Apocalypse How: Plausible Ways the World Could End

It may not happen Friday, but human existence could end in any number of ways, experts say.

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Experts don't believe a super volcano is likely to destroy mankind.

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With everyone from NASA to the federal government to even Mayan experts promising the world isn't going to end Friday, perhaps now the 2 percent of Americans who believe the world will end Dec. 21 can breathe easy. But beyond Friday, there are plenty of ways humans could end up biting the dust, experts say.

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Mark Van Stone, author of 2012: Science and Prophecy of the Ancient Maya, says the Maya did not predict the end of the world in 2012, and that he's found "no evidence to support" that the world will end Friday. He says he wrote his book to "shift the spotlight away from the one-dimensional Maya-as-mere-prophets-of-doom, to the real Maya, a much richer and subtler people."

Believers say that the Mayan "great cycle," which started 5,125 years ago, will end on Dec. 21, 2012. On that day, either the world will end, or some sort of "great change" will happen, they say. But according to Van Stone, the Maya not only didn't predict an end of the world, but recent discoveries suggest that the Mayans made calculations that go thousands of years into the future.

Assuming the end of humanity doesn't occur on Friday, there are other, scientifically plausible ways that, as one expert puts it, could "put an end to our residence on Earth."

Super Volcanoes

Deep under Yellowstone National Park lies the Yellowstone Caldera, a 34-by-45-mile "super volcano" that erupts every couple hundred thousand years, leaving ash over an area the size of Europe. Scientists think that it erupts about once every 600,000 years; its last eruption was 640,000 years ago.

A super volcanic eruption is believed to have caused the end of the Triassic period some 201 million years ago, killing much of life on Earth.

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"There were maybe a half million years with pulses of eruptions, carbon produced by the volcano caused rapid global climate change and acid rain on land that left a devastating effect on life," says Mike Benton, a fellow with the Royal Society of Edinburgh who studies so-called "mass extinction events" that kill off up to 90 percent of Earth's life.

"Normally with these past events, organisms of our size have gone out. Animals of up to a pound in weight survive reasonably well," he says. "But beyond that, you're likely to go extinct."

Stephen Self, a volcanologist with England's Open University, says recent discoveries paint a slightly better picture.

"The new science suggests a shorter period of cooling [caused by ash blocking out the sun], with some areas of the earth hit badly and others not—it might have serious consequences in various regions of the earth for about 3-5 years," he says.

Even more good news: Though scientists believe the largest super volcano lies under Yellowstone, there may be "tens" of others that haven't been discovered, Self says.

"These eruptions occur on a timescale of hundreds of thousands of years, we may well dispatch ourselves off Earth long before another one of these eruptions happens," Self says. "It's more likely disease, war, or something else ends humanity before that happens."

Asteroid Impact

It's been the subject of lots of movies (most notably Armageddon), but asteroids impact earth fairly often, including being the blame for dinosaurs' extinction 65 million years ago.

Benton says scientists consider a supervolcano or an asteroid impact as the "two dominant models of mass extinction that could happen any time in the future."

As scientists have learned more about asteroids, there is less evidence to suggest that they impact Earth on any sort of schedule.

"The common view of meteorites is that they hit randomly, and the larger they are, the more random they are," Benton says.

The science of predicting whether an asteroid will cause massive casualties is nearly impossible: There have been large meteorites that have hit Earth with very little damage.

"There have been many huge impacts with no extinctions," he says. "It depends on how they hit, where they hit, whether they land in the sea or not."