Greenland is covered in so much ice that if it all melted, it would raise sea level around the world by 23 feet. While it's not all melting, scientists say that climate change is turning a lot of that ice to water. But, how do we know?

Two GRACE (Gravity Recovery and Climate Experiment) satellites that orbit the Earth in tandem were launched in March 2002 by NASA. The pair measure the distance separating each other to an accuracy of 1 percent of the width of a human hair — and they orbit as far apart as Washington, DC and Philadelphia. Because each satellite accelerates or decelerates depending on the mass of the area beneath it (for example, a massive mountain range vs. flat lowlands), and because one satellite trails the other at some distance, the record of the shifting distance between them can be read like a giant planetary scale. And since they orbit over the same areas every ten days, the GRACE satellites provide a detailed record of mass changes in time, even tracking the seasonal accumulation and melting of Arctic snow.

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