New GPS measurements reported in the paper found that in some areas (where the mantle might be a little springier), Greenland's underlying land mass was rising by nearly half an inch (12 mm) every year as ice is removed. That's a big deal because many measurements of the thickness of the Greenland Ice Sheet are based on researchers measuring the elevation of the surface of the ice. The reasoning is if the ice is taller than it was last time measurements were taken, then the ice sheet is growing. If it's shorter, than the ice sheet is shrinking. And by taking many different measurements across Greenland, researchers can build up a picture of how much ice is being lost over time. These calculations do take postglacial rebound into account, but the new research suggests that Greenland is bouncing back faster then previous studies had accounted for.