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If you've watched the movies "2012" or "Armageddon" or read "On the Beach," you know about some of the threats that could end life as we know it. The Sun could do something nasty. A meteor could strike. We could nuke ourselves out of existence. These are only a few well-known extinction level events. There are so many more ways to die!
But first, what exactly is an extinction event? An extinction level event or ELE is a catastrophe resulting in the extinction of the majority of species on the planet. It's not the normal extinction of species that occurs every day. It isn't necessarily the sterilization of all living organisms. We can identify major extinction events by examining the deposition and chemical composition of rocks, the fossil record, and evidence of major events on moons and other planets.
There are dozens of phenomena capable of causing widespread extinctions, but they can be grouped into a few categories:
The Sun Will Kill Us
Life as we know it wouldn't exist without the Sun, but let's be honest. The Sun has it out for planet Earth. Even if none of the other catastrophes on this list ever happen, the Sun will end us. Stars like the Sun burn brighter over time as they fuse hydrogen into helium. In another billion years, it will be about 10 percent brighter. While this might not seem significant, it will cause more water to evaporate. Water is a greenhouse gas, so it traps heat in the atmosphere, leading to more evaporation. Sunlight will break water into hydrogen and oxygen, so it can bleed away into space. Should any life survive, it will meet a fiery fate when the Sun enters its red giant phase, expanding out to the orbit of Mars. It's not likely any life will survive inside the Sun.
But, the Sun can kill us any old day it wants via a coronal mass ejection (CME). As you can guess from the name, this is when our favorite star expels charged particles outward from its corona. Since a CME can sent matter any direction, it doesn't usually shoot directly toward Earth. Sometimes only a tiny fraction of particles reach us, granting us an aurora or a solar storm. However, it's possible for a CME to barbecue the planet.
The Sun has pals (and they hate Earth too). A nearby (within 6000 light years) supernova, nova, or gamma ray burst could irradiate organisms and destroy the ozone layer, leaving life at the mercy of the Sun's ultraviolet radiation. Scientists think a gamma burst or supernova might have led to the End-Ordovician extinction.
Geomagnetic Reversals May Kill Us
The Earth is a giant magnet that has a love-hate relationship with life. The magnetic field protects us from the worst the Sun throws at us. Every so often, the positions of the north and south magnetic poles flip. How often the reversals occur and how long it takes the magnetic field to get settled is highly variable. Scientists aren't completely sure what will happen when the poles flip. Maybe nothing. Or maybe the weakened magnetic field will expose the Earth to the solar wind, letting the Sun steal a lot of our oxygen. You know, that gas humans breathe. Scientists say magnetic field reversals aren't always extinction level events. Just sometimes.
The Big Bad Meteor
You may be surprised to learn the impact of an asteroid or meteor has only been connected with certainty to one mass extinction, the Cretaceous-Paleogene extinction event. Other impacts have been contributing factors to extinctions, but not the primary cause.
The good news is that NASA claims about 95 percent of comets and asteroids bigger than 1 kilometer in diameter have been identified. The other good news is that scientists estimate an object needs to be about 100 kilometers (60 miles) across to wipe out all life. The bad news is there are another 5 percent out there and not much we can do about a significant threat with our present technology (no, Bruce Willis cannot detonate a nuke and save us).
Obviously, living things at ground zero for a meteor strike will die. Many more will die from the shock wave, earthquakes, tsunamis, and firestorms. Those that survive the initial impact would have a hard time finding food, as the debris thrown into the atmosphere would change the climate, leading to mass extinctions. You're probably better off at ground zero for this one.
The Sea
A day at the beach might seem idyllic, until you realize the blue part of the marble we call Earth is deadlier than all of the sharks in its depths. The ocean has various ways of causing ELEs.
Methane clathrates (molecules made of water and methane) sometimes break from the continental shelves, producing a methane eruption called a clathrate gun. The "gun" shoots immense amounts of the greenhouse gas methane into the atmosphere. Such events are linked to the end-Permian extinction and Paleocene-Eocene Thermal Maximum.
Prolonged sea level rise or fall also leads to extinctions. Falling sea levels are more insidious, as exposing the continental shelf kills off innumerable marine species. This, in turn, upsets the terrestrial ecosystem, leading to an ELE.
Chemical imbalances in the sea also cause extinction events. When the middle or upper layers of the ocean become anoxic, a chain reaction of death occurs. The Ordovician-Silurian, late Devonian, Permian-Triassic, and Triassic-Jurassic extinctions all included anoxic events.
Sometimes the levels of essential trace elements (e.g., selenium) fall, leading to mass extinctions. Sometimes the sulfate-reducing bacteria in thermal vents get out of control, releasing an excess of hydrogen sulfide that weakens the ozone layer, exposing life to lethal UV. The ocean also undergoes a periodic overturn in which the high-salinity surface water sinks to the depths. Anoxic deep water rises, killing surface organisms. The late-Devonian and Permian-Triassic extinctions are associated oceanic overturn.
The beach doesn't look so nice now, does it?
And the "Winner" Is... Volcanoes
While falling sea level has been associated with 12 extinction events, only seven involved a significant loss of species. On the other hand, volcanoes have led to 11 ELEs, all of them significant. The End-Permian, End-Triassic, and End-Cretaceous extinctions are associated with volcanic eruptions called flood basalt events. Volcanoes kill by releasing dust, sulfur oxides, and carbon dioxide that collapse food chains by inhibiting photosynthesis, poison the land and sea with acid rain, and produce global warming. The next time you vacation at Yellowstone, take a moment to stop and ponder the implications when the volcano erupts. At least the volcanoes in Hawaii aren't planet killers.
Global Warming and Cooling
In the end, the ultimate cause of mass extinctions is global warming or global cooling, usually caused by one of the other events. Global cooling and glaciation are believed to have contributed to the End-Ordovician, Permian-Triassic, and Late Devonian extinctions. While the temperature drop killed some species, the sea level fall as water turned to ice had a much greater effect.
Global warming is a much more efficient killer. But, the extreme heating of a solar storm or red giant isn't required. Sustained heating is associated with the Paleocene-Eocene Thermal Maximum, the Triassic-Jurassic extinction, and the Permian-Triassic extinction. Mostly the problem seems to be the way higher temperatures release water, adding the greenhouse effect to the equation and causing anoxic events in the ocean. On Earth, these events have always balanced out over time, yet some scientists believe there is potential for Earth to go the way of Venus. In such a scenario, global warming would sterilize the entire planet.
Our Own Worst Enemy
Humanity has plenty of options at its disposal, should we decide it's taking too long for the meteor to strike or the volcano to erupt. We're capable of causing an ELE via a global nuclear war, climate change caused by our activities, or by killing enough other species to cause a collapse of the ecosystem. At any given time, multiple factors can be in play. For example, a volcano erupting and carbon dioxide emissions from fossil fuels can occur together, and countries could engage in nuclear conflict at the same time.
The insidious thing about extinction events is that they tend to be gradual, often leading to a domino effect in which one event stresses one or more species, leading to another event that destroys many more. Thus, any cascade of death typically involves multiple killers on this list.
Key Points
- Extinction level events or ELEs are calamities that result in the annihilation of most species on the planet.
- Scientists can predict some ELEs, but most are neither predictable nor preventable.
- Even if some organisms survive all other extinction events, eventually the Sun will eradicate life on Earth.
References
- Kaplan, Sarah (June 22, 2015). "Earth is on brink of a sixth mass extinction, scientists say, and it's humans' fault". The Washington Post. Retrieved February 14, 2018.
- Long, J.; Large, R.R.; Lee, M.S.Y.; Benton, M. J.; Danyushevsky, L.V.; Chiappe, L.M.; Halpin, J.A.; Cantrill, D. & Lottermoser, B. (2015). "Severe Selenium depletion in the Phanerozoic oceans as a factor in three global mass extinction events". Gondwana Research. 36: 209.
- Plotnick, Roy E. (1 January 1980). "Relationship between biological extinctions and geomagnetic reversals". Geology. 8(12): 578.
- Raup, David M. (28 March 1985). "Magnetic reversals and mass extinctions". Nature. 314 (6009): 341–343.
- Wei, Yong; Pu, Zuyin; Zong, Qiugang; Wan, Weixing; Ren, Zhipeng; Fraenz, Markus; Dubinin, Eduard; Tian, Feng; Shi, Quanqi; Fu, Suiyan; Hong, Minghua (1 May 2014). "Oxygen escape from the Earth during geomagnetic reversals: Implications to mass extinction". Earth and Planetary Science Letters. 394: 94–98.