I wrote recently here about the (really) enormous carbon sequestration potential of the terra preta approach to soil enrichment for agriculture. Well, you wouldn't be surprised to know that the earth already has sequestered many billions of tons of carbon dioxide and methane over time. The bad news is that as the world warms, the permafrost that has kept so much of these GHGs in its icy grip is also warming.
Permafrost is defined as ground that has been frozen for more than two years. Much of the earth's permafrost has been frozen for thousands of years. We have known about the disastrous potential of the release of ice-trapped GHGs for several years. Recent research, however, indicates that the amount of carbon stored in the Northern Hemisphere is even greater than previously thought. "The estimated 1,672 billion metric tons of carbon locked up in the permafrost is more than double the 780 billion tons in the atmosphere today. "It's bigger than we thought,'" says the lead researcher. See this from "ScienceDaily." If much of this permafrost melts, then the implications for accelerating global warming are momentous. See also the abstract for this paper.
How bad is the potential impact. An article from the "Discovery Channel" is titled Arctic Tundra Holds Global Warming Time Bomb. As you can see, there's a very bad feedback loop involved here. As the air warms further, more permafrost is thawed and more GHG is released, which further warms the atmosphere, etc., etc.
For another take on permafrost, see these stunning photographs from, where else, "National Geographic Magazine."