On ice - but for how long?
Global snapshots are great for taking pictures over the oceans, and the poles. Logically, polar ice is a focus of the latest EOS mission, IceSat. IceSat carries a laser altimeter to measure the height of ice. This gadget works like radar, sending out a light beam and recording how long it takes to bounce back off the ice.

IceSat is expected to explore a planetary catastrophe scenario. You'll recall that the Antarctic ice sheet contains 90 percent of the planet's solid water - about 3 million cubic kilometers. When ice on land slides into the ocean, it raises sea level, a major concern in the era of global warming. The current rate of rise is 1 to 2.5 millimeters per year.

If the relatively unstable ice in the West Antarctic Ice Sheet winds up in the ocean, sea level will rise by a few meters, and you can kiss low-lying coastal regions goodbye. They are home to billions of people.

Above: EOS satellite photo of the Larsen B ice shelf, a mass of floating ice on the east side of the Antarctic Peninsula, Jan. 31, 2002. Photo from NASA.

Below: Larsen B had largely busted up by Mar. 5, 2002, leaving thousands of icebergs drifting on the Weddell Sea. About 3,250 square kilometers of shelf area disintegrated in 35 days. Floating ice does not affect sea level, but the breakup is another sign of steady warming of the local climate. Photo from NASA .

See sea level rise
The Larsen ice shelf, shown above, was already floating, so it did not change sea level when it broke up and started to melt. But if the ice streams resting on land behind an ice sheet reach the ocean, sea level could rise quickly.

After the Larsen B bust-up, five of the six glaciers feeding it began moving twice as fast as before, says Charles Bentley, a professor emeritus of geophysics at the University of Wisconsin-Madison. He says similar rapid movement "could happen elsewhere" if ice shelves disappear, but that "is by no means certain."

Here is the working (telescope) end of IceSat, photographed from the comforts of a clean room here on Earth. Photo from NASA.

Bentley, who has studied the Antarctic for more than 40 years, says early warnings that the ice sheet on land might disappear in a century have been discounted. That's the good news. The bad news is that the ice seems much less stable than some had thought, so its overall fate is uncertain. "A lot of [research] has been coming out," Bentley says, "but it's not cleared up the mystery about how the ice will affect sea level. The more we learn about the ice sheet's past, the more ... we see change practically everywhere."

Predicting the future may be a fool's errand, but to Bentley, a strong history of change means "the likelihood of future change is higher."

High, and highly accurate
Bentley, who was a member of the science team for IceSat's laser altimeter, says early data indicate that each measurement will be within 10 centimeters of reality, about 10 times as accurate as today's measurements. When readings are averaged over a large area, he expects even greater accuracy.

And that should help IceSat track the rise and fall of Antarctic ice, leading to better forecasts of future sea level.

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