Freezing a Moving Target

Glaciers are supposed to be, well, glacial. For a time, glaciologists wondered if they moved at all. Recent studies of the world’s largest glaciers--the ice sheets and streams of Antarctica--show glaciers can act as if alive. They advance and retreat, “binge and purge,” lose weight and then rapidly grow thick around the middle.

The glaciers of Antarctica refuse to play by the rules--at least the rules glaciologists understand. Two studies from scientists at NASA’s Jet Propulsion Laboratory show adjacent parts of one of Antarctica’s ice sheets are acting in opposite ways.

One part of the west Antarctic ice sheet near the Amundsen Sea is retreating, thinning rapidly and breaking off into huge icebergs the size of Rhode Island, Delaware and Connecticut. Ice streams cutting through an area near the Ross Sea, meanwhile, are grinding to a halt and growing thicker and more stable as scientists watch.

“This is something new to glaciology,” said Eric Rignot, a JPL glaciologist and the author of a study soon to be published in the Annals of Glaciology showing rapid disintegration of some of the continent’s largest glaciers. “I don’t think anyone expected to see such changes in less than a decade.”

By contrast, Ian Joughin, who works just down the hall from Rignot at JPL, has just completed a study that shows stability in the ice sheet. “We’ve seen a wide range of behavior,” said Joughin, whose study is published in the current issue of the journal Science. “There’s a lot more variability than conventional wisdom would indicate.”

Understanding of the vast ice sheets of Antarctica has been a high priority for scientists for more than a decade. In the early ‘90s, there were indications that the ice sheet in western Antarctica, a layer 6,500 feet thick and the size of Mexico, was retreating rapidly.

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Gauging the Chances of a ‘Catastrophic Collapse’

The ice sheet has long been considered Antarctica’s “weak underbelly.”

The base of the sheet sits below sea level. Because of that, if it thinned, it could rise and begin to float. That would allow it to melt from its top and bottom, increasing the chances of a rapid disintegration--what scientists have termed a “catastrophic collapse.” The ice sheet has disappeared completely and re-formed at least once in its 20-million-year history.

Because Antarctica holds 90% of the world’s ice and 70% of its fresh water, some models suggested that the disintegration of the ice sheet could raise global sea levels by 20 feet within a century.

Fears of a catastrophic collapse have largely evaporated with more detailed studies of the ice sheet. British Antarctic Survey scientists recently estimated that the risk of a 3-foot sea level rise from an ice sheet collapse was just 5%.

But new data, including Rignot’s work showing disintegration of some of the continent’s largest glaciers--the Pine Island and Thwaites glaciers--still raise concerns about smaller but more rapid increases in sea level.

“We’re not out of the woods yet,” said Robert Bindschadler, a glaciologist at NASA’s Goddard Space Flight Center in Greenbelt, Md., who has studied Antarctic ice for two decades. Global sea levels have risen by about 2 millimeters per year for the last century, he said, and only half of that rise is accounted for. Lesser-known areas of Antarctica’s eastern side, he said, might play a role.

Predicting the future of any of the continent’s ice sheets remains nearly impossible. “Even just measuring ... whether the ice sheet is growing or shrinking has proved difficult,” says Penn State University glaciologist Richard Alley, an expert on rapid climate change and the behavior of ice.

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The Effects of Global Warming

Bindschadler was one of many involved in early attempts to describe the glaciers and their vagaries. That heroic early work involved camping on the ice in fierce winds and temperatures that routinely reached 40 below zero, dragging around bulky Doppler radar instruments and gingerly riding snowmobiles between yawing crevasses to drill deep holes in the ice.

Though such measurements still prove invaluable, they are spotty. Antarctica is so difficult to reach and to travel across that scientists have been able to visit only a tiny portion of the continent. A new generation of work relies on satellites with radars and lasers that measure the entire continent with high precision to detect even the smallest changes in the ice. “Radar is like having a million GPS units on the ground, wherever you want them,” said Rignot, who has yet to set foot on the continent he knows so well.

The new work using U.S., Canadian and European satellites, combined with decades of field work, could help glaciologists finally understand the strange behavior of the ice sheet and how it may be affected by climate changes. A new NASA satellite called Icesat will help once it is launched later this year.

The area Rignot studies, where the Pine Island and Thwaites glaciers meet the Amundsen Sea, may be affected by the human-induced global warming of the last century. Because global warming raises ocean temperatures around Antarctica, it could be increasing the rate at which icebergs break off, float north and melt.

“There the ice sheet is probably feeling very recent changes in climate,” said Bindschadler, who has used NASA and the U.S. Geological Survey’s Landsat and declassified CIA satellite images to study the ice sheets. But the majority of the ice sheet that is inland and atop solid ground does not appear to be affected by the current climate. “It’s so well insulated,” Bindschadler said, “it takes thousands or tens of thousands of years to see any effect.” The behavior seen in much of the ice sheet recently, he said, is probably in response to the end of the last Ice Age.

Or it might not be in response to climate at all. “Is this simply the ice sheet breathing?” Joughin asked.

Most glaciologists agree that the changes in glaciers are not due simply to melting or a decrease in snowfall but to “ice dynamics.” A key to the behavior of glaciers lies not in the ice that comprises them, but in the gravelly till that underlies them. When it gets colder, this till layer gets drier and acts as a brake, preventing the glacier from slipping quickly, Joughin said.

“We have to understand what’s happening at the bed,” Rignot said.

The report by Joughin and glaciologist Slawek Tulaczyk of UC Santa Cruz suggests that a glacial bed could also start to freeze after a long period of retreat. As the glaciers got thinner in past decades, they became less insulating and allowed more cold to creep through to the bed. Once glaciers stop moving and discharging ice, they can get thicker again.

A portion of the current sea level rise is attributed to global warming and is caused by the expansion of seawater as it heats and the melting of low-latitude glaciers such as the one atop Africa’s Mt. Kilimanjaro. Thickening of Antarctica’s ice sheets could help offset that rise.

“If I had beachfront property,” Joughin said, “I wouldn’t sell it just yet.”