h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

if the arctic icecap melted, why would sea levels rise??

all that ice has already displaced more water than it contains, thats why it floats??

if it all melted wouldn't sea levels go down??

or not change at all??

just like ice in a as it melts the level stays the same

Because the ice caps on Greenland and even more significantly Antarctica do not float on the water, they are land based and so do not displace any sea water.

What you say is true only about the north polar ice cap.

When floating ice melts, the level of the water stays the same.

Not only is the ice in Antarctica on land, but the weight of it has pushed the land downwards. If the ice melts, the water will flow into the ocean and raise its level, but also the land will rise, and this may displace water as well. So it's hard to estimate exactly how much the sea level will rise by.

There seem to be three main icecaps they're talking about:

Greenland - likely to melt
The Western Antarctic Icecap - likely to melt
The Eastern Antarctic Icecap - not likely to melt

The first two might raise the level of the sea by 5 - 10 feet. The third would raise it by 250 feet, which would wipe out most of the major cities of the world.

Mexico City would be safe.