Extinction level event

An extinction level event (ELE) is an event where the majority of species on the planet become extinct. This is opposed to normal extinctions of just single species, which happen all the time in the wild, at a background extinction rate. Things that cause extinction level events include supervolcanoes and mass climate change, which are events suspected to have happened in the past on several occasions. The most spectacular, of course, are asteroid impacts — which were popularized in the late 1990s by the movies Deep Impact and Armageddon.

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Historical extinction level events

The most famous — undoubtedly because it is among the most spectacular — historical extinction would be the asteroid impact that killed off the dinosaurs (the K-Pg boundaryFile:Wikipedia's W.svg) 65 million years ago.

However, this pales in magnitude compared to the P-T extinctionFile:Wikipedia's W.svg event, which ended the Permian period 252 million years ago. Also known as the Permian mass extinction or the "Great Dying", it resulted in the worldwide loss of 57% of all families, 83% of all genera, 96% of all marine species, and an estimated 70% of all land species. Its cause is unclear but may have involved gradual climate change made suddenly much worse by meteor impact or volcanoes.[1]

Similar explanations are suggested for the Triassic–Jurassic extinction eventFile:Wikipedia's W.svg around 201 million years ago.

The Late Devonian extinctionFile:Wikipedia's W.svg 360-375 million years ago only affected aquatic life; again its causes are unclear but oxygen levels in water seem to have dropped dramatically.

Finally, the extinction which is understood most clearly is happening now, the Holocene extinctionFile:Wikipedia's W.svg. The cause: people killing animals and plants—as much by habitat loss and climate change as by overhunting.

Plausible causes

It's generally hard to figure out what happened last night, let alone 300 million years ago. Explanations vary but the following mechanisms have been advanced for past extinction events:

  • Impacts from meteorites, comets, asteroids: typically causing tsunamis and blotting out the sun with dust.
  • Massive volcanoes, causing either global warming or global cooling.
  • Gradual climate change, which may have been caused by small continents mushing up into big continents (from Plate tectonics) and changing winds and ocean currents, or more complex mechanisms such as a runaway greenhouse effect, methane clathrate offgassing,File:Wikipedia's W.svg etc.
  • Changes in atmospheric oxygen, most obviously due to the widespread proliferation of photosynthesisFile:Wikipedia's W.svg: the Great Oxygenation EventFile:Wikipedia's W.svg
  • Astronomical effects: dark matter[2], supernovae,[3] or gamma-ray bursts, any of which could reduce sunlight, fry the planet with radiation, and provoke mass panic and suicide.
  • People killing stuff.

Implausible causes
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See also

References
  1. "The Great Dying", NASA, 2002
  2. "Does dark matter cause mass extinctions and geologic upheavals?", Astronomy Now, 2015
  3. "Supernova 'smoking gun' linked to mass extinctions", New Scientist, 9 January 2002
  4. "From cataracts and caterpillars to a planet overrun with males: The 10 strangest dinosaur extinction theories ever suggested by experts", Sarah Griffiths, Daily Mail, 8 August 2013
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