As answered in my earlier question How much methane clathrates are buried in continental deposits?, there is alot of methane trapped in the form of clathrates in continental deposits. Additionally, according to the National Snow and Ice Data Centre webpage Methane and Frozen Ground states that the
estimate that there are about 1,400 gigatons of carbon frozen in permafrost.
According to the webpage, this is more than exists in the atmosphere, but it is unlikely to be released at once (due to depth, local conditions etc).
The recent IPCC 5th Assessment Report stated that
Without additional efforts to reduce GHG emissions beyond those in place today, global emissions growth is expected to persist, driven by growth in global population and economic activities. Global mean surface temperature increases in 2100 in baseline scenarios – those without additional mitigation – range from 3.7 to 4.8°C above the average for 1850-1900 for a median climate response.
(GHG = Greenhouse gases)
They also state, in regards to permafrost,
It is virtually certain that near-surface permafrost extent at high northern latitudes will be reduced as global mean surface temperature increases, with the area of permafrost near the surface (upper 3.5 m) projected to decrease by 37% (RCP2.6) to 81% (RCP8.5)for the multi-model average (medium confidence).
What amount of permafrost carbon release could be expected with the IPCC worse-prediction of a 4.8C temperature rise?
(If any published studies are available, that would be good too).
Note: this carbon source is not explicitly covered in the question and answer for How is carbon distributed among the atmosphere, the oceans, the biomass and the unburnt fossil fuels?