The permanently frozen ground around our poles holds, it seems, a huge amount of methane, a very powerful greenhouse gas. Climate scientists wonder what will happen if there is a major melt. And there are signs of such a melt taking place, including methane bubbling up from Alaskan lakes (above).
The scary thing about climate change is that in the past it has not happened gradually, but by sudden jumps. If the planet is going to warm dramatically it will probably take place over the course of a decade or less, far too fast for us to do anything about it by the time it starts.