If you take the existing rate of sea level rise of 3.1 mm a year and double it every decade you get to a figure of around 1.7 metres rise which is still not “several metres” by mid-century. So what could it be?
Several things we do know and one is that the loss of ice mass is accelerating and as Hansen said the rate is doubling every decade. The second is that the ice shelves are thinning fast and as an example Pine Island ice shelf, which is still vast and hundreds of metres thick, is now in terminal decline.
With the effect of Global warming the earth’s temperature has raised 0.9C but this is an average of the whole planet and the poles are warming much faster than the average. In some places this is putting more snow on to the continent but around the edges and in West Antarctica in particular the warmer atmosphere is melting the ice from its surface. This has been thinning Larsen C ice shelf and it now has a big crack in it. Larsen A and B have already disintegrated so all eyes are on Larsen C to see what happens.
The amount of water held in ice in West Antarctica is enough to raise sea levels worldwide by six metres, East Antarctica sixty metres and Greenland seven metres so the figures are vast but we only need one metre to cause so much infrastructure and farmland to be flooded that it will cause economic collapse of most countries including the USA, the UK, Holland, China, Bangladesh and many small island nations..