Last time CO2 levels were this high ....

... was 5.3 to 2.6 million years ago, and there were trees in Antarctica. Sea levels were 20 metres (66 feet) higher than they are now, the world as a whole was 3-4 degrees C warmer, the south pole was 20 degrees C warmer, there was no Greenland ice sheet and no West Antarctic ice sheet.

Leaves of the extinct southern beech (Nothofagus beardmorensi) found at Oliver Bluffs, in the Transantarctic mountains, Antarctica. Photograph: J Francis, A Ashworth
Source: The Guardian

You can read The Guardian's whole piece here.

Of course, it will take centuries for the ice caps to melt.  But 66 feet!  And temperatures don't rise immediately—they'll also take time to respond to the forcing caused by greenhouse gases.  The worst of that is that even when we stabilise greenhouse gas levels in the atmosphere, temperatures will go on rising.  And when we stabilise temperatures, the sea level will go on rising.