Many climate scientists fear this one the most. The amount of greenhouse gasses locked up in the vast expanses of permafrost in the high northern latitudes is in the order of 100 years' worth of human emissions.
In the video attached to post 4, we saw the disproportionate amount of heating taking place within the Arctic Circle due to the melting ice and large land masses in this area. The permafrost time bomb is therefore very inconveniently located and is projected to explode irreversibly at 4 degrees of warming.
The term "irreversibly" means that we will be powerless to stop it. The massive source of carbon will release more greenhouse gasses than we ever could and, in order to have any chance of maintaining a functional society, we will have to do insane sounding things like launch millions of mirrors into space or paint our entire world white.
A much better idea would be to avoid the tipping points in the first place...