How many ways can the world end? We can think of at least five.
But before we get into detail, let's dismiss two things that won't cause the demise of the planet.
Global warming is bad for people who live in low-lying coastal areas and at the edges of deserts, but the truth is that Earth has been much warmer throughout most of the past 500 million years, and life did just fine.
On the other side of things, a new ice age would end most human habitation of Canada, northern Europe, the northern U.S. and Russia, but the tropics would stay about the same — and there'd be a lot more land to go around in south Florida as sea levels dropped.
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Since we're currently in the second half of an interglacial period, it's a pretty safe bet that the glaciers will indeed advance again within the next 10,000 years.
But at any given time, four of the five following scenarios really could cause the end of life on Earth — and the fifth almost certainly will.
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Massive asteroid impact: Asteroids and comets crash into our planet all the time, with varying degrees of damage. The last big one was 100 years ago in Siberia, but in such a remote area that no one died.
Yet scientists keep finding new evidence of medium-sized impacts that caused at least regional devastation — near New York harbor around 300 B.C., in eastern Canada about 11,000 B.C., the famous Meteor Crater in Arizona 50,000 years ago.
Much larger asteroids have been tied to mass die-offs in biological history — the end of the dinosaurs 65 million years ago, and the even more devastating Permian-Triassic extinction event 251 million years ago in which some 80 percent of animal species vanished.
It would take an asteroid the size of a small planet to really snuff out life on Earth.
Something very much like that seems to have happened when an object the size of Mars hit the Earth about 4.5 billion years ago, and the resulting debris formed the moon. Fortunately, there was no life on Earth yet.
Life on Mars, if it ever existed, might not have been so lucky. Most evidence indicates that the Red Planet was warm and wet in the distant past, and there are signs it had a strong magnetic field to shield the surface from solar radiation.
But recent studies indicate that Mars' entire northern hemisphere may be a gigantic impact crater, the result of a collision 3.9 billion years ago so huge it may have destroyed the planet's magnetic field.
Were that to happen on Earth, the few surface organisms that survived the impact and resulting earthquakes and fires would be fried by solar rays.
Massive volcanic eruptions: An alternate theory for the low, flat, featureless Martian northern hemisphere is that huge lava flows simply erased any previous features.
Similarly, there's good evidence that the dinosaurs back on Earth were killed not by an asteroid, but instead, or additionally, by enormous eruptions in what now is India.
Even moderate eruptions, which kick up huge amounts of soot and dust, blocking sunlight, can have climatic effects. The eruption of Mount Pinatubo in the Philippines significantly cooled the planet in 1991-92, as did Indonesia's Krakatoa in 1883.
More effective was Mount Tambora on the other end of Java in 1815, which cooled things so much that Europeans called 1816 "the year without a summer" — it snowed in June, and summer frosts killed crops across the Northern Hemisphere.
Moving up the scale, the Mount Toba supervolcanic eruption in Sumatra 75,000 years ago may have cooled the planet enough to force the early human population through a genetic "bottleneck" as most people died, leaving the few survivors to repopulate the world.
And the Yellowstone supervolcano — that's right, Yellowstone National Park sits atop a massive magma chamber — will probably take out most of the people living between the Rockies and the Appalachians next time it erupts, which could literally be tomorrow.
But neither of those would end life on Earth. For that, it would take something along the lines of the long-ago prolonged eruptions that created India's Deccan Traps and the Siberian Traps in Russia.
In both instances, giant fissures in the ground simply opened up, oozing lava that spread in every direction for hundreds of miles, releasing huge amounts of deadly gas, smoke and soot into the atmosphere. These events went on for tens of thousands of years.
The Deccan Traps eruptions took place just before the dinosaurs disappeared and formed much of the landmass of the Indian subcontinent. The Siberian lava flows happened 251 million years ago and are the likely cause of the aforementioned Permian-Triassic mass extinction.
In the latter case, 3 million square miles were covered by layer upon layer of lava. It doesn't take much extrapolation to conclude that an eruption event two or three times the magnitude of the Siberian one could end life on Earth.
Nuclear war: Few people have uttered the phrase "nuclear winter" since the end of the Cold War, but it was a very real fear during the 1980s.
The notion was that a full-scale nuclear war between the Soviet Union and United States would kick huge amounts of dust, smoke and soot up into the atmosphere and blot out sunlight for months or even years, causing mass extinctions as most plants died and most animals starved.
Life has squeaked by in such instances in the past, but the deadly post-nuclear radioactive particles carried around the world could land a deadly second blow on the surviving organisms.
Since the '80s, further research has indicated that the atmospheric soot would also destroy the ozone layer, letting in more extraterrestrial radiation and further cooling the planet.
The odds of total nuclear war between the U.S. and Russia seem remote now, and no other nations currently have the thousands of warheads it would take for such a doomsday scenario to occur. But there's always a chance of a full-scale nuclear exchange between future superpowers.
Black hole: Bottomless gravitational pits from which not even light can escape were first theorized in the 1960s, but since then they've been "spotted" throughout the universe.
It's now thought that every spiral galaxy, including our own, has a supermassive black hole at its center.
Smaller black holes are formed by the collapses of large stars, and can be expected to keep moving in the same orbits around galactic centers as they did before the collapse.
The problem is that we'd no longer be able to see them, and would have to watch the behavior of other astronomical bodies to figure out where they might be lurking.
Were a black hole to approach our solar system, we'd begin to notice changes in the light of other stars as it was bent by the black hole's massive gravity.
Then the orbits of the larger planets would begin to change as they were pulled toward it. The sun would become elongated, and the Earth's own orbit would shift.
Finally the sun, planets and asteroids would go into spiral orbits around the hole and gradually be sucked into it, one by one, like water going down a drain.
Thanks to the massive tidal disruptions on Earth, not to mention the lack of reliable sunlight, we'd already be dead.
Some think it's also possible that we could create our own black hole right here on Earth.
Last year, a flurry of lawsuits accompanied the firing up of the Large Hadron Collider outside Geneva, Switzerland, from people worried it could create a mini-singularity that would gobble up the planet.
Fortunately, the machine broke down after a few days. The end of the world will have to be pushed back to next summer at the earliest.
The expanding sun: If all else fails, the Earth will almost certainly come to an end in about 5 billion years when it falls into the expanding sun.
It's perfectly natural — stars like ours simply turn into red giants near the end of their lifespans, and their inner planets become toast.
Terrestrial inhabitants need not worry, since they'll be boiled off much earlier by the sheer heat of the growing star.
Some scenarios say we've got only a billion good years left on this planet — rather gloomy, since life in some form has been around for about 3.7 billion years and this means we're already close to the end.
Then again, it's also possible that scientists of an advanced future civilization could simply tow out Earth to a safer orbit, after having presumably rendered Mars and Venus inhabitable as well.
That's if they don't manage to accidentally destroy the planet first.
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