If Greenland goes, it is becoming clear that it won’t go quietly.
Scientists have already documented entire meltwater lakes vanishing in a matter of hours atop the vast Greenland ice sheet, as huge crevasses open beneath them. And now, they’ve cast light on the mechanisms behind another dramatic geophysical effect brought on by the rumbling and melting of this mass of often mile-thick ice: earthquakes.
In a new paper in the journal “Science”, a team of researchers from Swansea University in the Britain, the Lamont-Doherty Earth Observatory at Columbia University and several other institutions explains how the loss of Greenland’s ice can generate glacial earthquakes. In brief: when vast icebergs break off at the end of tidal glaciers, they tumble in the water and jam the glaciers themselves backwards. The result is a seismic event detectable across the Earth.
“These are all around magnitude 4.6 to 5.2, they’re all pretty close to magnitude 5,” says Meredith Nettles of the Lamont-Doherty Earth Observatory, a co-author of the study. “Which is a pretty big earthquake.”
Granted, these earthquakes aren’t caused by faults — they’re caused by massive movements of ice and how those impact the ground beneath. Compared with the early 1990s, Nettles says, scientists are now measuring seven times as many of these glacial earthquakes coming from Greenland — the rate has shot up as the ice sheet has begun to lose more mass from the calving of icebergs at the front end of glaciers.
To understand the dynamics behind how these glacial earthquakes are happening, the researchers put GPS instruments atop Greenland’s fast moving Helheim Glacier, which is located in the southeast part of Greenland, across the Denmark strait from Iceland. They also monitored the glacier’s calving front, where it meets the water, by camera, and used global seismic data to track earthquake occurrences.
To get a better sense of what they discovered, you first have to wrap your mind around how big these calving icebergs actually are. The amount of ice mass that breaks off in large iceberg calvings from Helheim Glacier, explains Nettles, is around a gigatonne, or a billion metric tonnes. “If you took the whole National Mall, and covered it up with ice, to a height about four times as high as the [Washington] monument”, says Nettles, you’d have about a gigatonne of ice. “All the way down from the Capitol steps to the Lincoln Memorial.”
Measured in space rather than mass, a big iceberg breaking off Helheim can be 4 kilometres in length. So maybe it is no surprise that a body this large can shake the Earth when it moves — and especially when it throws its weight against another solid object, as occurs during iceberg calving.
The iceberg, when it breaks off from the glacier front, is tall and relatively thin (compared with the glacier, anyway) and in effect “standing” up vertically. If you could see a cross section of what was happening when it breaks off, it might look as if cutting a fairly thick slice from a loaf of bread. But as soon as the iceberg is detached, it quickly begins to capsize.
As that happens, its top (above water) falls and pushes back against the glacier, even as its bottom (below water) rises up to the surface, eventually leaving the iceberg floating flat in the water. Or at least, that’s the usual process.
During this capsizing, two massive forces are generated. The first and most obvious one occurs as the tumbling top of the iceberg pushes against the glacier, and actually drives the sheet of ice backwards and inland. “During the earthquakes, the region near the calving front shows a dramatic reversal of flow, moving up-glacier for several minutes while simultaneously moving downward,” the authors write.
“The horizontal and vertical motion then rebound rapidly,” they continue. But in the meantime, it’s enough to shake the Earth. “That calving iceberg is pushing the remaining part of the glacier backwards hard enough that we can actually measure that, hard enough that it reverses the whole flow of the front of the glacier temporarily,” says Nettles. “And it’s that force pushing on the remaining glacier and the rocks beneath it that gives us the seismic [activity].”
There’s also a second force involved. As the iceberg separates from the glacier, that opens up a gap for water to rush into. The region has lower water pressure, so that lessens the push of water and ice downward on the Earth. And this results in “an upward force acting on the solid Earth, as observed in our seismic analysis”, note the researchers.
As if all of this not enough, the events also create a “big tsunami”, says Nettles. This occurs as the tumbling and rolling iceberg pushes water outwards through the fjord leading up to the glacier. “The tsunami is caused because the iceberg has to move a lot of water out of the way as it tips over,” Nettles explains.
Setting aside the drama of tsunamis and glacial earthquakes, what’s most fundamental is how all of this contributes to rising seas — because of course, it is happening over and over again. Over 55 days in the late summer of 2013, the researchers observed “ten large calving events and corresponding earthquakes” — and a total retreat of Helheim Glacier by 1.5 kilometres. And Helheim is just one of many Greenland glaciers that are losing ice.
Each gigatonne-sized iceberg would be equivalent to roughly a quarter of a per cent of Greenland’s estimated 378-gigatonne annual ice loss. It takes 360 gigatonnes to raise the global sea level by a millimetre, so Greenland is doing that roughly annually, as icebergs fall into the sea and glaciers retreat further. (In all, the ice sheet contains enough water to raise global sea levels by 6 metres.)
The loss of ice occurs both through iceberg calving — which explains almost half of Greenland’s total loss, according to the new study — and mechanisms such as simple drainage. Here, as meltwater atop the ice sheet finds ways of flowing down to its base (sometimes through sudden lake drainage), it then makes its way to the ocean. But both mechanisms have their explosive elements — earthquakes, tsunamis, lake vanishings — albeit in very different ways.
“The earthquakes are not themselves destabilising the ice sheet,” says Nettles, “but they are a marker of the fact that the ice sheet is getting smaller and retreating.”
And that’s where the admittedly tiny piece of good news from all of this comes in. Because these earthquakes are so big, and detectable anywhere with seismic equipment, they can actually be used to track how much ice Greenland is losing. They’re like the pulse of ice loss. So because Greenland will not go quietly, at least we will know how fast it is leaving us.