WE HAVE 20 YEARS BEFORE 4/5 OF OUR BIOSPHERE BECOMES EXTINCT ,^:
A short drive north of Fairbanks, Alaska, there's a red shed stuck right up against a hillside.
Behind the door is a geological time bomb, scientists say. No one knows exactly how big the bomb is. It may even be a dud that barely detonates. But the fallout could be so large that it's felt all around the world.

Now there's evidence that, in the past few years, the bomb's timer has started ticking.

The permafrost is packed with the remains of ancient life. From prehistoric grass and trees to woolly mammoths and woolly rhinoceroses, just about every creature that lived on the tundra over the past 100,000 years is buried and preserved down in the permafrost.

And all this life is made of carbon. So there's a massive amount of carbon buried down here.

"The permafrost contains twice as much carbon as is currently in Earth's atmosphere," Douglas says. "That's 1,600 billion metric tons."

In fact, there's more carbon in the permafrost, Douglas says, than all the carbon humans have spewed into the atmosphere since the Industrial Revolution — first with steam trains, then with coal plants, cars and planes.

They were ancient bacteria. And once they warmed up, they were hungry. The bacteria started converting the carbon that's in dead plants and animals into gases that cause climate change: carbon dioxide and methane.