The effort to determine how fast the ice sheets that blanket Greenland and Antarctica are melting is complicated by something called "postglacial rebound." As the earth's crust is relieved of its ­millennia-­long burden of ice, it recovers its original shape. The rebound of the bedrock underlying the ice can confuse measurements of the ice's thickness and mass.

To correct for this, a team of scientists from the U.S., Denmark, and Luxembourg installed 24 continuous GPS stations in bedrock around the coast of Greenland this summer. At year's end, they'll head for Antarctica to install 16 more. The project involves researchers from Ohio State University and engineers from Unavco of Boulder, CO. The stations, powered by solar panels and large battery packs, can measure lateral and vertical shifts of the earth's crust down to the millimeter scale. Equally important, they continuously beam out their readings. The data they generate should allow other sensors--which monitor elevation changes, glacial outflow rates, and the overall mass of the great ice sheets--to measure the rate of ice loss with greater accuracy.