The permafrost in the arctic prevents an enormous mass of peat from decaying. When it thaws, the carbon in the peat goes into the air as CO2 (if we’re lucky) or CH4 (if we’re not: methane is 25 times more warming than CO2). How about 1.5 trilllion tons, twice as much as is in the atmosphere now? Note that this process is self-accelerating: the warmer it gets, the faster the permafrost melts. Added to what we know about methane under the Siberian ice shelf, also melting and beginning to release that puff of trouble, it’s not exaggerating to call this terrifying. Not alarming or troubling; catastrophic. Decay of the arctic peat is irreversible; there’s no way plants can take up that much carbon, and it certainly can’t be put back into the melted tundra.
If that isn’t enough to spoil your day, consider that arctic sea ice, whose melting is a big part of permafrost thawing, is going faster than anyone thought it would (ht: Brad DeLong).