Web Extra Friday, December 12, 2003

A year of global ice observations

Scientists are now getting the most accurate view ever of changes in the Antarctic and Greenland ice sheets. The new maps, using NASA's Ice, Cloud and Land Elevation Satellite (ICESat), are shedding light on the processes controlling these ice masses, which comprise 75 percent of Earth's freshwater. But one year into the monitoring program, researchers say ICESat's primary goal is yet unfulfilled.

Launched on Jan. 12, 2003, ICESat has been measuring surface elevations of the polar ice to determine how its behavior, shape and size might be changing, especially in response to global warming (Geotimes Web Extra, January 2003). The satellite has had some technical problems from the start, with the first observation lasting only 36 days, far shorter than the three to six months the scientists had hoped for. And even a year is not much time to make conclusions about the state of the ice sheets. In order to detect any noticeable changes in the ice streams, researchers will need to obtain measurements over a several-year period, says Jay Zwally, a program scientist at NASA Goddard who presented the program's first year of results at the a American Geophysical Union meeting this week in San Francisco. With long-term funding for the program uncertain, the extra needed time could further complicate the mission.

Still, program scientists largely lauded the program. "Thus far, ICESat has made tremendous contributions to understanding how Earth's systems work," says Waleed Abdalati of NASA Headquarters in Washington, D.C. Despite the program issues, "we're accomplishing something that has never even been attempted," he says. NASA's satellites and monitoring platforms have for years been observing Earth in two dimensions, ICESat gives researchers a third dimension — the vertical component. This dimension is key in studying the atmosphere, land and water, Abdalati says, especially in areas like the poles, where cloud cover is high and typical observation difficult.

To obtain accurate maps, the satellite flew over the ice sheets, beaming down a laser that then bounced back up to create a profile of surface elevations and sea ice thickness to a "5-centimenter level on 50-kilometer spatial scales," Zwally says. "It is basically the equivalent of standing at one end of a football field, aiming the laser at a penny at the other end of the field and hitting the penny," he explains.

In addition to measuring sea ice changes, the satellite is giving researchers a new look at cloud cover, aerosols in the atmosphere — natural and anthropogenic — and boreal forest land cover. Precisely mapping the cloud density and frequency is important, says Jim Spinherne of NASA Goddard, because clouds trap solar radiation and thus determine the precipitation over the ice sheets; therefore, a small change in cloud cover would affect ice sheet balances.

Over the poles, cirrus clouds — thin, wispy, high-atmospheric clouds — have proven much lower and higher than the scientists expected. "It's always nice to get a few surprises," Spinherne says. The clouds, he says, might be associated with large weather systems, but he "doesn't quite know why" they are so different from what was expected and what has been observed for years over lower latitudes.

ICESat is also providing scientists with new information about aerosols in the atmosphere. These atmospheric particulates are the largest uncertainty in the global warming equation, Spinherne says. Knowing their vertical distribution, he says, "will help us figure out how they spread horizontally" and how they might be affecting global processes. For example, ICESat has provided images of the dust that blows off the Sahara into the Atlantic Ocean. Using those images, the researchers can better predict how far that dust will spread. And, in the fall, the satellite provided images of the smoke production from the fires in Southern California. The images of the structure within the smoke helped researchers figure out just how far the smoke would spread. They hope to use this technology in the future to predict how communities could be affected by nearby fires.

Megan Sever


ICESat Web site
"Polar satellite takes to the sky," Geotimes Web Extra, Jan. 13, 2003

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