Between 2003 and 2005, Greenland's low coastal areas shed 155 gigatons (41 cubic miles) of ice per year, while snow accumulation in the interior of the ice sheet was only 54 gigatons per year. The amount of ice lost in two years is roughly the same as the amount of water that flows through the Colorado River in 12 years.
The GRACE satellites sense changes in mass beneath them by responding to changes in gravitational force. As the twin satellites orbit the Earth in tandem, the distance between them changes as a result of changes in the concentration of mass on the Earth below.
(Now that part's kinda awesome: they used the satellites responsible for this gravity model, the data of which is normally intended for, basically, making sure your ICBMs land where you meant them to.)
Overall, Greenland lost 20 percent more mass than it received in snowfall each year. These results are consistent with overall trends in ice loss that other types of observations of Greenland have documented, including radar-based estimates of accelerating glacier flow off the ice sheet.
According to one of the study's authors, Jay Zwally of NASA's Goddard Space Flight Center, "This is a very large change in a very short time. In the 1990s, the ice sheet was growing inland and shrinking significantly at the edges, which is what climate models predicted as a result of global warming. Now the processes of mass loss are clearly beginning to dominate the inland growth, and we are only in the early stages of the climate warming predicted for this century."