Researchers have discovered the source of major seismic activity taking place around Greenland: Massive icebergs calving off the Greenland ice sheet and smashing back against it.
“Imagine that you could go and just push on the front of the glacier with your thumb, really hard,” Meredith Nettles of the Lamont-Doherty Earth Observatory at Columbia University told NPR, “so hard that you could reverse the direction that the front of the glacier is moving, and then you let it go. And that backward and then forward motion is actually recorded in the GPS data from the front of the glacier.”
Nettles co-authored a paper on the phenomenon that was published Thursday in the journal Science. Nettles and her fellow researchers were looking to discover what was causing the glacial earthquakes recorded around the region. Using GPS sensors placed on the ice, they were able to track 10 large calving events that coincided with the quakes.
“These are all around magnitude 4.6 to 5.2, they’re all pretty close to magnitude 5,” said Nettles. “Which is a pretty big earthquake.”
Greenland has one of only two ice sheets in the world (in effect, continent-sized glaciers). Together, the Antarctic and Greenland ice sheets contain about 99 percent of the world’s freshwater ice. Alone, the Greenland ice sheet (656,000 square miles) could raise the sea level by 20 feet if it completely melted. The Antarctic ice sheet (7.2 million cubic miles) could raise it 200 feet. Fortunately, even with global warming, such an event would take thousands of years.
Unfortunately, both sheets are currently melting at an “unprecedented rate.”
Scientists have already documented meltwater lakes forming atop Greenland’s ice sheets and vanishing in hours – a result of crevasses opening beneath them. Along with pollution darkening Greenland’s ice, this meltwater runoff is lowering the glaciers’ ability to reflect sunlight and increasing its melt rate. The volume of ice lost in Greenland has roughly doubled since 2009, which researchers believe is linked to global warming.
In an interview with The Washington Post, Nettles said that the rate of glacial earthquakes has increased by a magnitude of seven since the early 1990s. The quakes occur due to the enormous size of the icebergs that break away.
“We’re talking about something that is a gigaton of ice,” said Nettles, or one billion tons of ice. “That’s sort of the size of an ice cube you would have if you filled up Central Park in New York City to the top of the Empire State Building.”
Such an ice cube would be over two miles in length. When one detaches from the ice sheet, it “stands up” vertically and then slowly capsizes against the glacier. The glacier is also moving during this time (very slowly), but when the broken iceberg smashes against it, it temporarily reverses its flow. “[I]t’s that force pushing on the remaining glacier and the rocks beneath it that gives us the seismic [activity],” explained Nettles.
The event is accompanied by a tsunami that pushes water back up the fjord that leads to the glacier. “The iceberg has to move a lot of water out of the way as it tips over,” said Nettles.
What This Means for the Future
In her interview with the WP, Dr. Nettles explained that the increased rate of glacial earthquakes is not destabilizing Greenland’s ice sheet, “but they are a marker of the fact that the ice sheet is getting smaller and retreating.”
As the icebergs fall off the sheet, they enter the ocean, contributing minutely to a rise in sea level. At one gigaton apiece, these icebergs equal less than a percent of the estimated 378 gigatons of the sheet’s yearly ice loss. Yet it takes only 360 gigatons of ice to raise the sea level by a millimeter, and Antarctica is contributing its share as well. Between 2009 and 2012, just three Antarctic glaciers shed 204 gigatons per year – shifting the weight of the continent enough to slightly alter the Earth’s gravity.
Taken together with a recent NASA analysis that the West Antarctic ice sheet may completely disappear in the next few years, the increased rate of Greenland calvings could have a substantial impact on sea level rise by the end of this century.
Experts have warned that current greenhouse gas emissions are putting the planet on track to warm between two and four degrees Fahrenheit by 2100. As Professor Anders Levermann from the Potsdam Institute for Climate Change Research explained in 2014, “At one degree, we are already experiencing damages. Sea level rise in the long term…is somewhere in the vicinity of two meters. That puts cities like New York, Calcutta and Shanghai in difficult positions, and they need to protect themselves.”
A report from the Union of Concerned Scientists similarly warned that, by 2030, rising tides will likely cause 24 tidal floods per year for several communities along the U.S. east coast.
The economic damage of sea level rise, coupled with other climate change factors, could rack up a total of $1.1 trillion in damage by 2100, according to an assessment produced by the Environmental Protection Agency, the University of New Hampshire and others.