What would the earth look like if the both polar caps melted?

Answer 1

Since most of the northern polar cap is already floating, this would cause little effect. But if the Greenland caps melt, as appears to be happening, then this will contribute to sea level rise. But this will be partly counterbalanced in the long run by isostatic rebound of that landmass.

But as rock is several times as massive as ice, there will accordingly be a lesser volume of land rise than that mass of ice that melted. [Archimedes strikes again!] The isostatic rebound would take centuries - Scandinavia is still rising in response to melting at the end of the last ice-age .

The southern polar cap is mainly above the land mass of Antarctica, and could contribute as much, in an extreme position, as a 4% rise in sea level. (~160m)

But since the seas would flow over low lying land, the maximum credible sea level rise would be less than one half of that. The Antarctic Continent would also rebound, but once again, the time scale is centuries.

With the removal of the ice mass, and the rebound of the landmass, a greater and different space would be available for the magma, and the possibility of volcanic action is non-zero.