There were no scientists around circa 11,000 years ago, when the great Laurentide Ice Sheet, which once covered much of present day North America, began to collapse in numerous stages and eventually dwindled into a collection of much smaller ice caps across Alaska and Canada, raising seas by tens of feet in the process. So no one could observe all of the processes that, as the Northern Hemisphere warmed, led to its undoing.
Today, though, scientists studying the ice sheet covering Greenland – which survived when the Laurentide Ice Sheet didn’t – are watching something that may be highly analogous. And this time, they’re taking measurements.
As a result, what is coming into focus is that there appears to be a crucial interaction between ice melting on an ice sheet’s surface, forming into pools and lakes, and ice falling directly into the ocean where glaciers, extending out from the ice sheet’s center, terminate in often extremely deep waters. But precisely how they work together – and how much they could speed Greenland’s melt – is only beginning to reveal itself.
Two recent studies in Geophysical Research Letters each home in on different aspects of this linkage. And they do so by studying two apparently connected phenomena: The formation of sometimes vast lakes of meltwater on the ice sheet’s surface, and the release of huge “plumes” of meltwater beneath outlet glaciers that themselves are mostly submerged in the ocean.
One of the most remarkable features of Greenland’s melting – and this is probably true of all ice sheets – is the formation of what scientists term “supraglacial lakes” on its surface. In Greenland, these lakes can be very large – with an area of 2 square miles in one case – and can also suddenly disappear, draining down into the depths of the ice sheet below.
That’s very bad, because researchers suspect that this drainage has what they often refer to as a lubricating effect on the bottom of the ice sheet, helping it slide toward the sea. But Greenland’s future should include many more of these kinds of lakes, say Ádám Ignéczi of the University of Sheffield in the United Kingdom and a group of colleagues at British and Belgian universities in one of the studies.
Taking inventories and measurements of depressions in the current ice sheet where supraglacial lakes could potentially form, and then using a climate model to project future temperatures, the researchers found that the number of these lakes could expand greatly and that the volume of water they contain could grow by between 113 and 174 percent by the end of the century, depending upon the amount of greenhouse gases humans emit. By that time, the lakes could potentially hold between 10 billion and 12.5 billion tons of water.
“In the future, with warming, the meltwater volume could increase, and that can cause lakes to form,” Ignéczi said.
Moreover, the greatest potential for the lakes to grow is not in the southwestern region of Greenland, where temperatures are warmer and lakes are already abundant, but rather, in the ice sheet’s remote northeast, which will melt more and more as the years advance. The northeast ice stream of Greenland drains 12 percent of all of the ice sheet’s ice through two enormous glaciers named Zachariae Isstrom and Nioghalvfjerdsfjorden. Greenland would contribute 1.1 meters to total sea level rise if these glaciers were lost. And this broad sector of Greenland is expected to experience a 191 to 320 percent growth in surface lakes by 2100, the study finds, depending on the emissions scenario.
That’s bad enough, but a key issue is what happens when lakes drain – which some percentage do, sometimes quite rapidly – and where the water goes. And here we turn to the study of Greenland’s underwater “plumes,” taken up in a second paper by Dustin Carroll of the University of Oregon and his colleagues based at several universities in the United States, the United Kingdom and the Netherlands.
These plumes are huge pulses of meltwater that originate at Greenland’s surface but somehow make their way below the ice sheet and ultimately exit deep underwater at the bases of marine-terminating, or “tidewater,” glaciers. That’s right: For at least some percentage of Greenland’s surface lakes, when they drain the water ends up exiting the ice sheet not only far away from where it entered at the surface but also deep below the ocean surface.
We can’t easily observe where these plumes come out, because it is often hundreds of meters deep in waters that are dangerously cold – but the image at the top of this article captures a close analogue from a glacier that is perched on the rocky side of a fjord and doesn’t extend down into the sea. In essence, a vast flow of water has hollowed out a space beneath the glacier and enters the ocean as a waterfall.
According to Carroll, though, this is a small side-glacier in a fjord containing a much deeper and more vast main glacier. In other words, under water, the flow of fresh water could be more massive than this.
The key aspect of these underwater plumes, though, is how they behave – which is not dissimilar from other types of plumes with which we have more experience.
“Almost everybody knows from looking at plumes – you walk outside, you see a smokestack or fire – is that they tend to get larger as they rise; they tend to grown and spread,” Carroll said. “And that’s because as the plume rises, it’s mixing more and more of the surrounding air and water into it. The same thing happens at the base of these glaciers in Greenland. As the plumes rise, they incorporate more and more of this ocean water into the plume. It’s bringing in all that warm ocean water.” (In deep Greenland fjords, the warmest and saltiest water is found at the greatest depths.)
Carroll’s study finds that for really large and deep glaciers, whose ice fronts may extend close to a kilometer deep into the water, plumes can emerge very deep and melt the glacier base, causing it to become unstable and retreat. For shallower glaciers, though, rather than undercutting a glacier, plumes tend to have the opposite role, undermining the ice on top, rather than at the bottom.
This is bad news, because it means that the biggest, deepest glaciers get it the worst. “This kind of erosion at depth, this undercutting, could really lead to substantial retreat in the right conditions,” Carroll said.
“I think this is a potential feedback,” he continued. “The more melt we have on the Greenland ice sheet, the more water drains down to the bed, the plumes are more vigorous, and they’re going to draw in more ocean water and transport heat to the ice. This is a direct ocean feedback that’s really going to amplify as there’s more melting on the ice sheet.”
Scientists continue to learn about how melting on Greenland’s surface can also cause it to melt from below. The question now is whether, after taking all of this into account, they are able to predict a significantly faster rate of ice loss during this century – which, if they do, would have consequences for coasts around the globe.