The recent discovery of a subglacial water system beneath the West Antarctic ice sheet (WAIS) is causing scientists to rethink the mechanisms that control the flow of ice streams into the Ross Ice Shelf and ultimately into the Southern Ocean, according to Helen Amanda Fricker, a glaciologist at Scripps Institution of Oceanography, UC San Diego.
Subglacial Lake Engelhardt
Whillans Ice Stream
Western Antarctic Ice Sheet
Comparisons of elevation profiles taken by altimetry instruments on NASA's Ice, Cloud, and land Elevation Satellite (ICESat) revealed the draining of a subglacial lake some 1 km (3,290 feet) beneath the ice with an area of about 10 km by 30 km (6 x 18.5 miles).
Total water volume loss was about 2 km3 (5.28 1011 US gallons) to the ocean under the Ross Ice Shelf through a sublacial channel.
ICESat profiles over nearly three years (2003-2006) revealed 14 localized regions in the Western Antarctic Ice Sheet where the surface rose, fell or oscillated between tri-annual observations.
Reporting in the February 15, 2007, issue of Science magazine online, Fricker and co-authors describe the sighting of a previously unknown region of subglacial lakes lying under two fast-flowing ice streams about one kilometer (3,280 feet) thick. The study provides the first evidence that subglacial water is stored in a linked system of reservoirs underneath the ice and can move quickly into and out of those reservoirs. This activity may play a major role in controlling the rate at which ice moves off the continent.
Co-authors of the article are Ted Scambos of the National Snow and Ice Data Center, Robert Bindschadler of NASA Goddard Space Flight Center and Laurie Padman of Earth & Space Research.
The new findings were made possible by precise elevation measurements taken from NASA's Ice, Cloud, and land Elevation Satellite (ICESat) from 2003 to 2006 collected over the Whillans and Mercer ice streams in West Antarctica. The two ice streams are major feeders of ice to the Ross Ice Shelf, Antarctica's largest ice shelf covering an area of the ocean about the size of France with ice several hundred meters thick.
Ice streams are components of the ice sheet that move faster than surrounding ice and may be up to 50 kilometers (31 miles) wide and 2 (1.25 miles) kilometers thick, stretching for hundreds of kilometers. The ice streams are responsible for transporting most of the ice leaving the continent to the floating ice shelves and, ultimately, to the ocean.
Glaciologists have known for a long time that water exists under ice streams. The surprising thing about this discovery is the amount of water involved and the pace at which it moves from one reservoir to another, according to Fricker.
"We didn't realize that the water under these ice streams was moving in such large quantities, and on such short time scales," Fricker said. "We thought these changes took place over years and decades, but we are seeing large changes over months. The detected motions are astonishing in magnitude, dynamic nature and spatial extent."
During the three-year study period, the researchers observed one subglacial lake approximately 10 kilometers by 30 kilometers (6 miles by 18.5 miles) to drain a volume of 2 cubic kilometers (528 billion gallons) of water into the ocean under the Ross Ice Shelf through a subglacial channel. Another nearby subglacial lake steadily filled with1.2 cubic kilometers (317 billion gallons) of water. The data also indicate a net gain of water in the subglacial system over the three years, although no relationship can yet be determined to the motion of the two ice streams or the stability of the WAIS, Fricker said.
The WAIS is part of the ongoing debate on the stability of ice sheets and their possible collapse and contribution to raising sea levels worldwide as a result of global warming. Some 90 percent of the world's ice is located in Antarctica, most of it formed in an ice sheet over the land. If rising atmospheric temperatures increasingly melt the ice shelves, their ability to buttress the ice streams on the land would be reduced, allowing more ice to enter the oceans and raise sea levels. One concern is that as increased subglacial water accumulates, it may move ice off the continent faster.
"The links between ice stream activity and the climate are not well understood," Fricker said. "To predict how the ice sheets might respond to global warming, this new information is vital as it gives us a more complete picture of what is happening under the ice. We still don't know how the subglacial water system varies on longer time-scales from decades to centuries. To do this, we need to continue monitoring the ice streams with ICESat and future follow-on missions."
Launched in January 2003, ICESat carries a laser altimeter instrument to detect changes in the ice sheet surface elevation as small as 1.5 centimeters (0.6 inches) from nearly 645 kilometers (400 miles) above the earth. Its current operations plan activates it over Antarctica three times a year for periods of 33 days each. Antarctica is too large, too isolated and its environment too hostile to be surveyed in detail from the land, so satellites are essential to both investigating ice processes and discovering areas of interest that may warrant closer inspection. Studies of the subglacial environment are rare, being expensive, risky and labor-intensive.
"The approach used for this work provides glaciologists with a new tool to survey and monitor the nature of the subglacial water system and to link these observations to the motion of the ice sheet." Fricker said.
IICESat also measures cloud and aerosol heights, land topography, and vegetation characteristics.
The research for this study was supported with funding from NASA.
REFERENCE: "An Active Subglacial Water System in West Antarctica Mapped From Space" www.sciencemag.org/sciencexpress/recent.dtl