I just watched a very interesting episode of Nova called Artic Sinkholes with shocking revelations. Years ago I posted about how global warming can create a positive feedback loop that can accelerate global warming. This feedback loop is caused by gasses that previously were held in the earth. This episode showed that it's actually much worse. Beneath the permafrost in the artic circle are pockets of ancient methane that due to global warming are forming chimneys through the permafrost that are releasing large amounts of methane gas, a gas that is many 25 times more potent than carbon dioxide.
This is very scary stuff