Average Ocean Temperature Question for ACTUAL REAL Climatologists?

Let me start by saying I have a college level understanding of physics and I'm a relatively smart person, but I don't know the answer to this, so I'd put it here. Let me also state that I'd only like answers from people who have studied climatology in College. If you haven't studied this in college, don't chime in. My question is, how is it that the Earth's average ocean temperature has risen while there is still ice at the poles? I'll explain. In a Gl of water with Ice in it, the water temperature will remain right at 0� C as long as there is ice in the gl. As you add heat, the ice melts but the water remains at 0� until all the ice is consumed. Likewise if you remove heat from the gl, the water will remain at 0� but the ice will aculate until there is no more liquid water. I realize on a global scale not all the Earth's water is the same temperature but on an AVERAGE ocean temperature I would expect this to remain roughly true -- meaning as the Earth warmed, the AVERAGE ocean temperature would remain a constant, but the ice at the poles would deplete.