The clathrate gun hypothesis is the popular name given to the hypothesis that rises in sea temperatures (and/or falls in sea level) can trigger the sudden release of methane from methane clathrate compounds buried in the seabeds and their permafrost, which because the methane itself is a powerful greenhouse gas, in turn causes further temperature rise and further methane clathrate destabilization – in effect initiating a runaway process, as irreversible once started as the firing of a gun.[1]
http://en.wikipedia.org/wiki/Clathrate_gun_hypothesis