It's probably a good idea to hold off buying that beachfront cottage, at least until the effects of global warming are better understood. Using an aircraftmounted laser altimeter and a global positioning system, a group of NASA scientists has determined that the ice in Greenland is melting, contributing to rising sea levels. "Our measurements show little change in high-elevation parts of the Greenland ice sheet, but the margins are undergoing significant thinning," explained William Krabill of the Laboratory for Hydrospheric Processes at NASA's Wallops Flight Facility. In some places, the ice in Greenland is more than 4000 m thick. The group estimates that approximately 51 km3 of ice is lost per year, raising sea level by 0.13 mm per year -- or roughly 7 percent of the rise observed during the study. It estimates that the sea level could rise worldwide by as much as 7 m if the Greenland ice sheet were to melt. Krabill and his colleagues, on repeat flights over Greenland between 1993 and 1999, used a laser altimeter to monitor the thickness of the ice. The NASA groups are pursuing methods of mapping changes in the ice sheet that may indicate global climate change. The altimeter operates in the visible range, at 532 or 523 nm. "We have a heritage of working in the visible spectrum because of early studies in developing laser bathymetry," Krabill said. "We continued to use the hardware we already had." The researchers made time-of-flight measurements of the laser pulse from the aircraft to the surface and back. Measurements were converted to distance by including a range vector that accounted for the aircraft's pitch, roll and heading. Krabill said that the system has more precision than other ranging technologies, and that it is more time- and cost-effective than photogrammetry. Like other systems, however, it is unable to penetrate foliage and operate in inclement weather. Although their findings, published in the July 21 issue of Science, indicate that the margins of the ice sheet are thinning, the researchers cannot explain why. It is not caused by increased melting or less snow accumulation, because the ice sheet was in a state of equilibrium when yearly temperatures were even warmer. Moreover, regional temperatures in the late 1980s and early 1990s were actually cooler than the 96-year average temperature. The group's future projects include the use of a space-based laser altimeter and additional work on Antarctic ice.