Zimov, Schuur and Chapin point out that permafrost is a large carbon reservoir
Permafrost...is an additional large carbon reservor that is rarely incorporated into analyses of changes in global carbon reservoirs......Followed by considerable detail, which, among other things argues that paleoclimate changes in CO2 are more likely to be due to warming and cooling of the permafrost rather than carbon being absorbed in the oceans. They conclude that the permafrost is undergoing substantial thawing, and that in particular the large Siberian deposits might disappear by 2100. Because this permafrost is well studied they can make good estimates of how much carbon will be released.
Permafrost is a globally significant carbon reservoir that responds to climate change in a unique and very simple way. With warming, its spatial extent declines, causing rapid carbon loss; with cooling, the permafrost reservoir refills slowly, a dynamic that mirrors the past atmospheric record of CO2. In a warmer climate permafrost carbon is thus likely to become part of more actively cycling carbon reservoirs. Factors including high-latitude climate warming should be mitigated to minimize the risk of a potentially large carbon release that would further increase climate warming.