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.

Then Greenland might not be so good as real estate investment.

I think it also illustrates the general problem here. The most valuable real estate will be the one with sufficient rain fall (but not to much) close to the sea (but not submerged) and so on. And that is a question of just how the climate patterns will change, which we do not know.

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Except we are talking about a 100% certainty of doom for the whole icesheet.

A more reasonable threshold would be 50% chances of losing around a third. Because that would be enough to be a disaster. Actually 10% chances would already be rather a lot.

Earth provides enough to satisfy every man's need, but not every man's greed. Gandhi

So raising the sea wall another 5-7 meters would be no big problem?  How high are they now?  6C is not an unreasonable average ambient increase, given the collective ability of our species to respond to what are perceived as distant threats.  I understand that 6C is what we can expect unless current actual trends are significantly altered, and these trends have not yet really begun to be altered, especially outside of Europe.

Even if we reach 6C there is no certainty as to how long it will take for the ice cap to melt.  But there is also no certainty that the ice cap will not have already melted by then.  And then large scale ice melt from Greenland could conceivably alter the deep saline current and the Gulf Stream.  This could cause ice sheets to begin accumulating in northern Europe.  It might slow down the melting of Greenland, but it would hardly be an advantage to northern Europe.

There is only really one way to verify if the models are accurate and we don't want to go there.  To me the prudent thing is to assume a worst case and work to avoid it.  At worst we will have given people employment doing things that turned out not to have been necessary, but the world as we know it will have survived substantially.  But that is not how finance operates and, as we know, finance runs the world.

"It is not necessary to have hope in order to persevere."