Hang on a minute.

I have been thinking, and have reached a conclusion. Global Warming should in fact reduce sea levels.

When an object is placed in a fluid, water is displaced (Archimede's Principle: "any body fully or partially submerged in a fluid is buoyed up by a force equal to the weight of the fluid displaced.") Water is the only substance that gets larger when frozen. Around 90% of an iceberg is underwater. Therefore, when an iceberg (or the polar ice caps) melt, the 90% that is in the water is decreasing is size. The 10% that is above the water line does contribute to rising sea levels, but this is dramatically out-done by the 90% underwater to counter this. As a result, the volume of the water decreases, creating a decrease in sea levels.

Clearly, I must be wrong, because everyone else says that sea levels are going to drown and Norfolk will become the new Atlantis, but I think it's quite a good hypothesis, apart from the fact that it was poorly worded. If anyone can point out why I'm wrong, I would be most grateful.

Update: I've worked out why I'm wrong. Water expands as it heats in addition to when it freezes. Oops.