As Ted Schuur stepped along the edges of Smith Lake, a bubble surfaced from the muck underfoot. And then another. They're bubbles of methane, a potent heat-trapping gas made from decomposing plants and animals that had once been stuck in Alaska's frigid permafrost. But that frozen ground is thawing because the Arctic is warming four times faster than the rest of the globe. For thousands of years that frozen ground stored dead plants and animals, but the accelerating thaw is allowing them to decay into methane and carbon dioxide.