December 13, 1999
These latest findings by Antarctic researchers will be reported at the Fall American Geophysical Union meeting in San Francisco. The press briefing will be held on Dec. 13, 1999 at 3 p.m. PST (6 p.m. EST) in Room 112 of the Moscone Convention Center.
In unraveling the ancient history of the continent, scientists hope to be able to tell its future. Dr. Robert Bindschadler, a NASA Goddard Space Flight Center (Greenbelt, Md.) glaciologist, studies the way that the West Antarctic ice sheet flows along massive rivers of ice into the sea. "We have to understand the ice streams in order to predict the future and to know what their contribution will be to sea level change," he said, adding that if the entire West Antarctic ice sheet were to flow into the ocean, global sea level could rise as much as 20 feet (six meters).
Scientists have been aware of ice streams since Antarctic exploration in the mid-1970s. Vast streams of ice flow for hundreds of miles from the West Antarctic ice sheet into the sea, acting as giant conveyer belts dumping the continent's accumulated snow cover.
For a long time, scientists were mystified because the ice streams appeared to start from a near standstill and then flow hundreds of feet per year faster than the surrounding ice sheet.
The point where ice streams begin to flow rapidly is marked by deep crevasses in the ice between the fast flowing ice streams and the slower moving ice sheet. But upstream, there isn't anything on the surface to tip scientists off to where the ice streams are coming from, said Bindschadler.
Now, data taken by the Canadian Radarsat satellite in 1997 during NASA and the Canadian Space Agency's Antarctic Mapping Mission, has allowed researchers to see that the vast rivers of ice were actually the product of smaller tributaries flowing from further into the continent's interior.
"It's a huge amount of information that would have taken decades to collect, and we never would have the resources to make this stunning discovery without Radarsat," said Bindschadler. "What we see is that there are tributaries -- well-defined regions of flowing ice that are feeding the ice streams." Bindschadler said that the tributaries flow more slowly than the larger ice streams and therefore aren't marked by gaping crevasses along their borders.
Another interesting discovery, said Bindschadler, is that the tributaries flow within valleys on the continent's rocky surface, a factor that could be causing the ice to pick up speed. Ice funneled into the valleys becomes thicker than the surrounding ice, causing the base to become warmer and flow more easily. Then, two or more tributaries join and feed into the larger and faster-flowing ice streams, quickly moving the continent's accumulated ice to sea.
The Radarsat Antarctic Mapping Mission allowed Byrd Polar Research Center glaciologist Dr. Kenneth Jezek and colleagues to produce the first, high-resolution radar map of the entire Antarctic continent. The map is so clear, it can pick out features as small as a research bungalow but still outline with great precision the entire Antarctic coastline to help researchers see how the ice sheet ebbs and flows over time.
Jezek's team also identified areas of fast flowing ice on the ice sheet. Jezek said that the map shows the complexity of ice streams feeding the ice shelves in East and West Antarctica and small, fast-flowing glaciers scattered around the entire continent's perimeter.
Because of the rapid movement of ice streams, the surface of the ice only records their movement for a hundred years or so. To look further back in time, Nadine Nereson, a glaciologist from the University of Washington, studies the slow moving ridges between the ice streams that contain a record of ice flow for thousands of years.
"The important questions are," said Nereson, "have the ice streams always been where they are now, and how did they look in the past?" Nereson said that the ice ridges contain a record of their flow history, and the challenge is to interpret the record using ice flow models to discover how the fast moving ice streams that borders the ridges have changed over time.
By dragging ice-penetrating radar behind a snowmobile, Nereson and colleagues can see all the way through the 3,000-foot (1,000-meter) -thick ice to the rock below. The radar picks out different layers in the ice that deform in response to the movement of the neighboring ice streams.
Nereson said that a ridge bounded by two ice streams that flow very stably for thousands of years will show a very symmetrical bulge in the ice layers beneath the ridge divide, and analyzing the shape of the bulge can reveal past changes in ice stream elevations. In ridges bounded by rapidly changing ice streams, the bulge is skewed or warped. "The exciting part is that we could previously detect changes of only a 100 years or so," said Nereson, adding that the new method can tell what the ice streams have been doing over the last 1,000 to 5,000 years.
Across the continent, on the East Antarctic plateau, glaciologists Mark Fahnestock, from the University of Maryland, and Ted Scambos, from the University of Colorado, discovered from a satellite view something impossible to see from the ground. Three massive fields of snow dunes -- together covering an area larger than the state of California -- dot the plateau. "The dunes represent a very extreme environment on the surface of the ice sheet caused by very stable winds," said Fahnestock.
Using data from National Oceanic and Atmospheric Administration satellites and declassified military satellite images, Fahnestock found that the dunes have crests that are up to 60 miles (100 kilometers) long, lie 1.2 miles (two kilometers) apart and are only 10 feet (three meters) high from base to crest. Explorers have crossed the dunes by foot, but they were never identified because their faces are far less steep than more familiar sand dunes.
Fahnestock said that the dunes may be formed by recrystalization of snow caused by warm winds rather than snow drifting, but like many things about the mysterious continent, the dunes are something scientists are just beginning to understand.
Ohio State University
November 23, 1998
That's the conclusion drawn by an international team of scientists who analyzed five years of satellite radar measurements covering a large part of the southernmost continent. Their report was published in a recent issue of the journal Science.
The study is important in that it offers one of the best investigations so far of possible mass balance changes in the ice covering Antarctica. While global warming has been blamed for possible reductions in the Antarctic ice sheet, scientists have argued over whether there was definitive evidence of such ice loss. It's also unclear whether the west Antarctic ice sheet would be unstable in a warmer world.
The new study suggests that the answer is no, at least for the middle of the ice sheet.
"Based on our short, five-year period of observation of the interior of Antarctica, we do not seem to detect that the ice is melting more than one centimeter per year," explained C.K. Shum, an associate professor of civil and environmental engineering and geodetic science at Ohio State University.
"That would mean that the interior Antarctic ice sheet does not seem to be contributing to sea level rise more than 1 millimeter per year." Shum said that a one-centimeter (0.4 inch) decrease in Antarctic ice sheet volume roughly converts into a one-millimeter (0.04 inch) rise in global sea level.
Shum, along with other scientists from University College in London and the Delft University of Technology in the Netherlands, analyzed radar altimetry data retrieved from two European Space Agency remote sensing satellites -- ESA-1 and ESA-2 -- used to measure ice altitudes from 1992 through 1996.
The orbits of the satellites reached to 81.5 degrees N, allowing them to regularly monitor at least 60 percent of the continent's grounded ice.
The majority of the West Antarctic ice sheet sits atop dry land while the East Antarctic ice sheet is grounded below sea level. Changes in the East Antarctic sheet would have little effect on sea level since the ice displaces sea water. But a complete melt of West Antarctic ice would pour new water into the oceans, raising sea levels.
"We assume that global warming is underway now," Shum says, "and it may be enhanced by human activities but, until now, its effect on ice loss in Greenland and the Antarctic has been mostly speculation. We wanted to look at ice sheets to see how they contribute to sea level rise."
Researchers understand some causes for sea level rise, Shum said, but the role ice sheets may play "remains an uncertainty." During the last ice age -- 18,000 years ago -- sea levels were at least 100 meters lower than they are today.
Shum and his colleagues turned to the ESA satellites to look for ice sheet growth. The two, circling the globe in 800-kilometer (497-mile) orbits, were able to measure the height of the ocean surface to an accuracy of about 5 centimeters (two inches).
"But over ice, the readings are less accurate," Shum said. "The satellites also have problems accurately measuring coastal ice regions."
The researchers had to devise new algorithms to decipher the raw, ice sheet altimetry data and correct for several variables such as radar penetration below the ice surface and snow accumulation. Even with these limitations, the study represents the longest time series for which data is available.
Shum said the National Aeronautics and Space Administration is planning a new mission for the year 2001 called ICESAT. It would position a new satellite in a near-polar orbit, increasing the amount of ice sheet coverage, and use a more accurate laser altimeter to take measurements.
These, combined with the radar altimetry data, would give a much better assessment of mass balance changes, if any, in the Antarctic ice sheets.
This project was supported in part by the NASA Physical Oceanography Program and the United Kingdom Natural Environment Research Council.
ESA
Paris, 15 October 1998
Antarctica is not shrinking, the European Space Agency ESA reveals today. This result of the ERS (European Remote Sensing) satellites is reported 16 October 1998 by an international team of scientists in the leading American magazine, SCIENCE (See note). But the same investigation provides evidence that one part of West Antarctica may be rapidly losing its ice to the ocean.
The team of British, Dutch and American scientists, led by Professor Duncan Wingham at University College London, based their findings on ERS data collected over five years.
The data reveal that most of the ice stored in Antarctica is very stable. The icy continent now looks an unlikely source of rising global sea level this century, making thermal expansion of the ocean due to global warming, and the shrinking of mountain glaciers, more likely causes.
Prof. Wingham's team used ERS's radar altimeter instruments to determine if the thickness of the Antarctic Ice Sheet changed over the five-year period from 1992 to 1996. Transmitting over 4,000,000 radar pulses to the surface of the ice, and measuring the time taken for the echoes to return to the satellite, the average change of the height of 63 of the Ice Sheet was measured with an accuracy of 0.5 cm per year. The ice sheet has changed on average by less than 1 cm per year. Using previous measurements of changes in snowfall over the ice sheet, the team concluded that the interior of the Antarctic Ice Sheet had contributed only 1.7 cm to sea level rise this century.
Sea level has risen 18 cm over the past 100 years. Previously the accuracy of data could have meant that Antarctica were responsible for a raise or lowering of global sea level by 14 cm in either direction. Speaking today on the importance of the research Professor Duncan Wingham said: "Scientists have never really understood the role that Antarctica has played in this century's rising sea level. Our research makes it likely that the answer is very little."
The result supports the view that global warming, leading to a thermal expansion of the ocean and the melting of mountain glaciers, is responsible for rising sea level. Professor Wingham continued, "As a consequence of our research we should be able to produce more accurate predictions of future sea level rises. Indeed it is possible that the consequences of global warming on sea level rise has been underestimated."
Professor Wingham and his team also discovered a rapid reduction in thickness in one of West Antarctica's least known drainage basins. The Thwaites Glacier basin was shrinking rapidly at 12 cm per year. The result provides evidence that one of the most hostile regions of Antarctica may be undergoing rapid change.
Commenting on the wider importance of the research, ESA's Earth Observation Mission Manager Guy Duchossois said: "Knowledge about sea level rise matters for many million people in coastal areas whose lives depend on dykes that can withstand the forces of nature. ESA will continue to fly satellites that provide the data for a better understanding of our environment. Global warming may have daunting consequences that satellites in space can help to manage."
Note:
Antarctic Elevation Change 1992 - 1996, D.J.Wingham et al. Appears in the 16 October 1998 edition of SCIENCE
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