This summer there was a record melt of the Greenland ice sheet. Conditions were so extreme that there has been such a warm summer once every 150 years during the last 4000 years recorded in the Greenland ice core record. Not only was there unusually warm air coming up from the south but there was much low cloud. Though cloud reflects light the cloud over Greenland during the summer was thin enough to allow the radiation of the sun to pass through but critically prevented the heat radiating from the ice shet escaping, trapping it and so adding to the thaw.The Greenland ice cap contains 3 million cubic metres of ice, enough to raise the surface of the oceans by 7.4 metres if it all melted.
Clearly most major cities would be totally submerged. The timescale of this melting if the warm weather continues is not possible to predict.
Though the overall temperature is following the lower edge of the envelope predicted by the current climatic models the models are far from complete and cannot be relied on to provide accurate predictions.