One of the wildcards of global warming is how fast the world's massive ice sheets are melting. Efforts to measure the rate of melting on Greenland and Antarctica--and thereby predict how fast sea levels will rise--are complicated by something called "post-glacial rebound" of the earth's crust. When the crust is relieved of its millennia-long burden of ice, it shifts around a bit, and springs back.

In the first comprehensive effort to correct for this phenomenon, scientists from an international team are scurrying around Greenland this summer to install 24 continuous GPS stations into bedrock around the coast. They're also rigging them up with solar panels and large battery packs to keep them powered up through bitterly cold winters.

A one-meter-tall station was installed last Thursday near Ilulissat, on the west coast of Greenland, by members of the team from Denmark, Luxembourg, and the United States. Today, a team that includes Mike Willis, a PhD candidate at Ohio State, and Thomas Nylen, an engineer from the contractor UNAVCO of Boulder, CO, is flying out to the east coast to install one near Kulusuk.

The stations can detect lateral and vertical changes of the Earth's crust down to the millimeter scale. Equally important, they'll continuously beam out their readings. This data should allow other sensors--which monitor elevation changes, glacial outflow rates, and overall mass of the great ice sheets--to become far more accurate in measuring the rate of ice loss. The international team plans on installing 16 stations in Antarctica later this year as part of the project.

The units should be of particular help in complementing a NASA satellite system called GRACE--Gravity Recovery and Climate Experiment. GRACE takes readings on ice-sheet mass, as inferred by changes in gravitational pull, every 30 days. While GRACE has transformed scientists' understanding of ice sheet changes, it cannot tell with precision which areas of an ice sheet are losing mass. Nor can it directly correct for post-glacial rebound.

Greenland's and Antarctica's ice sheets contain enough ice to raise sea levels 70 meters, if it all melted. And in recent years, evidence has been mounting for accelerating ice loss. The additional information provided by the continuous GPS stations could help researchers improve the accuracy of their measurements.

David Talbot is reporting from Kangerlussuaq, Greenland.