a mountain on an island in the Arctic. It is designed to hold 2.5 billion seeds from more than 100 countries as a precaution to preserve the world's crops. The surrounding rock is 6C (21F) (as of 2015) but the vault is kept at 18C (0F) by refrigerators powered by locally sourced coal.[124][125]

More speculatively, if society continues to function and if the biosphere remains habitable, calorie needs for the present human population might in theory be met during an extended absence of sunlight, given sufficient advance planning. Conjectured solutions include growing mushrooms on the dead plant biomass left in the wake of the catastrophe, converting cellulose to sugar, or feeding natural gas to methane-digesting bacteria.[126][127]

Insufficient global governance creates risks in the social and political domain, but the governance mechanisms develop more slowly than technological and social change. There are concerns from governments, the private sector, as well as the general public about the lack of governance mechanisms to efficiently deal with risks, negotiate and adjudicate between diverse and conflicting interests. This is further underlined by an understanding of the interconnectedness of global systemic risks.[128]

The Bulletin of the Atomic Scientists (est. 1945) is one of the oldest global risk organizations, founded after the public became alarmed by the potential of atomic warfare in the aftermath of WWII. It studies risks associated with nuclear war and energy and famously maintains the Doomsday Clock established in 1947. The Foresight Institute (est. 1986) examines the risks of nanotechnology and its benefits. It was one of the earliest organizations to study the unintended consequences of otherwise harmless technology gone haywire at a global scale. It was founded by K. Eric Drexler who postulated "grey goo".[129][130]

Beginning after 2000, a growing number of scientists, philosophers and tech billionaires created organizations devoted to studying global risks both inside and outside of academia.[131]

Independent non-governmental organizations (NGOs) include the Machine Intelligence Research Institute (est. 2000) which aims to reduce the risk of a catastrophe caused by artificial intelligence and the Singularity.[132] The top donors include Peter Thiel and Jed McCaleb.[133] The Lifeboat Foundation (est. 2009) funds research into preventing a technological catastrophe.[134] Most of the research money funds projects at universities.[135] The Global Catastrophic Risk Institute (est. 2011) is a think tank for all things catastrophic risk. It is funded by the NGO Social and Environmental Entrepreneurs. The Global Challenges Foundation (est. 2012), based in Stockholm and founded by Laszlo Szombatfalvy, releases a yearly report on the state of global risks.[14][15] The Future of Life Institute (est. 2014) aims to support research and initiatives for safeguarding life considering new technologies and challenges facing humanity.[136] Elon Musk is one of its biggest donors.[137] The Nuclear Threat Initiative seeks to reduce global threats from nuclear, biological and chemical threats, and containment of damage after an event.[138] It maintains a nuclear material security index.[139]

University-based organizations include the Future of Humanity Institute (est. 2005) which researches the questions of humanity's long-term future, particularly existential risk. It was founded by Nick Bostrom and is based at Oxford University. The Centre for the Study of Existential Risk (est. 2012) is a Cambridge-based organization which studies four major technological risks: artificial intelligence, biotechnology, global warming and warfare. All are man-made risks, as Huw Price explained to the AFP news agency, "It seems a reasonable prediction that some time in this or the next century intelligence will escape from the constraints of biology". He added that when this happens "we're no longer the smartest things around," and will risk being at the mercy of "machines that are not malicious, but machines whose interests don't include us."[140] Stephen Hawking was an acting adviser. The Millennium Alliance for Humanity and the Biosphere is a Stanford University-based organization focusing on many issues related to global catastrophe by bringing together members of academic in the humanities.[141][142] It was founded by Paul Ehrlich among others.[143] Stanford University also has the Center for International Security and Cooperation focusing on political cooperation to reduce global catastrophic risk.[144]

Other risk assessment groups are based in or are part of governmental organizations. The World Health Organization (WHO) includes a division called the Global Alert and Response (GAR) which monitors and responds to global epidemic crisis.[145] GAR helps member states with training and coordination of response to epidemics.[146] The United States Agency for International Development (USAID) has its Emerging Pandemic Threats Program which aims to prevent and contain naturally generated pandemics at their source.[147] The Lawrence Livermore National Laboratory has a division called the Global Security Principal Directorate which researches on behalf of the government issues such as bio-security, counter-terrorism, etc.[148]

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