Another effect of climate change has surfaced, this time on Greenland. A NASA team reports today in Science  that the edges of the Northern Hemisphere's biggest ice cap shrank markedly between 1993 and 1998.
Computer models predict that Earth's polar regions will respond most quickly to changes in global climate. Some large chunks of ice have broken off Antarctica's ice shelves in recent years, although most researchers don't foresee runaway melting there. But in the Arctic, recent work suggests that the ocean's ice cover is melting each summer (ScienceNOW, 12 May 1998 ). Such changes hadn't yet been seen on Greenland, where the rugged coastal terrain makes satellite altitude surveys difficult to interpret.
A team led by hydrospheric scientist William Krabill of the NASA Goddard Space Flight Center's Wallops Flight Facility on Wallops Island, Virginia, used a laser altimeter to survey the ice height over southern Greenland by flying 400 meters above the massive ice cap. Taking measurements in 1993 and 1998, the researchers found that the cap's interior was stable or grew slightly at elevations above 2000 meters, a result also seen in satellite data from 1978 to 1988. But along the southern and eastern coasts, the ice was as much as 10 meters thinner.
Precipitation changes or surface melting alone could not account for such radical losses, the team believes. Rather, it's likely that meltwater is penetrating the glaciers to help them slide more quickly into the ocean, which would stretch them thinner. "Five years is an awfully short time frame" to suggest that a major melting trend is occurring, Krabill admits. But if further monitoring reveals an accelerated rate of ice thinning, "it ultimately could have an impact on global sea level."
The rates and geographic extent of ice loss are "very surprising," says radar altimetry expert Curt Davis of the University of Missouri, Kansas City. "But the number one question for me is whether this has happened for the last 50 or 100 years, not just the last five. There's no way to answer that right now." A new NASA ice-elevation monitoring satellite called ICESAT, scheduled for launch in 2001, will help scientists watch for climate-change trends near the poles over a longer time frame, he notes.