It's a guessing game and we're not very good at it. The "game" is trying to work out how quickly the planet's ice fields are melting. In essence, we're guessing how much time we have before the meltwater hits our shores.
The latest guesstimate to fall is the Greenland ice sheet. It's melting faster than we anticipated. What is it this time? It's these things:
See those dark spots on the surface of the ice sheet? They're called "supraglacial lakes." They're surface meltwater and, as the Arctic continues to warm, they're expected to grow both in numbers and in size. They would be a lot bigger already except that many of them manage to drain down through the ice sheet where the rivers of meltwater lubricate the ice above and speed its flow to the sea.
As those lakes grow and spread they also absorb more solar energy than the ice, accelerating the melt. Here are some images to show how the Greenland ice sheet is decomposing:
No one has an accurate estimate of what this will mean in terms of sea level rise. What is known is that the estimate of 8 inches of rise by 2100 caused by the Greenland ice sheet is way off the mark.