How do we know this?

In the 1960's the US government started a massive secret project in Greenland called camp century in which the military was digging ice tunnels to hide over 600 nuclear missiles.

The base never worked out. But while failing at tunneling through ice thousands of feet deep they also tunneled down until they hit dirt and then continued another 12 feet and got themselves a core sample .

This core sample went to Boston university where it was transfered into small jars and some of the jars were then sent to Denmark. The jars laid there in freezers all these years until someone finally got to analyzing them .

They found sticks and leaves and mosses with dating them to 50k years ago. Which means 50k years ago there was no ice in Greenland.

So, now we know that the ice will melt completely since there is no signs that the earth will cool . But we still do not know how fast.. more core samples from different areas of the ice sheet need to be drilled for and analyzed then we will know.

What we do know is that currently the melting ice sheet has caused a 1+ foot rise in sea level. And when the whole sheet is gone it will be 20+ feet  

If this doesn't wake up politicians in coastal areas nothing will until it's too late.

“We use the past to try to understand the future and understand the present,” Bierman continues. “And that makes the future a little frightening. Not that we should run from it—but to me, it's a call for action.”