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.