January 8, 2001
Ice streams in West Antarctic ice sheet hold clues to global climate change
By Tim Stephens
The West Antarctic ice sheet, which contains enough ice to raise global sea levels
by 5 to 6 meters, has been a focal point of concerns about global warming since the
1970s. According to a new analysis of ice streams that flow through the huge ice
mass, a complete collapse of the ice sheet may be unlikely, but the alternative scenario
suggested by the study could have equally dramatic consequences.
The West Antarctic ice sheet is thought to be inherently less stable than the larger
East Antarctic ice sheet, which covers most of the continent's land mass. Much of
the West Antarctic ice sheet rests on land that is below sea level, and parts of
it, called ice shelves, are floating on the sea. Ice streams are fast-moving riverlike
currents of ice that move through the ice sheet, carrying large volumes of ice out
into the ice shelves.
Slawek Tulaczyk, an assistant professor of Earth sciences, is investigating the behavior
of the Antarctic ice streams using mathematical models of the physical processes
involved in their flow. Tulaczyk and graduate student Marion Bougamont presented
their latest findings in December at the fall meeting of the American Geophysical
Union in San Francisco.
Other researchers studying the effects of global warming on the West Antarctic ice
sheet have proposed that the ice streams may accelerate their flow, increasing the
discharge of ice and contributing to the disintegration of the entire ice sheet.
According to Tulaczyk's model and recent observations, however, the ice streams may
instead slow down and stop moving altogether. Stoppage of the ice streams may lead
to thinning and shrinking of the ice shelves they nourish, most notably the large
Ross Ice Shelf that covers the Ross Sea. Loss of the ice shelf over the Ross Sea
could, in turn, trigger changes in global ocean circulation and climate, Tulaczyk
said.
"In the most extreme case, some models suggest that these changes could result
in a shift from the current interglacial climate into another glacial period,"
he said. "I don't want to go that far, because we're still learning how the
ice sheet would respond to changes in the ice streams; but it's an interesting possibility,
especially since people have focused for so long on the possible collapse of the
West Antarctic ice sheet due to global warming."
The ice streams can be seen in satellite images as large features within the ice
sheet about 500 kilometers (300 miles) long and 20 to 100 kilometers (10 to 60 miles)
wide. They move at a rate of 1 to 2 meters per day, sliding over a bed of sediment
saturated with liquid water. But if the bed becomes cold enough for the water in
it to start freezing, the loss of lubrication causes the ice stream to slow and eventually
stop moving, Tulaczyk said.
In his model, the temperature gradient at the base of the ice stream is critical
and is controlled by the thickness of the overlying ice. The heat that maintains
liquid water at the base of the ice stream comes from friction in the sediment bed
and geothermal flux from Earth's hot interior. This thermal energy dissipates into
the colder ice above at a rate determined by the steepness of the temperature gradient
between the bottom and the top of the ice sheet.
"As the ice thins, the temperature gradient becomes steeper and the amount of
thermal energy escaping into the colder ice above increases. As more heat escapes,
the bed reaches a threshold where it switches to freezing conditions," Tulaczyk
said.
One ice stream, known as Ice Stream C, is known to have stopped moving about 150
to 200 years ago. A second, Ice Stream B, has slowed by 50 percent over the past
four decades.
According to Tulaczyk, the stoppage of ice streams is not the result of global warming
caused by human activities within the past century, but rather it is a response to
changes in climate and in the geometry of the ice sheet over the past 10,000 years.
The West Antarctic ice sheet has been gradually thinning and retreating from its
maximum extent during the last ice age. The resulting changes in the temperature
gradients of the ice streams may only now be reaching the threshold point at which
the lubricating sediment beds freeze up.
"The ice streams have always been the big uncertainty in predicting the effects
of global warming on the ice sheet, because no one understood their dynamics well
enough to make good models," Tulaczyk said.
His model of the ice streams is largely based on his Ph.D. research with glaciologists
Barclay Kamb and Hermann Engelhardt at the California Institute of Technology, who
have been studying ice streams for the past 10 years. Their measurements of temperature
profiles from bore holes in the ice streams are consistent with Tulaczyk's findings,
he said. When Bougamont fed temperature data obtained from bore holes in Ice Stream
C into the model, the simulation showed freezing of the bed and shutdown of the ice
stream within 100 years.
"It's always a problem to take a complex physical phenomenon and boil it down
into a set of equations, but I think we have the most sophisticated model of the
ice streams to date," Tulaczyk said
