The high priest of the hotheads is NASA’s James Hansen, who preaches that, unless we dial back to 350ppm, we will lose, within a hundred years’ time, the vast majority of Greenland’s ice, which will raise sea levels about 20 feet. Hansen has testified that he thinks this could happen within a hundred years.
The hothead theory is that the ice on that gigantic island is much less stable than previously thought, and that with a tad more warming, lakes will form in the summer, drain thousands of feet down to the bedrock, and lubricate the flow to the ocean. It quickly melts, submerging a lot of Florida and Manhattan. The Washington Monument becomes an island.
Greenland has 10,000 feet of glacial ice above sea level, which is about 1/4 of the density of the granite underneath it. In order to maintain isostatic equilibrium, the ground underneath the ice sheet must be depressed about 2,500 feet in the center. In other words, the land surface of Greenland must be somewhat bowl shaped.
Additionally, Greenland has large mountain ranges which block the movement of deeper ice. The point being that the ice sheet can not slide off into the ocean as a block – because things don’t slide uphill and they don’t slide through mountain ranges.
Greenland’s glaciers move towards the ocean because snow builds up in the interior (above the mountain tops) and the excess weight causes the ice to flatten and spread outwards.
Hansen is often described by the press as the world’s greatest climatologist.