The permafrost, which holds lot of frozen vegetation and keeps it from decaying and contributing to atmospheric CO2, will melt enough to kick into a positive feedback of warming and melting.
Figure: Carbon emission (in billions of tons of carbon a year) from thawing permafrost.
The thaw and release of carbon currently frozen in permafrost will increase atmospheric CO2 concentrations and amplify surface warming to initiate a positive permafrost carbon feedback (PCF) on climate…. [Our] estimate may be low because it does not account for amplified surface warming due to the PCF itself…. We predict that the PCF will change the arctic from a carbon sink to a source after the mid-2020s and is strong enough to cancel 42–88% of the total global land sink. The thaw and decay of permafrost carbon is irreversible and accounting for the PCF will require larger reductions in fossil fuel emissions to reach a target atmospheric CO2 concentration.
This could triple atmospheric CO2. Translation: we’re screwed.
Science Notes 
February 20, 2011 at 15:58
chilling :(
February 22, 2011 at 15:18
And yet still the deniers rage on… :(
March 1, 2011 at 15:08
what is permafrost
March 1, 2011 at 23:08
Permafrost is permanently frozen ground. In the Arctic, houses have to be insulated on the bottom so that they do not melt (unevenly!) into the earth.
March 2, 2011 at 15:11
thanks for reply it was really helpful