4. Ice sheet melt feedback and sea level rise
This section explains the severe complacency that has infested the IPCC about the major risks of the rapid melting of the Greenland ice sheet and how the feedbacks went unnoticed and are accelerating.
All the glacier systems in the world are in retreat due to global warming. The Greenland ice sheet is 2-3 km thick, containing enough ice to raise sea level by seven metres if it melted. It used to be frozen solidly all year round, apart from a small amount of melt around the edges.
In the 1980s the top of the ice sheet started melting in summer, and by 2012, almost all the surface melted in high summer. Climate modellers at IPCC weren't worried though, as they expected the meltwater to refreeze in winter. They did not realise that this surface melt could turn the entire sheet into a fragile sliding lattice, with potential for much more sudden collapse.
A new surprising phenomenon appeared: drain holes that take water from the ice surface ponds down to bedrock 3 km below. These holes, known as moulins, had not been imagined before the ice started to melt. They increase the heat at all depths of the ice sheet, weakening the entire glacial structure by expanding tiny cracks. Like in a granite boulder, when the water melts and refreezes it gradually makes the whole glacier more fragile. The moulin holes then drain to the sea, lubricating the glacier on its rocky underside so it often flows twice as fast. The accelerating shrinking of the ice sheet is measured by satellites, which showed annual loss estimated at about 300 gigatonnes of ice from Greenland until 2016. More recent data is at
https://www.carbonbrief.org/guest-post- ... ed-in-2020
These unforeseen rapid melting processes in Greenland show how models that project a linear rate of sea level rise are wrong, since the warming processes create new instabilities that speed up the whole melting process.
Wadhams says the Antarctic ice sheet, holding enough water for 60 metres of sea level rise, was estimated to be losing only about a third as much ice as Greenland. But more recent studies measure how the Antarctic ice sheets are melting from both above and below. Like in Greenland, meltwater is seeping into the ice and causing it to fracture and weaken when it freezes again. But unlike Greenland, Antarctica has vast ice glacial shelves jutting over the ocean, and the ocean currents are warming, accelerating the speed of loss of these glaciers, with chunks breaking off as big as US states.
The IPCC has been complacent, ignoring the scale of the melting threat, leading many cities to fail to plan for the expected speed of sea level rise. Countries like Bangladesh and Vietnam have millions of people living in areas that are expected to flood this century and that will be very difficult and expensive to protect.
Working out how to stop Greenland from melting should be a primary planetary security priority, given the global danger of sea level rise, but instead the political focus is on speeding up the melt to encourage trade and mining. When the Arctic Ocean is blue water rather than white ice, the whole regional climate warms up, speeding the loss of ice from fresh water glaciers as well as from the salt water sea ice.