Ice streams and their smaller cousins, glaciers, don't just sit around looking frosty; they mosey along, albeit at different rates of speed. They also lose mass when snowfall cannot replace what is lost to melting. Global warming has hastened this loss, and may potentially raise sea levels to devastating heights. The big questions are: How important a player is climate change in the process—and what will its impact be in the future? In an effort find answers, scientists have created models based on what they know about global warming to help them predict its potential consequences.
Current models, however, are missing a major ingredient that a new study of Whillans Ice Stream in West Antarctica may provide: just how ice streams move. Finding out is key, because it may show what happens to slow glaciers down.
Until now, scientists had only observed glaciers creeping along. But the study of Whillans, which drains into the Ross Ice Shelf (the world's largest such ice mass spanning land and sea), shows that it moves quite differently, inching forward in fits and starts.
Douglas Wiens, professor of earth and planetary sciences at Washington University in St. Louis and lead author of the study published today in Nature, says his team has uncovered evidence that the mammoth 60-by-120-mile (95-by-193-kilometer) expanse of ice is powered by seismic energy in the watery, rocky terrain beneath it. He says researchers detected two seismic waves—each equivalent to powerful magnitude 7 earthquakes—daily, which apparently come from a single, "sticky spot"—glaciologist lingo for epicenter—on the southern end of Whillans.
Previously, scientists had no clue that energy from a single spot could budge an ice stream, much less that such bodies could move with such regularity.
Wiens initially discovered the daily tremors while monitoring seismic waves from 2001 to 2003 with seismographs (instruments that measure and record vibrations in the ground such as earthquakes) at the South Pole and in Antarctica's McMurdo Dry Valleys. In an effort to pinpoint their source and function, glaciologist Sridhar Anandakrishnan of the Department of Geosciences and the Earth and Environmental Systems Institute at Pennsylvania State University mounted global positioning system (GPS) monitors on poles placed in various locations on the Whillans Ice Stream.
Using a GPS system similar to that used for car navigation Anandakrishnan was able to track the poles' motion on Whillans, which he measured every 10 seconds for two weeks. The GPS indicated that the stream moved in what glaciologists call a stick-slip motion: Pressure builds on a single spot—that's the stick part. When this pressure becomes too strong, the ground moves below the stressed spot—the slip part. Translation: when the underlying surface moves, so does Whillans above it.
Co-author Matt King of the School of Civil Engineering and Geosciences at Newcastle University in England processed the data and confirmed that the seismographic signals were, indeed, coming from Whillans. Wiens then scoured a global network of seismographs for activity and detected waves generated by the ice sheet thousands of miles away in Australia and New Zealand.
So why have scientists never noticed such seemingly powerful seismic events before? Because they are virtually undetectable. Anandakrishnan says he has actually stood on Whillans during the quakes and not felt them. The reason: the ice sheet moves at a snail's pace of just 24 inches (61 centimeters) per half hour. He would have been able to feel the shift, Wiens says, if the ice sheet picked up the pace to, say, 24 inches per 10 seconds.
The team is not certain why the seismic waves occur like clockwork. But Wiens notes that the ice speed appears to take its cues from ocean tides in the Ross Sea and also beneath the Ross Ice Shelf—the part of the Ross Sea with a floating mass of ice. Stress builds as the tides fall; once a certain amount of stress builds, the ice slips.
Robert Bindschadler, head of the Hydrospheric and Biospheric Sciences Laboratory at NASA's Goddard Space Flight Center in Greenbelt, Md., agrees that the tides build pressure on the sticky-spot, prompting the ice to move. The reason why, however, remains a mystery.
He wonders: "How does a tide that only goes up and down 39.37 inches (one meter) influence a chunk of ice the size of a small New England state?"
Wien's team may have a better idea once the results are in from an array of GPS stations they plan to set up around Whillans's sticky-spot.