For our purposes, we won't care about the exact rise of sea level, for reasons that will become clear. We care that we're taking ice out of the polar regions and creating a thin global shell of water. Since the only thing that matters to the earth's rotation is how far the ice is from the pole, we can lump Greenland and Antarctica together and approximate them as a disk of ice 2500 km in radius. So the moment of inertia of this spinning disk of ice is 1/2 x (2,500,000 m)2 x 2.6 E+19 kg = 8.125 E+31 kg m2. That's about a millionth of the total earth's angular momentum.