We definitely have to stop before 6 (or 5, or 4) degrees. There are no safe levels of warming, but the effects will be managable at 2-3 degrees if there is no methane trigger at those temperatures.

Even if the Greenland ice sheet would 'irreversibly' melt that would take well over 1000 years.

The local impacts of Greenland ice sheet melting have however been underestimated for the Northern Atlantic coasts. 20 centimeters worldwide in a long-term equilibrium may translate in to two or three times that amount in Europe and ten or twenty times in the North-Eastern US and Canada in the short run.

Thermal expansion, especially the local impact of the expansion of the Arctic Ocean, also remains to be modelled with any kind of precision.