Just two weeks ago, we learned from twin scientific publications that the massive ice sheet of West Antarctica, which could cause over 10 feet of sea level rise, may be less stable than previously thought.
And now, this week, two studies suggest virtually the same thing about the still more massive ice sheet of Greenland, which, if it were to melt entirely, could raise global sea levels by as much as 23 feet -- an outcome that, while it surely would not happen in our lifetimes, would dramatically reshape the world's coastlines.
The reason for the increased worry is similar in both cases.
The researchers suggest that prior computer modeling efforts – including estimates from the United Nations’ Intergovernmental Panel on Climate Change – may be just too conservative. They may not capture the full dynamics of what is happening with these ice sheets in a warming world.
The Greenland ice sheet – the only one of its kind in the northern hemisphere -- is 656,000 square miles in extent and has an average thickness of over a mile. In places, though, it stretches as high as two miles into the air, its icy peaks reaching up to rarefied regions of freezing atmospheric temperatures.
But the ice sheet also spreads across the huge landmass underneath it, and slopes down to a large number of sea-front outlet glaciers -- 242 of which are at least 1.5 kilometers (or just under a mile) wide. Calving at these glaciers releases ice into the sea, where it can subsequently melt. Meanwhile, after each iceberg calves, new ice from the ice sheet flows in to fill the void.
So how fast is this process happening – and what does that mean for sea level around the world?
The first new study, just out in the Proceedings of the National Academy of Sciences by scientists at the University at Buffalo and several other institutions, uses a technology called laser altimetry to measure almost 100,000 elevation points atop the Greenland ice sheet, so as to determine how much their altitude decreased between 1993 and 2012. The resulting estimate is that the ice sheet is now losing about 243 gigatons of ice per year (a gigaton is equivalent to a billion metric tons), which works out to about .68 millimeters of annual sea level rise. (The shrinkage of ice was particularly dramatic in southeastern Greenland.)
That’s even more ice than West Antarctica is estimated to be currently losing -- a recent study put the melting from this ice sheet at about .29 millimeters per year -- and the research calls into question how computer models have, until now, attempted to understand the dynamics of Greenland’s ice. Those models have projected forward on the basis of the behavior of 4 individual glaciers, according to the researchers, but the new study says those aren’t necessarily representative. And that’s very concerning because the United Nations’ Intergovernmental Panel on Climate Change used that approach to estimate future sea level rise from Greenland by 2100, coming up with a range of between 14 and 85 millimeters -- roughly from half and inch to nearly 3 ½ inches.
“It is very likely that the IPCC projections are on the low side when it comes to mass loss associated with ice dynamics,” said Cornelis van der Veen, a professor of geography at the University of Kansas and a contributor to the report.
And then there’s the second study, out today in Nature Climate Change from another scientific group led by Amber Leeson from the University of Leeds in the UK. This research examined so-called “supraglacial lakes,” which form atop the Greenland ice sheet, and then sometimes suddenly vanish through “moulins,” or crevasses that plunge to depths far below the ice sheet.
You can see the lakes in this satellite image of Greenland:
These lakes are concerning for several reasons. First, in color they are darker than the surrounding ice, which means that they retain more heat, and can cause further warming. And when they drain through moulins, this warmer water has the potential to speed up the movement of the ice sheet by lubricating its base, deep below the surface.
The study finds that these lakes atop the Greenland ice sheet will become more extensive as global warming advances, and will appear in areas of higher elevation -- and if they drain, they could further enhance Greenland’s total ice loss.
IPCC scientists concluded in 2007 that this type of melting could add another 18 centimeters -- roughly seven inches -- to global sea level rise by the end of the century, but a later panel disputed the finding, according to Amber Leeson, the lead author on the study and a geophysical modeler at the University of Leeds. The new research suggests that the 2007 forecast was accurate after all, she said.
"Essentially this means that the  prediction of an additional 18 cm could be back on the table,” Leeson said.
The amount of sea level rise that would occur from a major loss of Greenland -- especially in combination with West Antarctica -- is pretty unfathomable. According to Benjamin Strauss of Climate Central, 12.8 million Americans live on land that is less than 10 feet above local high tide.
Joby Warrick contributed to this report.