Risks to civilization, humans, and planet Earth
An existential risk narrowly refers to any factor threatening the existence of humanity. Existential risks may also broadly refer to any of the various risks that have the potential to destroy, or drastically restrict, human civilization; to lead to human extinction; or even to cause the end of Earth. Severe events could cause the extinction of all life on the planet Earth, the destruction of the planet Earth, the annihilation of the solar system, to the annihilation of our galaxy or even the entire universe. Existential risks are distinguished from other forms of risk both by their scope, affecting all of humanity, and severity; destroying or irreversibly crippling the target.
Natural disasters, such as supervolcanoes and asteroids, may pose existential risks if sufficiently powerful, though man-made events could also threaten the survival of intelligent life on Earth, like catastrophic global warming, nuclear war, or bioterrorism.
Despite the importance of existential risks, it is a difficult subject to study directly since humankind has never been destroyed before; 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.
While individual threats, such as those posed by nuclear war or climate change, have been intensively studied on their own, very little systematic work in the area of existential risks was done before the beginning of the 21st century. A precise estimate of each individual risk may not be necessary when compiling the aggregate risk to mankind.
Types of risks 
Various risks exist for humanity, but not all are equal. Risks can be roughly categorized based on the scope (personal, regional, global) and the intensity (endurable, terminal). The chart to the right provides some examples.
The risks discussed in this article are at least Global and Terminal in intensity. These types of risks are ones where an adverse outcome would either annihilate intelligent life on Earth, or permanently and drastically reduce its potential. Jamais Cascio made an alternative classification system.
Chances of an existential catastrophe 
Some risks, such as that from asteroid impact, with a one-in-a-million chance of causing humankind extinction in the next century, have had their probabilities predicted with considerable accuracy (though later research suggested the actual rate of large impacts could be much higher than predicted). Similarly, the frequency of volcanic eruptions of sufficient magnitude to cause catastrophic climate change, similar to the Toba Eruption, which almost caused the extinction of the human race, has been estimated at about 1 in every 50,000 years. However, the relative danger posed by other threats is much more difficult to calculate. Though experts at the Global Catastrophic Risk Conference suggested a 19% chance of human extinction over the next century, the report cautions that the methods used to average responses to the informal survey is suspect due to the treatment of non-responses.
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, though modern probabilistic forecasting methods, like prediction markets, as well as more traditional approaches such as peer review could increase the accuracy of prediction.
Man-made threats such as nuclear war or nanotechnology are even harder to predict, due to the inherent methodological difficulties in the social sciences. During the Cuban Missile Crisis, President Kennedy estimated that there was between a third and a half chance of nuclear war. Despite this, 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 catastrophic events to occur in the past is not evidence against their likelihood in the future, because every world that has experienced one has no observers, so regardless of their frequency, no civilization observes existential risks in its history. 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.
Fermi paradox 
Many extra-solar planets have been discovered, and there are likely to be many more Earth-like planets, capable of supporting life. Given the relative rapidity with which life evolved on Earth, and the size of the observable universe, it seems a priori likely that intelligent life would have independently arisen on other planets. Therefore, the absence of any sign of intelligent life beyond the earth forms an apparent paradox. Especially relevant is the absence of large-scale astro-engineering projects, suggesting that few civilizations survive to colonize space.
While a variety of explanations for the Fermi paradox exist, such as that the Earth may be part of a galactic zoo, one plausible explanation is that a Great Filter exists; an evolutionary step between the emergence of life on an Earth-like planet and the colonization of space that is incredibly hard to take. Clearly, if this filter is ahead of us – perhaps most civilizations destroy themselves in nuclear wars – then unless humanity is very unusual, it is likely to prevent us from colonizing space.
Cognitive bias 
Research into cognitive biases reveals a number of ways in which humans fall short of unbiased rationality, many of which affect the prediction of existential risks. For example, availability bias may make people underestimate the danger of existential risks, as clearly no-one has any experience of them. Equally, hindsight bias makes past events appear to have been more predictable than they actually were, leading to overconfidence in our ability to predict the future.
Conjunction bias occurs when people overestimate the likelihood of conjunctions; for example, considering an activist more likely to grow up into a feminist bank worker than a bank worker.[clarification needed] Equally, people underestimate the likelihood of disjunctions. The threat of existential risks is heavily disjunctive; nuclear war or climate change or bioterrorism or asteroids or solar flares or artificial intelligence – so people tend to underestimate its plausibility.
There are many other biases that affect how likely people think existential disasters to be, such as overconfidence and anchoring, or how whether or not they get involved, such as bystander effect. A different type of bias is that caused by scope insensitivity. Rather than causing people to under- or overestimate the likelihood of an existential disaster, scope insensitivity affects how bad people consider the extinction of the human race to be. While people may be motivated to donate money to alleviate the ill, the quantity they’re willing to give does not scale linearly with the magnitude of the issue; for example, people are as concerned about 200,000 birds getting stuck in oil as they are about 2,000, rather than a hundred times more concerned. Similarly, people are often more concerned about threats to individuals than to larger groups.
Potential importance of existential risk 
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 a billion years before the increasing heat of the Sun makes the Earth become uninhabitable. Bostrom argues that there is even greater potential in colonizing space. If our descendants colonize space, we may be able to support a very large number of people on other planets, potentially lasting for trillions of years. Therefore, reducing existential risk by even a small amount would have a very significant impact on the expected number of people that will exist in the future.
Little has been written arguing against these positions, but some scholars would disagree. Exponential discounting might make these future benefits much less significant, and some philosophers doubt the value of ensuring the existence of future generations.
Some economists have also discussed the importance of existential risks, though most of the discussion goes under the name “catastrophic risk.” 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. Richard Posner has argued that we are doing far too little, in general, about small, hard-to-estimate risks of large scale catastrophes.
Future scenarios 
Many scenarios have been suggested. Some that will almost certainly end life on Earth are certain to occur, but on a very long timescale. Others are likely to happen on a shorter timescale, but will probably not completely destroy civilization. Still others are extremely unlikely, and may even be impossible. For example, Nick Bostrom writes:
Some foreseen hazards (hence not members of the current category) which have been excluded from the list on grounds that they seem too unlikely to cause a global terminal disaster are: solar flares, supernovae, black hole explosions or mergers, gamma-ray bursts, galactic center outbursts, buildup of air pollution, gradual loss of human fertility, and various religious doomsday scenarios.
Some threats for humanity come from humanity itself.
A category of existential risk are consequences of technology.
In 2012, Cambridge University created The Cambridge Project for Existential Risk which examines threats to humankind caused by developing technologies. 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.
Cambridge identified the "four greatest threats" to the human species: artificial intelligence, climate change, nuclear war and biotechnology.
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 themselves - in both cases, either deliberately or by accident.
Artificial intelligence 
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). 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. 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.
Vernor Vinge has suggested that a moment may come when computers and robots are smarter than humans. He calls this "the Singularity." He suggests that it may be somewhat or possibly very dangerous for humans. This is discussed by a philosophy called Singularitarianism.
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. 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.
Some experts and academics have questioned the use of robots for military combat, especially when such robots are given some degree of autonomous functions. There are also concerns about technology which might allow some armed robots to be controlled mainly by other robots.
The US Navy has funded a report which indicates that as military robots become more complex, there should be greater attention to implications of their ability to make autonomous decisions. One researcher states that autonomous robots might be more humane, as they could make decisions more effectively. However, other experts question this.
Nick Bostrom suggested that in the pursuit of knowledge humanity might inadvertently create a device that could destroy Earth and our solar system.
Warfare and mass destruction 
The scenarios that have been explored most frequently are nuclear warfare and Doomsday devices. There is difficulty in predicting whether such would exterminate humanity, however a nuclear winter would cause significant upheaval in advanced civilizations.
Man-made global warming 
Global warming refers to the warming caused by human technology since the 19th century. Global warming reflects abnormal variations to the expected climate within the Earth's atmosphere and subsequent effects on other parts of the Earth, such as in the ice caps, rising seas, melting glaciers, drought and so on.
According to the UN’s Office for the Coordination of Humanitarian Affairs (OCHA), climate disasters are on the rise. Around 70 percent of disasters are now climate related – up from around 50 percent from two decades ago. These disasters take a heavier human toll and come with a higher price tag. In the last decade, 2.4 billion people were affected by climate related disasters, compared to 1.7 billion in the previous decade and the cost of responding to disasters has risen tenfold between 1992 and 2008. Destructive sudden heavy rains, intense tropical storms, repeated flooding and droughts are likely to increase, as will the vulnerability of local communities in the absence of strong concerted action. Sea level rise may completely inundate certain areas.
It has been suggested that runaway global warming (runaway climate change) might cause Earth to become searing hot like Venus. In less extreme scenarios it could cause the end of civilization, as we know it.
According to a UN climate report, the Himalayan glaciers that are the sources of Asia's biggest rivers - Ganges, Indus, Brahmaputra, Yangtze, Mekong, Salween and Yellow - could disappear by 2350 as temperatures rise. Approximately three billion people live in the drainage basin of the Himalayan rivers, which is almost half of the current human population (see Environmental migrant). The Himalayan system, which includes outlying subranges, stretches across: Afghanistan, Bangladesh, Bhutan, China, India, Nepal, Burma, Cambodia, Thailand, Laos, Vietnam, Malaysia and Pakistan. These areas could experience floods followed by droughts in coming decades. In India alone, the Ganges provides water for drinking and farming for more than 500 million people.
The west coast of North America, which gets much of its water from glaciers in mountain ranges such as the Rocky Mountains, Cascade Mountains and Sierra Nevada, also would be affected. According to the California Department of Water Resources, if more water supplies are not found by 2020, California residents will face a water shortfall nearly as great as the amount consumed today.
Approximately 40% of the world's agricultural land is seriously degraded. In Africa, if current trends of soil degradation continue, the continent might be able to feed just 25% of its population by 2025, according to UNU's Ghana-based Institute for Natural Resources in Africa.
James Lovelock, creator of the Gaia hypothesis, in his book The Revenge of Gaia (2006), has suggested that the elimination of rain forests, and the falling planetary biodiversity is removing the homeostatic negative feedback mechanisms that maintain climate stability by reducing the effects of greenhouse gas emissions (particularly carbon dioxide). With the heating of the oceans, the extension of the thermocline layer into arctic and antarctic waters is preventing the overturning and nutrient enrichment necessary for algal blooms of phytoplankton on which the ecosystems of these areas depend. With the loss of phytoplankton and tropical rain forests, two of the main carbon dioxide sinks for reducing global warming, he suggests a runaway positive feedback effect could cause tropical deserts to cover most of the world's tropical regions, and the disappearance of polar ice caps, posing a serious challenge to global civilization.
Using scenario analysis, the Global Scenario Group (GSG), a coalition of international scientists convened by Paul Raskin, developed a series of possible futures for the world as it enters a Planetary Phase of Civilization. One scenario involves the complete breakdown of civilization as the effects of global warming become more pronounced, competition for scarce resources increases, and the rift between the poor and the wealthy widens. The GSG’s other scenarios, such as Policy Reform, Eco-Communalism, and Great Transition avoid this societal collapse and eventually result in environmental and social sustainability. They claim the outcome is dependent on human choice and the possible formation of a global citizens movement which could influence the trajectory of global development.
Global pandemic 
A less predictable scenario is a global pandemic. It has been hypothesised that an extremely virulent pathogen might not evolve. This is because a pathogen that quickly kills its hosts might not have enough time to spread to new ones, while one that kills its hosts more slowly or not at all will allow carriers more time to spread the infection, and thus likely out-compete a more lethal species or strain. This simple model predicts that if virulence and transmission are not linked in any way, pathogens will evolve towards low virulence and rapid transmission. However, this assumption is not always valid and in more complex models, where the level of virulence and the rate of transmission are related, high levels of virulence can evolve. The level of virulence that is possible is instead limited by the existence of complex populations of hosts, with different susceptibilities to infection, or by some hosts being geographically isolated. The size of the host population and competition between different strains of pathogens can also alter virulence. Interestingly, a pathogen that only infects humans as a secondary host and usually infects another species (a zoonosis) may have little constraint on its virulence in people, since infection here is an accidental event and its evolution is driven by events in another species. There are numerous historical examples of pandemics that have had a devastating effect on a large number of people, which makes the possibility of global pandemic a realistic threat to human civilization.
Ecological disaster 
An ecological disaster, such as world crop failure and collapse of ecosystem services, could be induced by the present trends of overpopulation, economic development, and non-sustainable agriculture. Most of these scenarios involve one or more of the following: Holocene extinction event, 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. A very recent threat in this direction is colony collapse disorder, a phenomenon that might foreshadow the imminent extinction of the Western honeybee. As the bee plays a vital role in pollination, its extinction would severely disrupt the food chain.
World population and agricultural crisis 
The 20th century saw a rapid increase in human population due to medical developments and massive increase in agricultural productivity made by the Green Revolution. 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. 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 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.
The authors of this study believe that the mentioned agricultural crisis will only begin to impact us after 2020, and will not become critical until 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.
Wheat is humanity's 3rd most produced cereal. Extant fungal infections such as Ug99 (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 then there would be a crisis in wheat availability leading to price spikes and shortages in other food products.
Natural climate change 
Climate change can refer to any long-term significant change in the patterns of average weather of a specific region (or, more relevantly to contemporary socio-political concerns, of the Earth as a whole) over an appropriately significant period of time, caused by natural forcing. In the past these have included periods of ice age's and periods warmer than today.
Ice age 
In the history of the Earth, 12 known ice ages have occurred. More ice ages will be possible at an interval of 40,000–100,000 years although engineers working for Posiva, a Finnish company currently constructing the Onkalo spent nuclear fuel repository, has planned the facility to withstand an Ice Age starting as soon as 20,000 years. 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/water. Currently, the world is existing in an interglacial period within a much older glacial event. The last glacial expansion ended about 10,000 years ago, and all civilizations evolved later.
A geological event such as massive flood basalt, volcanism, or the eruption of a supervolcano leading to the so called Volcanic Winter (Similar to a Nuclear Winter). One such event, the Toba Eruption, occurred in Indonesia about 71,500 years ago. According to the Toba catastrophe theory, 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. Massive volcano eruption(s) will produce extraordinary intake of volcanic dust, toxic and greenhouse gases into the atmosphere with serious effects on global climate (towards extreme global cooling (nuclear winter when in short term and ice age when in long term) or global warming (if greenhouse gases prevail)).
When the supervolcano at Yellowstone last erupted 640,000 years ago, the magma and ash ejected from the caldera covered most of the United States west of the Mississippi river and part of northeastern Mexico. Another such eruption could threaten civilization. Such an eruption could also release large amounts of gases that could alter the balance of the planet's carbon dioxide and cause a runaway greenhouse effect[dubious ], or enough pyroclastic debris and other material might be thrown into the atmosphere to partially block out the sun and cause a volcanic winter, as happened 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 worldwide, due to the failure of the monsoon, resulting in major crop failures causing starvation on a massive scale. Supervolcanoes are more likely threats than many others, as a prehistoric Indonesian supervolcano eruption may have reduced the human population to only a few thousand individuals, while no catastrophic bolide impact, for example, has occurred since long before modern humans evolved.
Another possibility is a megatsunami. A megatsunami could, for example, destroy the entire East Coast of the United States. The coastal areas of the entire world could also be flooded in case of the collapse of the West Antarctic Ice Sheet. While none of these scenarios are likely to destroy humanity completely, they could regionally threaten civilization. There have been two recent high-fatality tsunamis—after the 2011 Tōhoku earthquake and the 2004 Indian Ocean earthquake, although they were not large enough to be considered megatsunamis. A megatsunami could have astronomical origins as well, such as an asteroid impact in an ocean.
Meteorite impact 
Earth has collided with several asteroids in recent geological history. The Chicxulub asteroid, for example, is theorized to have caused the extinction of the dinosaurs 65 million years ago at the end of the Cretaceous. If such an object struck Earth it could have a serious impact on civilization. It is even possible that humanity would be completely destroyed; for this to occur the asteroid would need to be at least 1 km (0.62 mi) in diameter, but probably between 3 and 10 km (2–6 miles). Asteroids with a 1 km diameter have impacted the Earth on average once every 500,000 years. Larger asteroids are less common. Small Near-Earth asteroids are regularly observed.
In 1.4 million years, the star Gliese 710 is expected to cause an increase in the number of meteoroids in the vicinity of Earth by passing within 1.1 light years of the Sun and perturbing the Oort cloud. Dynamic models by García-Sánchez predict a 5% increase in the rate of impact.
Alien invasion 
Extraterrestrial life could invade Earth either to exterminate and supplant human life, enslave it under a colonial system, harvest humans for food, 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 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. Scientists consider such a scenario technically possible, but unlikely.
Distant cosmic future 
There are a number of cosmological theories as to the universe's ultimate fate that exclude the indefinite continuation of life. Most involve time periods and distant futures much greater than the 13.8-billion-year age of the universe. A long-established and widely accepted theory is the eventual heat death of the universe.
Calculations indicate that the Andromeda Galaxy is on a collision course with the Milky Way. Andromeda is approaching at an average speed of about 110 kilometres (68 mi) per second and thus impact is predicted in about four billion years. An actual collision with the Earth is unlikely, but this merging could eject the solar system in a more eccentric orbit.
The theory of stellar evolution predicts that our sun will exhaust its hydrogen core and become a red giant in about five billion years, becoming thousands of times more luminous and losing roughly 30% of its current mass. Ignoring tidal effects, the Earth would then orbit 1.7 AU (250,000,000 km) from the Sun at its maximum radius. This would allow the Earth to escape being enveloped by the Sun's now expanded and thin outer atmosphere, though most life, if not all, would perish due to the Sun's proximity. However, a more recent study suggests that the Earth's orbit will decay due to the effects of tidal drag, causing it to enter the Sun's expanded atmosphere and be destroyed in 7.6 billion years. Before being swallowed by the Sun, the Earth's oceans would evaporate, and the Earth would finally be destroyed by tidal forces. However, this fate is not inevitable—it appears possible to move the Earth to a more distant orbit, using repeated close encounters with asteroids.
Before this happens, Earth's biosphere will have long been destroyed by the Sun's steady increase in brightness as its hydrogen supply dwindles and its core contracts, even before the transition to a Red Giant. After just over 1 billion years, the extra solar energy input will cause Earth's oceans to evaporate and the hydrogen from the water to be lost permanently to space, with total loss of water by 3 billion years. Earth's atmosphere and lithosphere will become like that of Venus. Over another billion years, most of the atmosphere will become lost to space as well; ultimately leaving Earth as a desiccated, dead planet with a surface of molten rock.
Other cosmic threats 
A number of other scenarios have been suggested. Massive objects, e.g., a star, large planet or black hole, could be catastrophic if a close encounter occurred in the solar system. Gravity from the wandering objects might disrupt orbits and/or fling bodies into other objects, thus resulting in meteorite impacts or climate change. Also, heat from the wandering objects might cause extinctions; tidal forces could cause erosion along our coastlines. Another threat might come from gamma ray bursts. Both are very unlikely. One especially deadly hypothesized source is a hypernova, produced when a hypergiant star explodes and then collapses, sending vast amounts of radiation sweeping across hundreds of lightyears. Hypernovas have never been observed; however, a hypernova may have been the cause of the Ordovician–Silurian extinction events. The nearest hypergiant is Eta Carinae, approximately 8,000 light-years distant. The hazards from various astrophysical radiation sources were reviewed in 2011.
In April 2008, it was announced that two simulations of long-term planetary movement, one at Paris Observatory and the other at 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 the Earth, all life on earth would be obliterated and the impact might displace enough matter into orbit to form another moon. Note that an asteroid just 15 km wide is said to have caused the extinction of the dinosaurs; Mercury is some 5,000 km in diameter.
- Dark nebula - The solar system passing through a cosmic dust cloud, leading to a severe global climate change.
- A drastic and unusual decrease or increase in Sun's power output; a solar superstorm leading to partial or complete electrical or technological death of human civilization. (See solar flare.)
- Near-Earth supernova
Other scenarios 
- Peak oil: Fossil fuels attain a level of scarcity before an economically viable replacement is devised, leading firstly to economic strain, followed by the collapse of modern agriculture, then to mass starvation.
- Antibiotic resistance: Natural selection would create super bacteria that are resistant to antibiotics, devastating the world population and causing a global collapse of civilization.
- Gulf Stream shutdown: There is some speculation that global warming could, via a shutdown or slowdown of the thermohaline circulation, trigger localized cooling in the North Atlantic and lead to cooling in that region. This would affect in particular areas like Ireland, the Nordic countries, and Britain that are warmed by the North Atlantic drift.
- Mutual assured destruction: A full scale nuclear war could kill billions, and the potential for subsequent nuclear winter would effectively crush any form of civilization.
- Famine: As of late 2007, increased farming for use in biofuels, along with world oil prices spiking to more than $140 per barrel, had pushed up the price of grain used to feed poultry and dairy cows and other cattle, causing higher prices of wheat (up 58%), soybean (up 32%), and maize (up 11%) over the year. Food riots have recently taken place in many countries across the world. An epidemic of stem rust on wheat caused by race Ug99 is currently spreading across Africa and into Asia and is causing major concern. Scientists say millions of people face starvation.
- Experimental accident: Investigations in nuclear and high energy physics could conceivably create unusual conditions with catastrophic consequences. For example, scientists worried that the first nuclear test might ignite the atmosphere. More recently, others worried that the RHIC or the Large Hadron Collider might start a chain-reaction global disaster involving black holes or false vacuum states. These particular concerns have been refuted, but the general concern remains.
- Hypercane: A theoretical hurricane the size of a continent. The theory estimates that ocean temperatures would have to reach almost 50°C before storms of this size could occur.
- Geomagnetic reversal: The magnetic poles of the Earth shifted many times in geologic history. The duration of such a shift is still debated. Theories exist that say that during that time, the magnetic field around the Earth would be weakened or nonexistent, threatening electrical civilization or even several species by allowing radiation from the sun, especially solar flares or cosmic background radiation to reach the surface.
- 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. Named after Jules Verne.
Historical fictional scenarios 
The belief that the Mayan civilization's Long Count calendar ended abruptly on December 21, 2012 was a misconception due to the Mayan practice of using only five places in Long Count Calendar inscriptions. On some monuments the Mayan calculated dates far into the past and future but there is no end of the world date. There was a Piktun ending (a cycle of 13,144,000 day Bak'tuns) on December 21, 2012. A Piktun marks the end of a 1,872,000 day or approximately 5125 year period and is a significant event in the Mayan calendar. However, there is no historical or scientific evidence that the Mayans believed it would be a doomsday. Some believe it was just the beginning of another Piktun.
The cataclysmic pole shift hypothesis was formulated in 1872. Revisited repeatedly in the second half of the 20th century, it proposes that the axis of the Earth with respect to the crust could change extremely rapidly, causing massive earthquakes, tsunamis, and damaging local climate changes. The hypothesis is contradicted by the mainstream scientific interpretation of geological data, which indicates that true polar wander does occur, but very slowly over millions of years. Sometimes this hypothesis is confused with the accepted theory of geomagnetic reversal in which the magnetic poles reverse, but which has no influence on the axial poles or the rotation of the solid earth.
Precautions and prevention 
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. Solutions of this scope may require megascale engineering.
Some precautions that people are already taking for a cataclysmic event include:
Organizations formed to study, prevent or mitigate existential risks
- Cambridge-based Centre for the Study of Existential Risk - scientific study of the "four great" technological risks: artificial intelligence, biotechnology, global warming and warfare.
- Center for Responsible Nanotechnology — for safe, efficient nanotechnology.
- Machine Intelligence Research Institute — for developing Friendly AI
- Foresight Institute — for safe nanotechnology and a society prepared to handle the consequences of such
- Center for Genetics and Society — for the relinquishment of genetic technologies which may irrevocably change the definition of "human"
- The Svalbard Global Seed Vault is a vault buried 400 feet inside a mountain in the Arctic with over ten tons of seeds from all over the world. 100 million seeds from more than 100 countries were placed inside as a precaution to preserve all the world’s crops. A prepared box of rice originating from 104 countries was the first to be deposited in the vault, where it will be kept at minus 18C (minus 0.4F). Thousands more plant species will be added as organizers attempt to get specimens of every agricultural plant in the world. Cary Fowler, executive director of the Global Crop Diversity Trust said that by preserving as many varieties as possible, the options open to farmers, scientists and governments were maximized. “The opening of the seed vault marks a historic turning point in safeguarding the world’s crop diversity,” he said. Even if the permafrost starts to melt, the seeds will be safe inside the vault for up to 200 years. Some of the seeds will even be viable for a millennium or more, including barley, which can last 2,000 years, wheat 1,700 years, and sorghum almost 20,000 years.
- Future of Humanity Institute
- The Lifeboat Foundation, "a nonprofit nongovernmental organization dedicated to encouraging scientific advancements while helping humanity survive existential risks and possible misuse of increasingly powerful technologies, including genetic engineering, nanotechnology, and robotics/AI, as we move towards the Singularity."
- Our approach to existential risks cannot be one of trial-and-error. There is no opportunity to learn from errors. The reactive approach — see what happens, limit damages, and learn from experience — is unworkable. Rather, we must take a proactive approach. This requires foresight to anticipate new types of threats and a willingness to take decisive preventive action and to bear the costs (moral and economic) of such actions.
See also 
- Apocalyptic and post-apocalyptic fiction
- Doomsday argument
- Doomsday Clock
- Extinction event
- New tribalism
- Outside Context Problem
- Snowball Earth
- Societal collapse
- Timeline of the far future
- Ultimate fate of the universe
- 10 Ways to End the World
- Bostrom 2002, section 1.2
- Weitzman, Martin (2009). "On modeling and interpreting the economics of catastrophic climate change". The Review of Economics and Statistics 91 (1): 1–19.
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Further reading 
- Derrick Jensen (2006) Endgame. ISBN 1-58322-730-X
- Jared Diamond (2005). Collapse: How Societies Choose to Fail or Succeed. ISBN 0-670-03337-5
- Joel Garreau, Radical Evolution, 2005
- John Leslie (1996). The End of the World. ISBN 0-415-14043-9
- Martin Rees, Our Final Hour (UK title: "Our Final Century"), 2003, ISBN 0-465-06862-6
- Alexei Turchin, Structure of the global catastrophe. Risks of human extinction in the XXI century, 2010, ISBN 978-1-4457-5658-5
- Last Days On Earth (TV documentary) ABC News 2-hour Special Edition of 20/20 on 7 real end-of-the-world scenarios (Wed. Aug 30 2006)
- "What a way to go" from The Guardian. Ten scientists name the biggest danger to Earth and assesses the chances of it happening. April 14, 2005.
- "Confronting the New Misanthropy", by Frank Furedi in Spiked, April 18, 2006
- Ted.com (video) - Stephen Petranek: 10 ways the world could end
- Armageddon Online, A collection of doomsday scenarios and daily news
- Doomsday Guide, a directory devoted to end times theories
- Top 10 Ways to Destroy Earth
- Several potential world ending scenarios
- "Countdown to Doomsday with Today Show Host Matt Lauer". SciFi.com (Syfy). 2006.
-  - A website about existential risk by Nick Bostrom.
- Cognitive biases potentially affecting judgment of global risks - A paper by Eliezer Yudkowsky discussing how various observed cognitive biases hamper our judgement of existential risk.
- Why the future doesn't need us, Wired.com, April 2000 - Bill Joy's influential call to relinquish dangerous technologies.
- Being present in the face of existential threat: The role of trait mindfulness in reducing defensive responses to mortality salience.