See Inside October 2008

Researchers hone seismic skills to peer inside glaciers

Seismic data enable scientists to peer inside melting glaciers before they calve

Glaciers at the earth’s poles are melting, calving and surging toward the seas at alarming speeds. With few exceptions, global glaciers have been getting smaller since the early 20th century, according to the National Snow and Ice Data Center in Boulder, Colo. The suspected cause of all this shrinkage, of course, is warming temperatures. The consequences are not surprising: a warmer world could mean melting ice, rising seas and flooded coastlines.

To learn more about what is happening, researchers in the bursting field of glacier seismology are refining techniques to track changes inside the ice in real time. Specifically, they are using seismic instruments to listen to ice movements, like physicians use heart rate monitors to learn about a patient’s health. With such information, these ice doctors could better determine how glaciers are changing over short periods—a sharp contrast to more traditional methods in which glaciologists relied on photographs, satellite images and direct measurements to document large-scale, long-term ice movements.

Glacier seismology exploded onto the scene after 2003 with the surprise discovery of a new class of ice movements in Greenland by Columbia University’s Göran Ekström and Meredith Nettles. The strong seismic signals were recorded across the globe; researchers suggested that they were caused by Greenland’s glacial ice surging forward by as much as 10 meters in less than 60 seconds and that the late summer events had been increasing since at least 2000—clearly showing a link to large-scale climate change. Since then, scientists have watched other small-scale, short-term movements and features inside the ice: the opening of crevasses; calving at the glacier terminus; water surging underneath the ice and into cracks at the bottom; and friction points below the glacier.

Before scientists can use the data to predict ice behavior and climate change, they still need to determine exactly how to interpret glacial seismic records accurately. For instance, what was initially identified in publications as an “ice quake” in Greenland now appears to be a different kind of ice movement. At the June workshop of the Incorporated Research Institutions for Seismology (IRIS) in Stevenson, Wash., Nettles suggested that the apparent ice quakes might not be surges at all but could actually be major ice-calving events.

When a glacier calves, huge pieces of ice—some approaching half a cubic kilometer in size—suddenly break off, transferring large masses of freshwater from land to sea and raising sea level instantly. According to a study in the August 24, 2007, issue of Science, melting and calving of glaciers (as opposed to icebergs or ice sheets) account for more than half of the ice lost to the sea since 1996. Shad O’Neel, a co-author of the paper and a glaciologist with the U.S. Geological Survey in Anchorage, studies glacier-calving trends in Alaska with local seismic instruments. Here the leading edge of a glacier called Columbia has shrunk by about 16 to 18 kilometers because of calving in the past 25 years. At the same time, the glacier as a whole is surging forward. “The ice is moving faster in the forward direction but is calving even faster,” O’Neel explains. “Sea-level rise is a really strong motivator” for under­standing this glacier’s movements, he adds.

At the southern tip of the world, glacial surges are on the examination table, too. By analyzing seismic records, Washington University seismologist Douglas Wiens and Pennsylvania State University glaciologist Sridhar Anandakrishnan discovered that the stick-slip surges of Antarctica’s Whillans glacier happen twice a day with the tides and that the surges have been slowing down since at least 1994. One explanation, Anandakrishnan says, is that climate-induced sea-level rise is affecting movements of the ice.

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