I DON'T BELIEVE IT. WATER LEVEL LOWERS WHEN ICE MELTS. FILL UP A GLASS WITH ICE. POUR WATER UNTIL ITS AT THE TOP. COME BACK IN 2 HOURS AND SEE THE WATER LEVEL DECREASE.
WATER EXPANDS WHEN IT FREEZES.
I NEED SOMEBODY TO EXPLAIN HOW THIS IS DIFFERENT FOR GLACIERS.
In your scenario, the big blocks/cubes of ice takes up more space as ice than it does as water which is why it doesn't overflow when it melts.
With global warming, several low lying countries like The Maldives, Indonesia, Japan, parts of the US, etc. etc. can't take a drastic increase in sea level. Those places are already on the water. The ice shelves keep the glaciers in place. Warm up the water and they become brittle and break apart and eventually float out into open sea and melt further in the sun which leads to higher sea levels. For your example, fill a cup with water to the top and add ice. It probably won't overflow with the added ice. Take those same ice cubes and put them in another cup until they melt. Then add that second cup of melted ice/water to the first cup and it probably will overflow this time. This is what's happening.