Why does ice float?
What other observations or facts are known?
Because ice floats, we can infer that ice must be less dense than water.
Soo if it gets warm and ice melts there will be less ocean levels and Miami NYC Boston loose beach front.
Ice does float but the ice these dire predictions are talking about is the 99% of frozen fresh water in two places, Greenland and Antartica. This is all on land masses and if it melted the seas would rise.
It would almost all have to melt for the oceans to rise that much, which is very unlikely.
Anyway the Dutch have handled it and given time we could too.
First, I am not claiming man-made global warming is happening. The earth has warmed and cooled many times with ice-ages and corresponding rises and falls in the sea level.
But to your point of ice being less dense so melting lowers sea levels, that is not the complete picture. Much of the world's ice exists as glaciers on land. Melting those places has more water running into the ocean, such as Greenland, Antarctica, etc.