by Anna Bisaro
Oct 24, 2013
Icebergs the size of Manhattan’s Central Park break off of Greenland’s outlying glaciers and trigger earthquakes that can be detected thousands of miles away. About half of the mass lost from Greenland every year is due to these earthquakes.
Seismologist Meredith Nettles of Columbia University monitors the force of the earthquakes and knows they are increasing in number.
While these earthquakes won’t knock any buildings down, they are tearing away at the ice mountains of the glaciers. And, with more recorded every year, that’s cause for alarm, Nettles said. The increasing number of quakes is adding to rising global sea levels, she told top climate scientists gathered at the annual Comber Abrupt Climate Change Conference this month in Wisconsin.
Nettles goes to Greenland year after year for stints of field research that show the increasing glacial calving in Greenland that causes glacial earthquakes.
The earthquakes themselves contribute to the collapse of the glaciers and sea level rise, according to Nettles.
“We had the same number of earthquakes that we have by now last year in early December,” she said. “So we’re more than a month ahead of schedule.”
Nettles said combining her study of earthquakes and climate change happened by accident. A seismologist by training, in 2003 Nettles started to notice seismic activity coming from Greenland, but the signals were not indicative of normal earthquakes.
“We didn’t’ know what they were when we first saw them,” Nettles said of the phenomena she termed glacial earthquakes. “Maybe we would have chosen a different name.”
Traditional earthquakes occur with a massive release of energy as tectonic plates move past each other along a fault. This type of quake can happen in glaciers if walls of the ice move past each other along a fault, and Nettles said these quakes are aptly termed ice quakes.
Nettles described glacial earthquakes to be more like landslides except instead of large rocks falling, they are icebergs breaking off from the edge of an outlet glacier into the ocean. The seismic waves occur when the icebergs break off.
As glaciers move across a calving margin – the point at which the ice breaks– the pressure on the ice becomes too great and an iceberg breaks off. The earthquakes happen when the calving line of an iceberg is close to the grounding line. The grounding line is like a foundation for the glacier, the boundary between the rock underneath the glacier and the ocean. When the retreating glacier’s calving front is close to the grounding line, earthquakes result.
“Those calving lines have retreated closer and closer into Greenland,” Nettles said. “More and more of them are moving across grounding lines and starting to generate earthquakes.”
The icebergs that break off may be the size of a cubic kilometer, or the equivalent of the size of New York City’s Central Park, with the height of the Empire State Building.
“It’s so big that it’s really hard to visualize,” Nettles said.
Most of the icebergs that break off are about that size. A cubic kilometer equates to a giga-ton, or one billion tons of water.
Traditional earthquakes have been studied for more than 100 years and Nettles admits the limitations in making conclusions based on her research because glacial earthquakes have not been studied for very long.
“We have a basic idea of how it works, but we’re really far from having the understanding that we have of those [tectonic] earthquakes,” Nettles said.
Once Nettles and her team figured out where the seismic waves came from, their mission shifted to figuring out the effects of the quakes on the glaciers themselves, and then the greater implications for the planet.
SEA LEVELS RISING
The latest U.N. Intergovernmental Panel on Climate Change report estimates global sea levels will rise up to 3.3 feet by 2100 with high carbon fuel emission levels, more than 50 percent higher than the estimate in the previous report. That’s enough to swamp coastal regions and island countries. Sea levels could rise about 1 to 2 feet even with significant emisssions reductions, the report estimates. But the projections of sea level by the IPCC are actually very uncertain, according to Nettles. Part of the uncertainty comes from not knowing how many icebergs will break from the glaciers in Greenland, Nettles said. These icebergs make large contributions to sea level rise.
“Everything we have done in society now has really been optimized for a very particular climate and a very particular sea level,” Nettles said. “Even if you don’t live on the coasts, there will be an impact in your life from changing sea level.”
Nettles explained the importance acknowledged that a drop in sea level would present its own suite of problems as well.
“There’s a lot of ice there [Greenland]. Understanding how fast that ice can come out of the ice sheet and go into the ocean is important,” Nettles said. About half of the ice lost from Greenland is from these earthquakes. “In other ways these earthquakes are really nice. No one feels them, they don’t knock any buildings down.”
From 2003 to 2006 Nettles and her team collected data on these glacial earthquakes. By 2005, they started to notice a great increase in the number of quakes every year as well as an increase in the speed of the glaciers experiencing the quakes.
Because of the trends seen in the data, Nettles felt a great sense of urgency in finding out more about what was happening with these glacial earthquakes in Greenland and quickly applied for grants to be able to start more closely monitored research and fieldwork on the glaciers.
The five years of data collected from 2006 to 2010 has been analyzed and used to attempt to understand the affects of the glacial earthquakes on the glaciers themselves.
As the iceberg accelerates and breaks off, it triggers a reaction force on the glacier. The reaction force, in addition to causing seismic waves, also causes the glacier to accelerate in the opposite direction. That force is increasing too because ice loss from the glacier is the greatest contributor to short-term velocity, Nettles said.
To measure the change in velocity of the glaciers, Nettles uses GPS data.
Columbia University geochemist, Wallace Broecker, a climate change expert, asked Nettles during her presentation if the ice cores of the glaciers contribute to heat flow of the glaciers, thus affecting the movement. Nettles responded that she and other scientists believe ice cores do affect the heat flow, but how is beyond their level of understanding right now. The information will be integrated into the final model of the moving and quaking glaciers.
Quantifying the size of glacier that moves or the distance earthquakes is another story. From the seismic stations around the world, Nettles cannot tell if the entire glacier moves a couple meters or if a smaller mass moves a greater distance as a result of the quake.
It is clear that glaciers do speed up as a result of the earthquakes. While the speed that glacier moves as a result of the quake remains consistent there is no direct relationship between the size of the quake and how much faster the glacier will move as a result.
Right now Nettles and her team cannot predict how fast a glacier will move in response to a quake, no matter the size of it. At this point, they can only provide a range of how much faster the glacier will move given the size of the iceberg and glacier.
“It would be simple if you could say the bigger the ice loss the bigger the speed up,” she said. “It is not at all consistent.”
Nettles showed in her presentation how the biggest and smallest speed-ups that happened as a result of glacial earthquakes actually came from the two largest earthquakes, proving that there is no direct correlation between size and speed. Other factors like thickness of the glacier and melting rate.
In addition to the speed of the glacier as a result of the quake, climate scientist Klaus Lackner wanted to know the velocity of the seismic waves in the ice. You can measure velocity of seismic waves in rock, so why not ice, Lackner wondered.
Nettles said that because the ice is only a few kilometers thick – a fact they know from extensive prior research – scientists can only measure the velocity of very high frequency waves in ice.
Since the 1990’s the number of glacial earthquakes in Greenland have been increasing in number according to Nettles, but it is important to keep that increasing number in perspective.
“You want to make sure you don’t have an apparent but artificial increase in the numbers that comes from being better able to detect the earthquakes,” Nettles said. Nettles said it is important to account for bias when looking at older data of the earthquakes that may come from the equipment simply not being as good as it is today.
“Presumably these kinds of earthquakes have been happening all along,” Nettles said. “But the increase in number that we see, in the sense that it is related to the retreat of the ice, and the retreat of the ice is linked to changes in the atmosphere and the oceans, whatever’s happening in the climate system affects what that ice is doing.”
Nettles said warmer ocean water and warmer temperature cause retreat of the glaciers. As the glacier melts it gets thinner and then calves faster, she said. The evidence that the number of glacial earthquakes is increasing due to climate change. But without data from further back than 1980, it is impossible to say with absolute certainty that this phenomena may have occurred anyway.