The danger of global warming is not rising sea levels. I mean, that's not good but it's not going to just flood overnight. We're not in any immediate danger of that. The more immediate danger is that with the poles melting and the freshwater ice runoff flowing into the oceans, the salinity content of the ocean will become more diluted. Even a 1% change in salinity content could be pretty catastrophic.

The salinity content has a direct bearing on earth's ability to regulate its temperatures. Normally, the winds sweeping over the icecap on Greenland chills the waters off Norway--a place called Lofoten. That causes the seawater to want to freeze but it can't because it's holding so much salt. So the salt begins to leech out of the freezing seawater and falls to the bottom of the ocean in what are called "chimneys" taking a great deal of chilly water down with it.

This very salty water at the bottom moves southward hugging the Eastern Americas due to the earth's rotation. When it gets to Antarctica, it joins more heavy, cold, salty water there and it all flows into a huge faultline in the ocean floor. The earth's rotation propels this water through this seam or trough that takes the water to the Pacific and Indian Oceans where it surfaces. This cools the waters of the tropical regions and prevents the tropics from overheating. It flows around Africa and works its way North until it hits the Arctic waters near Lofoten. Now the water is warm because it came from the balmier southern climes. Because it's warmer, it retains its salinity quite well until the winds sweeping over the Greenland icecap chill it and cause its salt to leech out and the whole cycle starts again.

Lowered salinity levels in the seawater due to polar melting causes smaller, weaker chimneys to sink and not nearly as much cold, heavy, salty water is moving south and hence the tropics won't be cooled sufficiently causing the waters to heat up which will kill a great deal of ocean life.

Couple this with the fact that Greenland's icecap is melting, the waters off Lofoten are not being sufficiently chilled to form chimneys anyway. So the Arctic waters heat up but don't get chilled anymore and that overheats the Arctic. That also destroys a great deal of marine life.

Ultimately, the earth will survive this change but life will be dramatically altered and there is no guarantee that humans will survive...or want to.