One MAJOR feedback loop that's always been on the back of my mind has been the possibility of the release of the 500-2500 GIGATONS of methane stored as methane clathrates on the ocean floor. Not only is this substantially larger than the total amount of CO2 in the atmosphere (700 gigatons) but methane is a much stronger greenhouse gas than CO2. So potentially dangerous is the global warming effect from the worldwide release of it that it has been proposed as a major factor in the Permian extinction which killed 96% of all sea life (http://en.wikipedia.org/wiki/Clathrate_gun_hypothesis) and most (90%) of all terrestrial life (as described in the book "When Life nearly died").
So anyway I was not very happy to read of the discovery of "more than 250 plumes of bubbles of methane gas are rising from the seabed of the West Spitsbergen continental margin in the Arctic, in a depth range of 150 to 400 metres". Evidently caused by a warming ocean current, it shows that the methane is coming out of its crystalline ice cages at deeper and deeper depths. While the authors of the study say that if it becomes widespread it will (only) release an additional 5-10% of the methane coming from other global sources, they only give that figure for the methane from the artic sources. Unfortunately methane clathrates are located throughout the deep oceans (it's cold down there; everywhere). Let's hope it stays that way, for our lives and almost every other living creature on earth (remember when scientists say 90% of all life went extinct during the Permian extinction, they mean 90% of all species vanished, not individual creatures. The death rate of individual creatures was likely much higher.) This is one feedback loop that could kill us all.