Annie Snider/MEDILL

Still photos courtesy of Meredith Nettles

As Arctic sea temperatures warm, glacial earthquakes offer important insight about climate changes, says seismologist Meredith Nettles.


Seismic messages of global warming

by Annie Snider
Oct 02, 2009

Helheimcalvingfrong


Courtesy of Meredith Nettles.

Ice breaking off the front of glaciers can cause earthquakes that register on the global seismic scale. Climate scientists have installed monitoring equipment on several Greenlandic glaciers, including Helheim Glacier, pictured here. The increasing number of earthquakes point to climate change.

glaciercamp

Courtesy of Meredith Nettles

The giant ice field dwarfs a research camp at Kangerdlugssuaq Glacier.

Seismologist Meredith Nettles’ office is just north of New York City, but her mind is more often on Greenland where she spends part of the year doing field research.

As glacial ice melted to its seasonal minimum this year, she watched, not just with satellites and GPS data, but also with seismic readings.

Nettles’ specialty is earthquakes caused by large chunks of ice breaking off glaciers and crashing into the sea. Climate scientists say these earthquakes offer information about glacial melting that could dramatically change earth’s climate.

“The earthquakes essentially give us a way to monitor the health of glaciers in Greenland,” says Nettles, assistant professor of Earth and Environmental Sciences at Columbia University’s Lamont-Doherty Earth Observatory. “They give us a very valuable tool for monitoring and understanding the ice sheet and its future.”

It’s a future of intense interest to climate scientists, who warn that the melting of the ice sheet would bring rising sea levels and coastal flooding. It could also impact the global ocean circulatory system that makes our climate habitable for human beings over much of the globe.

Scientists monitored a six-fold increase in earthquakes from Greenland between 1993 and 2005. Nettles says the simultaneous changes happening in many glaciers around Greenland are a good indication that these glacial retreats are linked to the larger scale process of climate change.

Nettles’ work with glacial earthquakes, or icequakes, began six years ago when she and colleagues discovered a new class of earthquakes occurring mostly in Greenland. Despite their intensity, the earthquakes had been hidden because they happen at a very slow speed. Whereas a magnitude five earthquake along a fault takes about two seconds to happen, the Arctic earthquakes occur over as much as 60 seconds.

Intrigued by the phenomenon, Nettles and a handful of other climate scientists headed to Greenland to install GPS equipment that would give them a closer look. It is dangerous and expensive work. Glacial ice in these areas moves at a speed of about 3 feet an hour, and the boxes must be installed at points generally only accessible by helicopter.

Over the years, Nettles’ team has perfected the process for installing the equipment, which involves drilling a 9-foot hole in the ice and connecting a box of monitoring equipment. When her team placed 14 GPS monitors on Helheim Glacier this summer, they could install one in less than 10 minutes.

This monitoring equipment isthe secret to how the scientists discovered that the earthquakes were being caused by enormous blocks of ice breaking off the front of glaciers and falling into the water far below in the process called “calving.” They also found that the flow of the glacier immediately increases after a large-scale calving event.

Nettles says understanding how this happens could provide critical information about major global changes in climate.

“If we understand how fast we can put more ice from the ice sheet into the world’s oceans, it has an important effect on ocean circulation, sea-level rise, and many other inter-related aspects of the climate-ocean system,” she says.

According to Nettles, the scientific community is now starting to see a “clear link” between glacial retreats marked by earthquakes and two key elements of climate change: ocean temperature and ocean circulation.

“We see a link, basically, through the ability of that warmer water to erode sea ice and floating ice mélange in front of the glaciers that otherwise helps restrict calving," Nettles says. That ice in front of the glacier had been providing resistance, slowing the forward movement of the glacier that causes more ice to break off into the ocean.

“Greenland is a very sensitive responder to changes in climate,” Nettles says. Although scientists aren’t currently finding similar changes in Antarctica, she says the process there would be the same and “could have very significant implications because Antarctica is so big and contains such a large mass of ice.”

As global leaders work toward a political framework for preventing further climate change and adapting to the changes already in the pipeline, far-away Greenland will be front-of-mind.