The surest sign of a warming Earth is the steady melting of its ice zones, from disappearing sea ice in the Arctic to shrinking glaciers worldwide. Now, scientists are using increasingly sophisticated satellite technology to measure the extent, thickness, and height of ice, assembling an essential picture of a planet in transition.
by michael d. lemonick
After carbon dioxide, the substance most crucial in determining how climate change will play out over the next century and beyond isn’t a greenhouse gas — it’s the solid state of the molecule H20. Summer melt in the sea ice that covers the Arctic Ocean exposes heat-absorbing seawater to the sun, accelerating global warming in a phenomenon known as Arctic amplification. Summer melting in the land-based ice that covers Greenland is increasingly responsible for the sea-level rise that has already begun to endanger many thousands of miles of coastline. Glaciers moving more rapidly to the sea, in both Greenland and Antarctica, threaten to raise sea level even higher, while disappearing mountain glaciers around the world could choke off water supplies to many hundreds of millions of people.
That being the case, it’s crucial to know exactly what’s happening to the ice as the planet warms up. So it’s easy to understand why the European Space Agency was horrified when its ice-measuring CryoSat satellite failed on launch back in October 2005 — and that the agency was thrilled when CryoSat-2 lifted off successfully this April from the Baikonur Cosmodrome in Kazakhstan. The satellite is now in its shakedown phase, where scientists check out the instruments and solar panels, and it seems to be working perfectly. “There’s no precedent for such a rapid recovery,” says CryoSat Mission Scientist Mark Drinkwater. “We’re up there, collecting great data.”
It’s not that ice-watchers have been flying blind since 2005. A virtual flotilla of satellites, some dedicated entirely to looking down on the planet’s white places and others only partly devoted to sensing ice, has been whirling overhead for more than three decades. The result has been an unbroken record of observations, starting with NASA’s Nimbus-7 satellite in 1978, showing such key phenomena as the steady, three-decade disappearance of Arctic sea ice. Since the late-1970s, NASA and space agencies from other nations have devised an array of detectors to measure ice from space, using wavelengths spanning the electromagnetic spectrum, from microwaves, to infrared, to visible light.
Just as cell phones, computers, DVD players, and other once-exotic devices have plummeted in price and soared in capability over the years, so have the electronics at the heart of high-flying satellites. The satellites’ vision has in effect become sharper, and the data they return richer in detail. Satellites have even managed to transcend light entirely: the GRACE satellite array now routinely measures changes in local gravity resulting from mass losses in ice sheets.
But each orbiter has a limited lifetime even when it’s working perfectly, and some break down prematurely. NASA’s ICESat — which used lasers to measure the height of ice formations and had already exceeded its expected five-year lifetime — stopped working last October. That’s potentially a huge problem, says Claire Parkinson, of NASA’s Goddard Spaceflight Center, because “we really want a continuous record.” The reason: ice-watchers don’t want to miss any unexpected events, such as the spectacular break-up of Antarctica’s Larsen B Ice shelf in 2002 or the extreme plunge in late-summer Arctic sea ice in 2007. Scientists also want to be sure that observations from different satellites can be calibrated against one another.