Global Warming (Greenhouse Effect), normal cycle of Earth?

The other day I was talking to a friend at a party and we got into the global warming situation. The argument was this: I said that the world is definatly heating up and the ice caps are melting away. She said that the polar caps melting only cool the atmosphere back down again.
I was wondering if her theory of the air being heated and melting the ice only causing the oceans to get cooler and cool off the atmosphere is a true and valid.