"One ice stream slowed considerably about 150 years ago and another has been slowing down over the past few decades, but the present slowing and past near-shutdown are probably due to local conditions, says Byron Parizek, graduate student in geosciences. "Regionally, there is still sufficient water generated up glacier to maintain ice streaming along the Siple Coast."
Unlike the Arctic, where most of the ice is formed over water and therefore already floating, most Antarctic ice is formed over land. Any portion of the Antarctic ice sheets that begins to float, increases the volume of the oceans and raises their level.
"If all of the West Antarctic ice sheet melted or floated, it would raise the oceans by 15 to 20 feet vertically, so that anything less than 20 feet above sea level would be underwater," says Dr. Richard Alley, the Evan Pugh professor of geosciences. "While there is no need for panic, Byron's work suggests that we still have to worry about what the West Antarctic ice streams are doing."
Working with Alley, Howard Conway of University of Washington, and Sridhar Anandakrishnan, University of Alabama and now at Penn State, Parizek utilized his numerical flowline model to look at the regions of rapid ice flow near the Ross Sea, collectively known as the Siple Coast ice streams. Simulations of the flow of ice stream D were compared to the geological evidence to decipher what makes the streams slow, stop or speed up.
About 13,000 years ago, the West Antarctic ice sheet solidly covered West Antarctica out to the edge of the continental shelf, the farthest it can ever cover. Since then, the grounded portion of the ice sheet has retreated more than 800 miles across the Ross Sea. The grounding line, the line where the ice sheet stops floating and retains contact with the continent is far back from the ice's edge. Recent information on the slowing and stopping of some of the ice streams has led some to suggest that the migration of the grounding line and the resulting rise in sea level will stop, but the Penn State researchers believe the ice stream changes are local events and that the grounding line may continue to retreat.
"The question is whether the grounding line retreat and variability of the ice streams are part of the long-term dynamics of the ice sheet," says Parizek. "Or whether the present variability is a sign of imminent ice-stream shutdown and grounding-line readvance. "
Looking at the geological history of this area, it appears that there have been ice streams dating back to at least 13,000 years ago; and the streams change speeds and start and stop routinely because of complex interactions between the ice, basal sediments, heat flow, topography and the atmosphere.
The researchers reported in a recent issue of Geophysical Research Letters that the contribution of geothermal heat beneath the ice and frictional heat generated by ice sliding over the ground, provide more heat to the bottom of the ice sheet than is removed by heat flow through the ice to the cold air above, causing the interface between ice and ground to have a thin layer, or stream of water. Ice flows downhill to the ocean from a topographic dome on the ice sheet. The pressure from the overlying ice load drives the flow of these very shallow water streams. At times, the water beneath the ice may freeze locally, temporarily slowing the ice stream but regionally, there is sufficient up-glacier melt-water production to maintain rapid ice motion with the overall ice-stream system.
"The history contained in the off-shore bedforms says that this system had variability thousands of years ago, the flow patterns recorded within the ice say that the system exhibited variability throughout the last millennium, and the modern measurements of the system say that variability continues," says Parizek. "There is no reason to believe that the future of the ice streams will not be variable as well. Our research suggests that amidst these changes, the ice streams are still thermodynamically healthy and retreat can continue."
Journal
Geophysical Research Letters