User:Otolemur crassicaudatus/End of the planet Earth

Risks to civilization, humans and planet Earth are existential risks that could threaten mankind as a whole, have adverse consequences for the course of human civilization, or even cause the end of planet Earth. The concept is expressed in various phrases such as "End of the World", "Doomsday", "TEOTWAWKI", and others.

Future scenarios
Many scenarios have been suggested. Some that will almost certainly end humanity are certain to occur, but only 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 of bangs 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, supervolcanoes, buildup of air pollution, gradual loss of human fertility, and various religious doomsday scenarios.

Space
It is certain that events in the universe will cause life on Earth to come to an end. The certain events, however, will happen at an extremely long timescale measured in billions of years. Calculations indicate that the Andromeda Galaxy is on a collision course with the Milky Way. Andromeda is approaching at an average speed of about 140 kilometres (87 miles) per second and thus impact is predicted in about 3 billion years. This merging could eject the solar system in a more eccentric orbit and an unwanted position in the merged galaxy causing our planet to become uninhabitable, even if an actual collision does not take place.

In about 5 billion years, stellar evolution predicts our sun will exhaust its core hydrogen and become a red giant. In so doing, it will become thousands of times more luminous. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, the Earth will be in an orbit 1.7 AU from the Sun when the star reaches it maximum radius. Therefore, the planet is thought to escape envelopment by the expanded Sun's sparse outer atmosphere, though most (if not all) existing life would have been destroyed by the Sun's proximity to Earth. However, a more recent simulation indicates that Earth's orbit will decay due to tidal effects and drag, causing it to enter the red giant Sun's atmosphere and be destroyed. Some scientists predict that the sun will eventually make the Earth uninhabitable in as little as 500 million years.

On an even longer time scale, the universe may come to an end. The current universe is estimated as being 13.73 billion years old. There are several competing theories as to the nature of our universe and how it will end, but in all cases, there will be no life possible. These scenarios take place on a considerably longer timescale than the expansion of the sun.

Meteorite impact
In the history of the Earth, it is widely accepted that several large meteorites have hit Earth. The Cretaceous-Tertiary asteroid, for example, is theorized to have caused the extinction of the dinosaurs. If such an object struck Earth it could have a serious impact on civilization. It's even possible that humanity would be completely destroyed: for this, the asteroid would need to be at least 1 km (0.6 miles) in diameter, but probably between 3–10 km (2–6 miles). Asteroids with a 1 km diameter impact the Earth every 500,000 years on average. Larger asteroids are less common. The last large (>10 km) impact happened 65 million years ago. So-called Near-Earth asteroids are regularly being observed.

Some scientists believe there are patterns in the number of meteorites hitting Earth. An interesting explanation of such a pattern is given by the hypothetical star Nemesis. This hypothesis states that a star named Nemesis regularly passes through a denser part of the Oort cloud, causing meteorite rains to collide onto Earth. However, the very existence of this pattern is not widely accepted, and the existence of the Nemesis star is highly contested.

A star passage that will cause an increase of meteorites is the arrival of a star called Gliese 710. This star is probably moving on a collision course with the Solar System and will likely be at a distance 1.1 light years from the Sun in 1.4 million years. Some models predict that this will send large amounts of comets from the Oort cloud to the Earth. Other models, such as the one by García-Sánchez, predict an increase of only 5%.

Less likely 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. Another threat might come from gamma ray bursts; some scientists believe this may have caused mass extinction 450 million years ago. Both are very unlikely. Still others see extraterrestrial life as a possible threat to mankind; although alien life has never been found, 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 US Apollo Space Program. It was removed in 1991. Scientists consider such a scenario technically possible, but unlikely.

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 colliding with the Sun, colliding with Venus, or being ejected from the solar system altogether).

Earth
Ice age: In the history of the Earth, many ice ages have occurred. More ice ages will almost certainly come at an interval of 40,000–100,000 years. This 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 technically existing in a warm period between such ice ages (the last ending c. 10000 years ago), and all civilizations (save a few hunter-gatherer populations) have come into existence within that time.

Global pandemic: A less predictable scenario is a global pandemic. For example, if HIV mutates and becomes as transmissible as the common cold, the consequences would be disastrous, but probably not fatal to the human species, as some people are immune to HIV. This particular scenario would also contradict the observable tendency for pathogens to become less fatal over time as a function of natural selection. A pathogen that quickly kills its hosts will not likely 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 outcompete a more lethal species or strain. A real-life example of this process can be found in the historical evolution of syphilis towards a less virulent form. Also as a virus mutates in a direction of being easily transmittable it will likely give up much of its virulence in the process. Though this is not to say that a highly destructive and highly transmissible disease is not possible. Of course, a pandemic resulting in human extinction need not arise naturally; the possibility of one caused by a deliberately-engineered pathogen cannot be ruled out.

Megatsunami: Another possibility is the megatsunami. A megatsunami could, for example, destroy the entire east coast of the United States of America (see La Palma). The coastal areas of the entire world could be flooded in case of the collapse of the West Antarctic Ice Sheet. While none of these scenarios could possibly destroy humanity completely, they could regionally threaten 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 advances 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. 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.

Supervolcano: When the supervolcano at Yellowstone last erupted, 600,000 years ago, the magma and ash covered roughly all of the area of North America west of the Mississippi river. 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, or enough pyroclastic debris and other material may be thrown into the atmosphere to partially block out the sun and cause a natural nuclear winter, similar to 1816, the Year Without A Summer. Such an eruption may cause the death of millions several hundred miles from the epicenter and overall hundreds of millions of deaths worldwide due to the failure of the monsoon, causing starvation on an unthinkable scale.

In the end, Earth will be rendered lifeless by no later than about 7.6 billion years ; before this the oceans will evaporate, and even if Earth is lucky not destroyed by tidal forces or dragged into the Sun by friction with the solar atmosphere, Earth will be too frigid to sustain life when the Sun shrinks to a white dwarf star. However, newer research shows there is the tidal interaction is more possible, the Earth can possibly be engulfed anyway.

Humanity
Some threats for humanity come from humanity itself. The scenario that has been explored most is a nuclear war or another weapon with similar possibilities. It is difficult to predict whether it would exterminate humanity, but very certainly could alter civilization, in particular if there was a nuclear winter event.

Another category of disasters are unforeseen consequences of technology.

It has been suggested that learning computers that rapidly become superintelligent may take unforeseen actions or that robots would out-compete humanity. Because of its exceptional scheduling and organisational capability and the range of novel technologies it could develop, it is possible that the first Earth superintelligence to emerge could rapidly become very, very powerful. Quite possibly, it would be matchless and unrivalled: 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 desires. It could eliminate, wiping out if it chose any other challenging rival intellects, alternatively it might manipulate or persuade them to change their behaviour towards its own interests, or it may merely obstruct their attempts at interference.

Biotechnology could lead to the creation of a pandemic, 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. It has also been suggested that physical scientists might accidentally create a device that could destroy the earth and the solar system. In string theory, there are some unknown variables. If those turn out to have an unfortunate value, the universe may not be stable and alter completely, destroying everything in it, either at random or by an accidental experiment. This is called Quantum Vacuum Collapse by some. Another kind of accident is the Ice-9 Type Transition, in which our planet including everything on it becomes a strange matter planet in a chain reaction. Some do not view this as a credible scenario.

It has been suggested that runaway global warming might cause the climate on Earth to become like Venus, which would make it uninhabitable. In less extreme scenarios it could cause the end of civilization. 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 2035 as temperatures rise. Approximately 2.4 billion people live in the drainage basin of the Himalayan rivers. India, China, Pakistan, Bangladesh, Nepal and Myanmar 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.

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 worlds 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 climate change 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.

Other scenarios

 * 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 localised 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.
 * Demography : Demographic trends create a "baby bust" that threatens the order of civilization.
 * Mutual assured destruction : A full scale Nuclear war could kill billions, and the resulting nuclear winter would effectively crush any form of civilization.
 * Dysgenics : The ideas of dysgenics suggest that the average individual in a civilization may eventually become weaker, because the most intelligent reproduce least leaving the population less able to perform higher functions.
 * Finance : Markets fail worldwide, resulting in economic collapse: mass unemployment, rioting, famine, death, and cannibalism. (This scenario happened, to some extent, in the 1930s and 1940s, helping to cause World War II which has, so far, been the only war to involve nuclear weapons. That war (along with its its immediate precursor in Asia, the second Sino-Japanese War) also involved major atrocities and genocide. Such a scenario, in an age of ICBMs and hydrogen bombs, might be even worse in the future.)
 * Overpopulation : Some scenarios of simultaneous ecological (food & water production) and economical (see f.e. below) collapses with overpopulation are presumed to lead to a global civil war, where the remaining habitable areas are destroyed by competing humans (so called 'Mad Max'-scenario).
 * Famine : As of late 2007, increased farming for use in biofuels, along with world oil prices at over $100 a barrel, has 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.
 * Peak oil : Oil becomes scarce before an economically viable replacement is devised, leading to global chaos and discomfort.
 * Experimental accident : The unlikely creation of a hypothetical microsingularity or exotic matter in particle acceleration experiments, or some unanticipated experimental accident, resulting in destruction of the planet or a large scale disaster. An (unrealised) example was that the first test of an atomic weapon might lead to ignition of the atmosphere and global destruction (see Trinity test). Concern currently exists over the Large Hadron Collider triggering a disaster, although majority of the scientists do not accept this theory.