what has melting polar ice to do with the water supply of a billion people??? Ok it´s not good because it raises the tide of the Ocean so for example the dutch may get wet feet in future ;-) but this has nothing to do with the general fresh water supply.
Actually the salinity of seawater would be lowered by melting ice caps meaning the water would evaporate at a lower temp creating more rain. This would create, OMG, a greener planet.
That just my humble opinion. I'm not a professional meteorologist, but I did stay at a Holiday Inn last night.
If the polar ice cap up north completely melts the rise in the oceans level will be zero cm. The northern ice cap is floating ice. It displaces exactly as much water to keep it floating as it would replace when it melts. To prove this fill a glass with ice and water so some ice is floating above the glasses lip. What happens when the ice melts? Meanwhile the southern ice cap is getting thicker.