jc42 writes, "A recent study published in Nature documents the accelerating release of methane from melting permafrost. Methane is a greenhouse gas 23 times more potent than carbon dioxide, so this may signal more rapid warming in the near future. If you don't subscribe to Nature, the Guardian has a good summary of the piece."
It's not just Siberian permafrost. One of the major concerns is bogs
— they account for a relatively small percent of total surface space, but have a large amount of carbon locked up. No one is sure if the greenhouse effect will cause them to lock up more, or to release more carbon.