What is a far more potent greenhouse gas than carbon dioxide?

Much as carbon dioxide levels in the atmosphere contribute to global warming, there is another substance which is even more potent when it is released.  Presently there are trillions of tons of it embedded in a type of ice slurry in the Artic and in the seas around continental shelves from North America to Antarctica.  What is it?

CLICK HERE to read "The 'Ticking Time Bomb' That Could Cause Such Rapid Global Warming We'd Be Unable to Prevent Extinction" by Thom Hartmann, 11-26-2013, in http://admin.alternet.org