Melting Permafront

A study of Siberian caves indicates that a temperature rise of 1.5 degrees Celsius above the pre-industrial mean could release massive amounts of carbon from melting permafrost.  It's important to note that this is below the 2 degrees Celsius target set by the United Nations Framework Convention on Climate Change to avoid catastrophic warming.

Update (April 30, 2015):  There's a lot of concern about permafrost.