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 at the University of California, Santa Cruz, 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 on December 15 at the fall meeting of the American Geophysical Union.
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.