[wideminded]

much of the glacier is already underwater; its melting will have no effect on sea level.

If any of the glacier is resting on the bottom, this is not true. Obviously a sufficiently large piece of ice will overflow a glass of water when it melts.

[DuncanWaring]

I was referring to the melting of the part underwater.

[norton]

Obviously a sufficiently large piece of ice will overflow a glass of water when it melts.

Oh Noes!
Does that mean I'm drinking too fast?

Oh, Yeah...You're wrong.

[Check out how much of that significant ice is below versus how much is above the waterline, then reflect on displacement and on water's expanding when it freezes/contracting when it melts.]