Greenland’s ice is thinner at the margins Two new NASA studies show that the

central portion of Greenland’s ice sheet isn’t, on the whole, getting any thinner. However, one of the investigations finds substantial thinning along most margins of the ice sheet-changes that are con- tributing to rising sea levels.

Scientists at NASA’s Wallops Flight Facil- ity in Wallops Island, Va., led both studies, which they report in the July 21 SCIENCE.

One group used the Global Positioning System to measure the downhill flow of ice at elevations of about 2,000 meters above sea level from 1993 to 1997. Survey points were typically about 30 kilometers apart. Together, these roughly 150 points bordered the approximately 1 million square kilometers of Greenland’s central ice sheet.

On average, the high-altitude portion of the ice sheet is neither gaining nor losing thickness, glaciologist Robert H. Thomas and his colleagues report. Ice flowing down past the 2,000-m contour is almost balanced by the accumulation of snow above that level. Particular areas, howev- er, show significant net gains and losses.

For example, the extreme southwest- ern portion of the central ice sheet thick- ens by some 210 millimeters each year. Although increased snowfall accounts for some of this growth, the scientists sug- gest that this area also accumulates ice flowing from higher elevations, a result of warming temperatures.

The thick bars bordering Greenland’s 1-million-square-kilometer central ice sheet show the relative magnitude and direction of ice flow.

During the same period, the far south- eastern portion of the ice sheet has been thinning by almost 300 mm annually, pri- marily due to increased melting. North- ern portions of the ice sheet experience smaller changes due to lower precipita- tion and colder temperatures.

The second research team studied Greenland’s ice between 1993 and 1999, using airplane-mounted laser altimeters to calculate its thickness. William B. Kra- bill and his colleagues confirmed that the central ice sheet remained stable. Howev- er, their research also shows that about 70 percent of the ice sheet’s margins are thinning substantially-in some places, Krabill notes, up to 1 meter per year. Moreover, they find, thickening of the margins elsewhere is not compensating for these losses.

Overall, more than 50 cubic kilometers of Greenland’s ice melts away annually, Krabill says. That’s enough to raise Earth’s sea level by about 0.13 mm each year, he adds, or about 7 percent of the rise observed during the study. Melting Greenland’s entire ice sheet, which is more than 4,000 m thick in places and covers almost all the island, would raise sea level about 7 m.

Currently, researchers can’t separate the effects of recent warming from those due to long-term trends in climate.

The key to predicting future sea level changes is distinguishing the effects of in- creased average temperatures during the 1990s from those due to long-term trends, says Dorthe Dahl-Jensen of the University of Copenhagen, in a commen- tary accompanying the articles. “[Tlhe records need to be extended to several decades before the long-term trend can be estimated,” she says. -S. Perkins

Edible vaccine sDawns antibodies to virus Scientists have genetically engineered

potatoes for the first time to deliver an edible vaccine against a common virus.

The researchers added to spuds the capsid protein that forms the shell around diarrheacausing Norwalk virus. When eat- en, the product stimulated volunteers’ im- mune systems to create a flood of antibod- ies. The response prevented Norwalk virus from latching onto cells lining the in- testines, which causes disease.

Twenty volunteers ate two or three portions of raw, diced, genetically modi- fied potatoes over 3 weeks. Four other volunteers consumed unaltered pota- toes. Blood samples taken over the next 2 months revealed that 19 of 20 people getting the edible vaccine had elevated concentrations of antibodies against Nor- walk virus, although the rise varied be- tween individuals.

The researchers also detected antibod- ies in the feces of six of the people who had received the edible vaccine. That in- dicates that the immune agents were so plentiful in the gut that some were being excreted, and that’s a good sign, says study coauthor Charles J. Arntzen of the Boyce Thompson Institute for Plant Re- search at Cornell University.

Antibody concentrations remained unchanged in the volunteers receiving the untreated potatoes, Arntzen and his colleagues report in the July JOURNAL OF INFECTIOUS DISMFS.

“It’s a great study, very forward think- ing,” says Dennis R. Lang of the National Institute of Allergy and Infectious Dis- eases (NIAID) in Bethesda, Md.

The intestinal virus is named for Nor- walk, Ohio, where researchers first iden- tified it in 1968. Norwalk or a related virus infects more than 23 million people in the United States every year, causing di- arrhea, nausea, and stomach cramps, ac-

cording to the Centers for Disease Con- trol and Prevention in Atlanta.

Scientists consider Norwalk virus no more than a nuisance in the United States. It’s distressing but survivable. Natural antibodies form during a bout with the virus, and some of these linger indefinitely. “We’ve never seen a volun- teer without any,” Arntzen says. Unfortu- nately, the antibodies decline to useless concentrations over a few years, leaving a person susceptible again.

In developing countries around the world, Norwalks occurrence can be more deadly, especially if children getting it be- come dehydrated, Amtzen says.

In those countries, injectable vaccines are costly to deliver and needles can transmit diseases, says Arthur 0. Ander- son of the US. Army Medical Research Institute of Infectious Diseases in Freder- ick, Md. An edible vaccine, therefore, might present a better approach to bat- tling Norwalk virus outbreaks in devel- oping countries, he says.

The scientists designed this initial ex- periment to gauge the safety of the edi- ble vaccine. They won’t be able to mea- sure its effectiveness until they give it to people who are then exposed to the actu- al virus. NIAID, which funded Arntzen’s study, is currently working out guidelines for such a test, Lang says.

Meanwhile, Arntzen and his colleagues plan to trade in potatoes for tomatoes as their vaccine carrier-possibly dried for better storage. They may eventually try to develop banana-based vaccines. They’re also targeting papilloma virus, which can lead to cervical cancer, as well as hepati- tis B virus. A reliable injectable vaccine already exists for hepatitis B, but a less expensive, edible vaccine would be bet- ter, particularly for use in the developing world, Arntzen says. -N Seppa

54 SCIENCE NEWS, VOL. 158 JULY 22,2000