Melting ice effects may be more chilling than first thought
Scientists say they may have underestimated the global rise in sea levels caused by ice sheet loss. File photo |
The fourth IPCC report released in January says sea levels will rise by 28-43 centimeters by 2100. This would happen even if greenhouse gas emissions are immediately reduced to zero, it says.
"But our studies have shown that this prediction is probably much too low," said Robert Bindschadler, a chief scientist with the National Aeronautics and Space Administration (NASA), at the annual meeting of the American Association for the Advancement of Sciences.
In a recent study using NASA's satellite for ice, cloud and land elevation, Bindschadler and Helen Fricker with the University of California, San Diego, found a dynamic subglacial water system beneath the West Antarctic ice sheet.
The study was published in the February 15 online issue of Science magazine.
During the three-year study, scientists observed a previously unknown region of subglacial lakes lying under two fast-flowing ice streams feeding 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.
The observation 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 significant role in controlling the rate at which ice moves off the continent.
One of the observed lakes approximated 10 kilometers by 30 kilometers and drained a volume of 2 cubic kilometers of water into the ocean under the Ross Ice Shelf through a subglacial channel. Another nearby subglacial lake steadily filled with 1.2 cubic kilometers of water.
The data also indicated a net gain of water in the subglacial system over the three years.
Glaciologists have known for a long time that water exists under ice streams. But this is the first discovery of the involvement of such huge amounts of water and the pace at which it moves from one reservoir to another, Fricker said.
Co-author Bindschadler added: "We still don't know the real interactive mechanism between the observed subglacial lakes and ice sheets. Are these lakes simply collecting and storing ice loss or they are dryers of ice themselves?"
He said that working out the subglacial water network mechanism will enable scientists to better predict the ice change.
Some 90 percent of the world's ice is located in Antarctica, most of it as ice sheets covering 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.
Bindschadler said the IPCC drafters had not considered the studies on the subglacial water movement because there was no way to quantify the movement and project the change it caused to sea levels.
"We cannot understand the clear impact of climate change on the sea level until we have better understanding of the subglacial water movement," Bindschadler told China Daily.
In the IPCC report, scientists mention that there will be an extra 20 centimeters' rise of sea level caused by unpredicted total ice sheet loss by 2100.
But the number also results from reconciliation among scientists, Bindschadler said. Considering many uncertainties, including subglacial water movement in Antarctica and tidewater glacier outlets, there is a great possibility that the sea-level rise has been much underestimated, he noted.
(China Daily 02/28/2007 page19)
