An extinction level event is a catastrophic occurrence which has the potential to terminate entire species of animals and plants: eg, to cause a mass extinction. Such events are decidedly rare, but geological evidence shows that they have happened on many occasions since multicellular life became abundant on the planet almost a billion years ago.

This Entry does not attempt to catalogue the great extinction events of the past, but instead lists out the types of events that, were they to happen, could precipitate extinctions on a massive scale, including perhaps the discontinuation of the human race. Probabilities of occurrence are also included, lest we feel compelled to drill big holes in mountains to minimise exposure to Gamma Ray Bursts.

Human extinction, or for us, the 'End of the World', has been a common theme in literature, religious and scientific thought since time immemorial. Television documentaries revel in informing us of our impending doom as a race, and extinction worries tend to follow fashions and fads which are closely linked to the major preoccupation of society at any one time.

Higher Probability Events (Hundreds of Years)

Nuclear War

Humanity is unique among the Earth's fauna for its ability to have devised, through technological means, the ability to wipe itself out quickly. The mechanism is nuclear fusion: ironically the exact same process that powers the Sun - the ultimate source of all life on Earth. In the mid-20th Century this power was harnessed and weaponised, and after World War II vast stockpiles of weapons were built up by Russia, the USA and China as a means of maintaining an uneasy peace between these world powers. In more recent times, many other countries have acquired or amassed nuclear weapons.

Nuclear bombs have been built¹ that are 3,000 times more powerful than the Hiroshima bomb of 1945. While death in the immediate vicinity of a blast is nearly a certainty² humanity itself could be threatened if a significant amount of dust from the explosions and their resultant firestorms were blown into the stratosphere. Less sunlight would reach the surface of the Earth, resulting in a 'nuclear winter' which would devastate plant growth on a worldwide basis, collapsing the food chain as a result.

Worldwide Pestilence

One of humanity's greatest threats is from enemies we cannot see, hear or smell; who can build up vast armies within days; and who can rapidly take advantage of the weaknesses of their adversaries. These enemy are disease microbes.

In 1346, the Black Death arrived in Europe, quickly taking away nearly a quarter of the population of the continent. It reappeared on a regular basis for centuries afterwards, exacting huge death tolls each time. In 1918 a strain of influenza appeared, killing 50 million people within a year. Diseases such as measles and smallpox had a devastating effect on the native inhabitants of America after it was discovered by Europeans. New diseases such as AIDS and Ebola have caused mayhem in recent times. New strains of influenza and diseases such as variant CJD lurk threateningly in the background. Even worse, old diseases of the past such as TB, are developing a resistance to even our most powerful antibiotics. Certain diseases such as anthrax have been weaponised and could be released in vast quantities should someone have the inclination and means to do so.

Climate Change

Records collected from ice cores and deep-sea sediment have shown that the world's climate can change dramatically, often within the timeframe of a few decades, leading to dramatic consequences. This is particularly a concern for our age, considering the amount of fossil fuel that has been burned since the Industrial Revolution began in the 18th Century. Scientific research amassed over the past 20 years links this increase in greenhouse gases to an above-normal increase in world temperatures.

Climate change, whether natural or man-made, could cause mass extinctions. In some habitats it could lead to extensive droughts in places once fertile, or tundra conditions where flowing water was once abundant. If it occurs fast enough, many plants may find that they cannot survive, leading to their destruction before new habitats can be found. If these plants are food for animals, the survival of these animals is also threatened, and so on up the food chain.

We human depend on a relatively small number of highly cultivated strains of crops and vegetables for many of our basic foodstuffs. A rapid change in climate could decimate these food sources, triggering famines. Loss of land due to rapid sea-level increases would put further stress on living conditions, leading to mass-migration and potentially, war. Furthermore, ever-adaptable microbes might find new territories to start new assaults on humanity. Not necessarily species threatening, but not very nice at the same time.

Habitat Destruction

Perhaps we are currently living through a mass-extinction, caused by none other than ourselves? We humans, ever since we have invented the technology to batter other animals over the heads with sticks, have already seen off plenty of other species of animals, the dodo³ being the most famous. Not only that, but as our numbers increase, the populations of other plants and animals decrease due to habitat loss. The World Conservation Union announced in 2003 that of 40,000 species assessed, 12% of all birds, 13% of flowering plants, and 25% of all mammals faced an extinction risk.

Medium Probability Events (Thousands of Years)

The Return of the Ice Age

Technically speaking, we are still in the Ice Age. The last 12,000 years is part of a warm period (an interstadial), which sooner or later will end with the onset of a new cold-phase (a stadial). That's how the Ice Age has worked, alternating between warmth and cold many times since its onset some two million years ago. A returning deep freeze would kill all but the hardiest plants in the affected zones. There would be large-scale climate change across the world putting stress on all animal and plant populations. The onsets of warmth and cold also tend to occur quite rapidly, leading to even greater stress. At the very end of the last cold spell, many large animals such as the Woolly Mammoth, the Giant Sloth and the Great Irish Deer disappeared, never to return.

There is some speculation that the current warming period and subsequent diminution of the Greenland ice cap could precipitate the closing down of the North Atlantic Current, causing an Ice Age to re-assert itself within years. This is a subject of great debate amongst scientists.

Supervolcanoes

Scientists in Yellowstone Park recently discovered something very worrying. They realised that the entire park is an enormous volcanic caldera, and that it has erupted with devastating consequences on a regular basis every 600,000 years or so⁴. The potential force of the blast would be tremendous, some 1,000 cubic kilometres of material being propelled into the atmosphere, resulting in a catastrophic drop in temperatures across the globe.

The most recent supervolcano to erupt was Toba in Indonesia, some 70,000 years ago. Scientists reckon it may have come very close to killing off all humans as there is a distinct genetic 'pinch' in our DNA⁵ which has been attributed to this time. There is another dormant supervolcano, Taupo, in New Zealand.

Magnetic Pole Reversal

Analyses of iron crystals in rock show that the Earth has changed its magnetic pole⁶ on many occasions, averaging 200,000 years. A magnetic flip is not a sudden occurrence. It can take thousands of years to complete. While there is no correlation between a reversal and mass extinctions, reversals do involve a significant weakening of the protective magnetic field around the planet. This could expose many animals and plants to above-normal exposure of damaging solar and cosmic rays at certain times. The last reversal was over 700,000 years ago, and we are long overdue another magnetic flip. Some scientists believe that one is already happening, as the Earth's magnetic field has weakened in recent decades. Bottom line: nothing to worry about (we think).

Low Probability Events (Millions of Years)

Bolide Impact

It is now well known that the Earth is not moving through purely empty space. Our planet shares its orbit around the Sun with thousands of other bodies, many small, some very big indeed. On occasion, one of these big objects crash into the planet, triggering mass extinctions on a global scale. The last time an impact of this size occurred was 65 million years ago, when a 10km sized object, travelling at a speed of 30km per second, hit the region of Yucatán peninsula in Mexico. It is believed to have contributed to the rapid demise of the dinosaurs, who had previously been around in one form or another for a whopping 170 million years.

Flood Basalts

Interestingly, the demise of the dinosaurs co-incides with a lesser-known catastropic event. As the super-continent Pangaea slowly ripped itself apart to form the current continents, large cracks developed on the crust in the areas of rupture exposing the atmosphere to the underlying mantle. In the region of western India, an area known to geologists as the Deccan Traps, basaltic magma and noxious gases were released in vast quantities. The living conditions of plants and animals worldwide would have been degraded substantially by this event. Flood Basalts in an area known as the Siberian Traps are also associated with the worst mass-extinction in geologic history: the Permo-Triassic extinction some 250 million years ago. This particular extinction killed off 90% of all marine species and 70% of all land species.

Supernovae

A supernova is an explosion that occurs when a very large star collapses under its own gravity, leaving a small, dense neutron star in its wake. A supernova in the Milky Way is a rare occurrence by human standards⁷ - happening once every 400 years or so. For a few days, it would outshine every other object in the galaxy. It might even be possible to see it during daytime. One effect of a supernova is the high velocity emission of gases and particles formed from within the star. Were the solar system to collide with this debris field, it is envisaged that the Heliosphere - the protective electrically charged region around the Sun and the planets - would contract in size, potentially exposing the Earth to harmful cosmic rays. A more ominous threat would be from vast clouds of rocks and metals moving at high speed through the solar system. A few very large stars⁸ exist close to the Sun, but even if they went supernova tomorrow it would take thousands of years for their debris fields to reach the Earth. The current threat is not seen as significant.

Runaway Greenhouse

In the 'runaway greenhouse' scenario, the level of greenhouse gases in the atmosphere reaches a point where more heat is trapped in the atmosphere than can be radiated back out into space. The result is a unstoppable temperature rise over a period of years or tens of years, which would be sufficient to wipe out most forms of life very quickly. Our neighbouring planet, Venus, is affected by this phenomenon, with temperatures on the surface high enough to melt lead, and the rain is sulphuric acid.

The Earth's seas contain large quantities of gas hydrates, a type of water-methane crystal pinioned to the ocean floors by water pressure. Methane is a very potent greenhouse gas. It is theorised that if water temperatures were to increase, these hydrates could release methane gas into the atmosphere, potentially triggering a runaway greenhouse. Many people believe that the current sustained levels of fossil fuel burning could also lead to a runaway greenhouse effect over time.

Very Low Probability Events (Billions of Years)

Snowball Earth

An unstoppable cooling effect is also possible to the extent that the entire globe would be covered in ice. Were an ice age to be severe enough, the increased albedo, whereby energy is reflected back into space by white ice and clouds, would cause the temperature of the Earth to drop precipitously. Some geologists think that the Earth may have gone through such a phase around 800 million years ago. Factors such as extreme volcanism, the energy output of the Sun and even the position of the continents on the planet are hypothesised to contribute to the onset of such an effect.

Gamma Ray Bursts

Some stars are believed to be so massive that when they collapse they emit incredibly high levels of gamma rays, the most energetic (and dangerous) form of radiation⁹. Astronomers detect the signatures of these hypernova events by monitoring the levels of gamma rays hitting the Earth. Sudden, short-lived increases in gamma ray activity have come to be known as Gamma Ray Bursts or GRBs. Were a GRB to be directed at our Sun from a star within the Milky Way it would completely irradiate the planet, destroying all forms of life on it. Some scientists think that GRBs are a remnant from an earlier part of the Universe's history, and not something associated with our present galaxy. However, there is recent evidence of GRBs from galaxies much closer to home¹⁰. Some scientists suggest that the Ordovician mass extinction 450 million years ago was caused by a GRB.

Solar Expansion

As the Sun uses up all its hydrogen fuel, the result is a gradual increase in size. This process will continue until, around five billion years from now, the Sun will be truly vast - 100 million miles across - encompassing the entire orbit of the Earth. Life will have been extinguished on our planet long before that, as the seas boil off and all gas is blown away from the Earth due to the action of solar storms. In fact it is estimated that in a mere one billion years from now, the temperature of the Earth will be similar to present-day Venus.

Planetary Impact

It just so happens that the Earth has been involved in at least one major collision with a large planet sized body. The evidence of this impact is right above our heads, orbiting our world every 28 days. The Moon is a chunk of ancient Earth ejected into space as a result of a collision with a body around the same size as Mars. This collision occurred 4.5 billion years ago when the solar system was in its infancy. Since then, luckily for us, things have settled down, and we are now under no threat from our Moon¹¹ or any of our planetary neighbours.

Other Events (but Interesting Nonetheless)

Alien Invasion

If you watch the movies, or read enough science fiction, this one is bound to tickle your fancy. The Universe is a big place, and with the recent discovery of planetary systems around neighbouring stars the odds have shortened somewhat that life, and even alien civilisations, could exist even in our own galaxy. However, to destroy life on the planet they would need the ability to cross vast distances in space first of all, a tricky proposition¹². Alien doomsday scenarios come in many forms, from hyperaggressive monsters, to alien viruses, to a multi-craft invasions, to a single mother ship, to rapid evolution following arrival, to the demolition of the planet by alien bureaucrats in order to make way for a hyperspace bypass¹³.

Robot Takeover

This is extinction with a twist. In this scenario, we humans create our technological replacements, only to be rewarded by rapid extinction when our creations turn on us. Another nightmare possibility is that envisioned by nanotechnology, where tiny robots, no more than a few atoms in size, replicate themselves with incredible speed, turning the world and everything on it into a 'grey goo'¹⁴.

Last Judgement

Many religions have a doomsday scenario: the return of God to the world where the good people are separated from the evil ones. In one version of this account known as the Rapture, the good people are spirited away from the Earth just before things start getting really bad. In the Biblical account of the Last Judgement, the End of the World is preceded by War, Conquest, Pestilence and Famine, the so-called 'Four Horsemen of the Apocalypse'.

Time Travel Slip-ups

In this account, humans develop the means to go back in time, whereupon we soon start making changes that have huge repercussions for the human race. In some versions, we step on a small mammal who just happens to be a distant ancestor. In other versions, it is the humans who kill off the dinosaurs: only for us to find that the same is done to us by creatures from our future.

Selected Reading

Alien Invasion/Intervention

• The Hitchhiker's Guide to the Galaxy (Douglas Adams). Earth gets blown up to make way for hyperspace bypass.

• The War of the Worlds (HG Wells).

• Childhood's End (Arthur C Clarke). Humans fuse with Overmind to become one single entity.

• The Forge of God / Anvil of God (Greg Bear). Humans are killed off by aliens through misdirection and self-replicating machines; a handful survive – most are bent on finding the killers (in Anvil of God).

Creations of Mankind Gone Foul

• The Day of the Triffids (John Wyndham). Mutant, mobile plants, thanks to mankind's tinkering with genetics, terrorise the Earth, killing and eating humans.

• Red Dwarf: Backwards (Rob Grant). Agonoids, built by humans for the purpose of sport-killing, take revenge on their creators, wiping out all of humanity except for one man.

Depression

• The End of Eternity (Isaac Asimov). Mankind destroys its own chance for interstellar travel by controlling its past, present and future; dies out in despair.

Global Catastrophes

• A Wrinkle in the Skin (John Christopher). Earthquakes of epic proportions, of global scale, decimate the Earth.

Hot Viruses, Superbugs and Plague

• The Third Pandemic (Pierre Oulette). A superbug is 'naturally' created when various dangerous bacteria exchange virulence and antibiotic-resistance genes.

• The Plague (Albert Camus). The return of the Bubonic Plague (well... It's not the end of mankind but...)

• The Stand (Stephen King). A rapidly-mutating virus accidentally released from a US military facility kills off most of the world population.

• The Andromeda Strain (Michael Crichton). Microbes from outer space!

• The Fourth Horseman (Alan Nourse). Bubonic plague.

• Some Will Not Die (Algis Budrys). Super-plague wipes out most of life on Earth.

• Eternity Road (Jack McDevitt). Plague wipes out most of humanity, destroys Earth.

• Earth Abides (George R Stewart). Pandemic wipes out all of Earth except one man.

Nuclear (or Other) War

• The Martian Chronicles (Ray Bradbury). Earth is destroyed by war. Two human families survive and escape to Mars.

• Walk to the End of the World (Suzy Mckee Charnas).

• The Foundation series (Isaac Asimov). Nuclear holocaust causes Earth to become radioactive; by which time mankind has travelled so far out in space that 'Earth' is nothing more than legend.

• This is the Way the World Ends (James Morrow). Nuclear holocaust.

• Alas, Babylon (Pat Frank). Nuclear holocaust.

• Fiskadoro (Denis Johnson). Yet more nuclear war.

• The Last Ship (William Brinkley). Nuclear war kills off all but 178 people, who are on a ship.

Cosmic Catastrophes

• Shiva Descending (Gregory Benford). A hail of meteors bombard earth, caused by a giant 30 billion-ton comet.

• Moonseed (Stephen Baxter). A nanovirus that blew up Venus devastates Earth.

• Aftermath; Starfire (Charles Sheffield). A cosmic blast from the Alpha Centauri supernova decimates Earth.

Rapture, Divine Intervention etc.

• The Nine Billion Names of God (Arthur C Clarke). Mankind discovers all nine billion names of God, thus achieving its ultimate destiny: bringing the world to an end (or so it is implied).

Evolution and Natural Selection

• Darwin's Radio; Darwin's Children (Greg Bear). Retrovirus dormant in human genome for thousands of years wake up and transform foetuses into a genetically 'enhanced' super-human species.

• The Children of Men (PD James). People become infertile overnight causing population decline and threatening human existence.

• Greybeard (Brian Aldiss). Another variation on human infertility.

Technology

• Cat's Cradle (Kurt Vonnegut). A scientist 'invents' a way of making water freeze at room temperature. Following an accident, all water on earth freezes.

• Oryx and Crake (Margaret Altwood). A mad scientist creates genetically-engineered hybrid humans to create a society that will live as one with nature; releases epidemic to kill off non-engineered humans – which leaves only one 'survivor' (who was intended as guardian of these creatures).