A second mission, ICESat (Ice, Cloud and land Elevation Satellite), ran until 2009 and measured, among other things, Earth's elevation across the globe. These elevation measurements also captured changes in ice thickness.
Bringing it all together Gardner and his colleagues compared the data from these satellite missions directly. The two have different strengths, he said. GRACE, for example, is not sensitive enough to tell the difference between ice lost from ice sheets and ice lost from glaciers right next door to ice sheets - which include about 30 percent of the world's glaciers.
For large regions, Gardner said, GRACE and ICESat's measurements are nonetheless in close agreement. In contrast, in icy regions of less than about 1,900 square miles (5,000 square kilometers), the satellite measurements didn't agree as clearly.
That lack of agreement is because GRACE doesn't always catch melt in small glaciers, which are often in mountainous regions surrounded by lakes and groundwater.
"What if the glaciers are melting but the lake is filling?" Gardner said. "GRACE sees that nothing happened, because no mass was actually removed from that region."