What if all the world's supervolcanoes erupted at once

• There are about a dozen supervolcanoes on Earth - each one at least seven times larger than Mount Tambora, which had the biggest eruption in recorded history.
• If all of these supervolcanoes erupted at once, they'd likely pour thousands of tons of volcanic ash and toxic gases into the atmosphere.
• This ash would linger for months, blocking sunlight and setting off a volcanic winter. The gas would also likely fall back down to Earth as acid rain, devastating agriculture and leading to global famine.
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Following is a transcript of the video.

In 1815, Mount Tambora erupted in Indonesia, killing an estimated 92,000 people. It was the biggest eruption in recorded history. And yet, Tambora was about one-seventh the size of the smallest supervolcano. There are about a dozen of these monsters worldwide, and the last one went off 26,500 years ago in New Zealand, blanketing most of its island in a layer of thick ash. Now, it's extremely unlikely that one of these supervolcanoes would go off anytime soon, let alone multiple eruptions at once. But, for curiosity's sake, what would happen if all the world's supervolcanoes erupted at once?

If every supervolcano went off, you'd have a hard time finding a safe place to flee to, because almost every continent is home to at least one supervolcano. For example, there's Yellowstone in the US, Ngorongoro in Tanzania, and Toba in Indonesia. So, basically, no matter where you are, you're out of luck. But at least you'll have a warning, because weeks or months beforehand, the ground will tremble with earthquakes. Then, when E-Day arrives, the earth-shattering sound would be a dead giveaway that something wasn't right.

When Krakatoa, which wasn't even close to the size of a supervolcano, erupted in 1883, it released a roar so loud the sound traveled nearly 4,800 kilometers across the Indian Ocean, shattering windows and deafening people in its path. And since even the smallest supereruption would dwarf Krakatoa's eruption, there is really no telling how much damage this would cause, but it would be epically catastrophic.

And let's say you were lucky enough to survive this first wave of destruction. After that, it would be time to find a fallout shelter, because these supereruptions spew billions of tons of ash, volcanic glass, and rock thousands of meters into the air. Not something you want to inhale or be in the path of, since this cloud of ash doesn't just travel up; it also expands out, crashing across the landscape at jet-fighter speeds. It would collapse buildings, contaminate water supplies, and bring down any power grids in its path. And the fallout would extend for hundreds of kilometers, so any cities nearby a supervolcano are immediately toast. As are any planes that attempt to fly people away from the danger.

And keep in mind that ash travels. When Toba erupted 74,000 years ago, winds blew ash all the way to India. So if all the supervolcanoes go off at once, volcanic debris would spread across the globe. When the eruption ends, the disaster will have only just started. Because for the next six months, much of that supervolcanic ash would linger in the stratosphere and block sunlight, causing global temperatures to plunge by as much as 15 degrees Celsius. That's close to the difference between summer in Rio versus Anchorage, Alaska. In fact, just "little" Mount Tambora's eruption alone set off the "Year Without a Summer," where frost and blizzards plagued much of the Northern Hemisphere. So, multiply that by the 12 or so supervolcanoes all spewing thick black dust, and you've got a worldwide volcanic winter for the next few years. Tropical forests, which can't handle cold weather, would wither and die, bringing down the millions of animal species that live there.

And it's about to get even bleaker. You know that saying "out of the frying pan and into the fire"? Well, in addition to ash, those volcanoes also belch toxic gasses, like sulfur dioxide, into the atmosphere. And after a few years, just after the winter finally ends, those gasses would start to fall from the sky as acid rain. When the Laki volcano erupted in 1783 in Iceland, it rained down so much sulfuric acid that it devastated farmland and wiped out half of all livestock. The next year, a full quarter of Iceland's population died in the resulting famine. So imagine that, but everywhere. And since Laki wasn't even a supervolcano, we're looking at acid rain for the next decade. Say goodbye to civilization, because it probably can't survive a decade-long global famine.

Now, there are a few places on Earth that, while still suffering from cold and acid rain and famine, would at least be free from the actual explosions themselves. Like volcanic islands such as the Galapagos, or even Hawaii, which ironically is famous for its volcanoes. But the thing is, these volcanoes slowly release flows of lava rather than violently exploding. So, you could at least enjoy a nice view as civilization implodes.