Various risks exist for humanity, but not all are equal. Risks can be roughly categorized into six types based on the scope (personal, regional, global) and the intensity (endurable or terminal). The following chart provides some examples:
|Typology of risk|
|Global||Plate tectonics||Gamma ray burst|
|Regional||Flash flooding||Permanent submersion under sea level|
|Personal||Being disoriented by a nearby bolide||Being decapitated by a sudden fall off a cliff|
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, or permanently and drastically reduce its potential. Jamais Cascio made an alternative classification system
Many scenarios have been suggested. Some that will almost certainly end humanity 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:
Events in the universe will certainly cause life on Earth to come to an end. There is, however, uncertainty about what the events will be. At the latest, 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 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.
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, life as we know it will not be possible. These scenarios take place on a considerably longer timescale than the expansion of the sun.
Long before the sun is predicted to expand into a red giant, there may be bolides hitting the Earth. 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.
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%.
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.
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).
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. This is not to say that a highly destructive and highly transmissible disease is not possible. Ebola, for example, is highly contagious and 90% fatal; the only reason it hasn't caused a worldwide crisis is because outbreaks occur in rural Africa. 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 a megatsunami. A megatsunami could, for example, destroy the entire east coast of the United States of America. 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 has been one recent high-fatality tsunami, although it was not big enough to be a megatsunami.
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, 640,000 years ago, the magma and ash ejected from the caldera covered most of the United States and part of northeastern Mexico 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 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, as well as destruction of the "American breadbasket", causing starvation on a massive scale.
Modification of the Sun's properties: The Earth will likely be dragged into the sun when it becomes an enlarged red giant by no later than about 7.6 billion years; before actual collision with the sun, the oceans would evaporate, and Earth could be destroyed by tidal forces. Alternatively, if the Sun shrinks to a white dwarf before consuming Earth, the Earth would be too frigid to sustain life.
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. The west coast of North America, which gets much of its water from glaciers in mountain ranges such as the Rocky 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 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.
Many believe that the Maya civilization's Long Count calendar ends abruptly on December 21 2012. This misconception is due to the Maya practice of using only five places in Long Count Calendar inscriptions. On some monuments the Maya calculated dates far into the past and future but there is no end of the world date. There will be a Piktun ending (a cycle of 13 144,000 day Bak'tuns) on December 21st, 2012. A Piktun marks the end of a 1,872,000 day or approximately 5125 year period and is a significant event in the Maya calendar.