[lizma]

The main reason is that the melting of the ice caps has caused the mantle - the thick layer of nearly-molten rock between the crust and the Earth's core - to rebound.

Supposedly a change in the molten flow of the earth's core, a gazillion years ago, reversed the earth's magnetic poles. That wouldn't be any fun.

But I was under the impression that the overall volume of the ice caps has not changed.

[Doctor Stochastic]

The North magnetic pole has moved about 800 miles since it was first visited. The Earth's magnetic field is rather weak. It does switch though, providing one of the direct evidences of continental drift.

[Doctor Stochastic]

The Arctic ice cap seems to have shrunk by about 40% since first measured. (It's thinner rather than being less widespread.) There's no known connection with climate change; it may just be a fluctuation. Of course, melting of the Arctic cap has no effect on ocean levels; melting of the Antarctic ice cap or Greenland ice cap would have an effect.