A new study that employs global positioning system measurements has revealed that the Greenland ice sheet is melting much faster than what previous estimates indicated, by roughly 7.6 percent. Previous estimates pegged the amount of ice loss between 2003 and 2013 at 2,500 billion tons, but the new study corrects this to 2,700 billion tons — a major factor in estimating the rate of future sea level rise.

But what went wrong with previous measurements? 40 million years ago, the portion of the planet’s crust that Greenland is part of passed over a hot region of the mantle, the region currently under Iceland that gives the island its famous volcanic activity. When Greenland passed over this hot spot, it softened the crust, making it relatively pliable. After having passed over the hot spot, the area began to accumulate ice that proceeded to weigh down the soft crust, pushing the island’s elevation lower.

However, after the last glacial maximum 20,000 years ago, Greenland began to lose a massive amount of ice. The reduced weight from this ice loss allowed the crust to slowly rebound upward, something that it has been doing so ever since.

Today, ice thickness is typically measured by satellites, recording the elevation of the ice’s surface, but this method hasn’t taken into account the possibility of the slow rising of the underlying crust. The result is that despite the ice sheet’s expected drop in height as it melts, the ground below the sheet is pushing upward, making it look like the ice isn’t melting as fast as it actually is.