The oceans are the roach motel of global warming. In the case of the oceans, it is heat which checks in but doesn't check out. Water has an extraordinary heat capacity. In other words, it holds heat better than almost anything else. That means that when the oceans warm, they lose that heat only slowly and reluctantly. Ocean heat stays around for a long time. And most of the heat from global warming -- about 90% -- goes into the oceans.
source:
http://members.dcn.org/dorritie/now.html