Keep Watching the Ice
Meet the satellites bringing data to the discussion of global warming
- By Ben Iannotta
- Air & Space magazine, September 2006
(Page 3 of 7)
When the lead GRACE satellite reaches a point in its orbital path above an area of increased mass, such as a mountain or ice sheet, the corresponding increased gravity tug slows it down and changes ever so slightly the distance between it and its twin. Microwave signals that are continuously passed between the two satellites now arrive at slightly different points along their wavelengths than they did before the slight disruption. GRACE’s electronics detect the change, and software measures the change in distance between the satellites to an accuracy of one micron, or a millionth of a meter.
A team of researchers at the University of Texas’ Center for Space Research in Austin converts that data into monthly maps that are color-coded to show changes in the distribution of Earth’s mass—primarily the movement of water. Other researchers use the data to explore the rate of the planet’s ice loss. Isabella Velicogna and John Wahr, satellite researchers at the University of Colorado in Boulder, wrote what is perhaps the most stunning GRACE-based report. The two examined the data and reported in the March 2, 2006 issue of Science that Antarctica is losing 36 cubic miles of ice a year. That’s enough to fill Lake Erie in three years.
The job facing Velicogna and Wahr was not quite as simple as placing the entire Antarctic ice sheet on the equivalent of a bathroom scale. Earth’s crust is still rising in some places and subsiding in others in reaction to the weight lifted by the melting of the glaciers at the end of the last ice age, more than 10,000 years ago. The scientists had to factor out the post-glacial rebound. “We use some models to remove this signal, and then what’s left is the mass change of the ice,” Velicogna explains.
Zwally doesn’t doubt that Antarctica’s ice cover is shrinking, and the effect over the centuries could be devastating, but he thinks Velicogna and Wahr have the scale wrong. “Too big a number for Antarctica,” he says tersely over the phone, while preparing to leave for Greenland. He later adds that a true accounting of ice being lost will be calibrated by ICESat, aiding scientists’ calculations of the resultant rise in sea levels.
The strength of GRACE is that it accurately measures mass, providing hard numbers for the amount of ice lost, but only on geographic scales large enough to produce a gravity tug that can be sensed.
ICESat works nothing like GRACE, which is why glaciologists are curious to see whether the findings from the two projects corroborate each other.
When Zwally started advocating for ICESat among his colleagues, he knew the potential of using altimeters to measure ice, but also saw the limits of flying them on airplanes, which was the common method. Only a satellite orbiting from pole to pole could provide the coverage necessary to monitor content across an entire continent or subcontinent. Radar satellites were on the horizon, but Zwally knew those readings would be much less precise because radar wavelengths are longer than those of laser light. “Satellite radar can’t get all the way to the edges of ice,” Zwally says. “There’s penetration too. The radar penetrates some distance into the snow. And that has some variability depending on the properties of the snow. The laser measures directly to the surface.”
ICESat measures the height of Earth’s surface every 660 feet, so it can examine relatively small features. “What ICESat can do is provide very high resolution on a particular track: What’s happening on this glacier?” says Jim Abshire, a soft-spoken native of Tennessee whose job is to ensure the accuracy of ICESat’s laser