The ice sheet covering Greenland is expansive. Located in the north Atlantic Ocean, Greenland is the largest island in the world and has the second largest mass of frozen fresh water on Earth. The ice and snow, covering 85 percent of the island, may provide important clues on global climate change.

Because of the importance of this ice sheet to Earth, NASA has been conducting aerial surveys of the island since 1992 using an aircraft carrying lasers and radars. In May 2003 crews will return once again to Greenland to take a "snapshot" of large areas of the glaciers.

"The measurements obtained during the aircraft flights will help scientists better understand glacial changes caused by global climate change," said Bill Krabill, the lead NASA investigator for the mapping missions.

Some computer models show that increased global temperatures will partially melt polar ice sheets, such as those in Greenland, and raise sea level. Others show that rising temperatures will result in increased snowfall and expand the size of the ice sheets.

"Knowledge of any change in these glaciers may indicate trends in world climate and provide an indirect measure of sea-level changes," Krabill said.

Measurements by NASA and universities over the past 11 years have shown the glaciers shrinking along the southeast coast of Greenland. Scientists have surmised that melting ice flows down to the rock below the glaciers and acts like a lubricant between the ice and rock. Thus, it becomes easier for the glacial ice to flow into the ocean. However, at the same time there has been an increase in the size of the ice sheet in the central portion of the island.

Krabill said, "We have to remember that these measurements only cover a 11-year life span of an island that is thousands of years old. We really don't know if this is a normal cycle of events for these ice sheets or if this is a true sign of permanent change for the island, and thus sea-levels."

It has been estimated that a 10-inch (25-centimeter) decrease in the average height of the central Greenland ice sheet would result in a 0.04-inch (1 millimeter) increase in the sea level of the world's oceans.

Scientists are scheduled to depart for Greenland on a NASA P-3B aircraft from the NASA Goddard Space Flight Center's Wallops Flight Facility, Wallops Island Va., on May 7. During the next four weeks, through early June, the scientists will survey the Greenland ice sheet in the southern and northern parts of the island.

"You can take ice measurements from the ground, however because the area you sample is small you don't get an accurate look at the big picture of what's happening with the ice sheets," Krabill said. "The aircraft allows us to cover much larger areas, and thus, obtain a more accurate picture of the ice sheets."

Advances in technology are what make it possible to develop maps of the ice sheets with very good accuracy, Krabill said. Using Global Positioning System (GPS) receivers, the aircraft is able to fly flight lines year after year and keep within feet of the original flight line. This allows the mapping instruments to take measurements over the same area each year.

Using three different instruments this year, the scientists will be able to get a clear picture of the thickness of the ice sheet and information on depths of snowfall within the past year.

NASA will fly two laser systems on the aircraft that provide the scientists with the height or elevation of the ice. The instrument sends a laser pulse to the surface of the ice, scanning an area about 650 feet (200 meters) wide below the aircraft. The ice then reflects the laser pulse back to the aircraft. Knowing the speed of the laser light and the position and altitude of the aircraft, scientists can then develop a map of the ice sheet elevations. The laser system can help researchers develop maps of the ice sheet surface to an accuracy within 4 inches (10 centimeters).

Researchers from the University of Kansas, Lawrence, will fly two radars. One is an ice penetrating radar that will give scientists a map of the bedrock below or the location of the bottom of the ice sheet. Combining the radar information with the laser data, researchers can then determine the thickness of the ice sheet.

The second Kansas radar is a newly developed snow accumulation radar. This radar will measure the top 60 feet (20 meters) of the snow pack. This will help researchers in trying to measure the annual snowfall.

The data from this year's flights will be added to the information gathered from previous missions, providing researchers valuable information in further understanding the effects of global climate change on the world's ice systems.

More information on the NASA laser system, including uses for mapping other terrains, can be found at: http://aol.wff.nasa.gov/aoltm.html