"Permafrost—the ground that stays frozen for two or more consecutive years—is a ticking time bomb of climate change. Some 24 percent of Northern Hemisphere land is permafrost. That's 9 million square miles (23 million square kilometers) found mostly in Siberia, the Tibetan Plateau, Alaska, the Canadian Arctic, and other higher mountain regions."
"Unfortunately, thawing permafrost releases massive amounts of methane and/or carbon dioxide. The question is whether that would happen over the course of decades or over a century or more. This short video from the Yale Climate Forum explains the current scientific thinking on just how close we might be to the lethal tipping point."