Since 2002, the twin satellites of GRACE have made a complete map of Earth’s gravitational field every 30 days. Gravity is determined by mass. While most of the planet’s mass — its land and core — doesn’t move much in 30 days, its water and ice do, causing Earth’s gravity to shift. By tracking these changes, GRACE and GRACE-FO can identify how much ice sheets and glaciers are shrinking.

GRACE data are used extensively to determine mass changes of the world’s land ice (ice sheets, icefields, ice caps and mountain glaciers). Land ice continually adds mass through precipitation and loses mass via meltwater runoff and calving of solid ice into the ocean. If losses are greater than gains, land ice loses mass, causing sea level to rise. Over the last decade or so, losses from land ice have been implicated in causing two-thirds of the observed rise in sea level.

Researchers seek to further scientific understanding of ice-sheet dynamics and land-ice response to changes in atmospheric conditions and how these processes influence rates of sea-level change. GRACE data also provide information about seasonal and sub-seasonal variations in water transport between the continental land masses and oceans – of which the cryosphere (Earth’s frozen places) is an important component.

Data from the GRACE satellites show that ice sheets in both Antarctica and Greenland are losing mass. The continent of Antarctica has been losing about 118 gigatons of ice per year since 2002, while the Greenland ice sheet has been losing an estimated 281 gigatons per year. One gigaton equals a billion metric tons.

"Earth is losing a huge amount of ice to the ocean annually, and these new results will help us answer important questions in terms of both sea rise and how the planet's cold regions are responding to global change," said onetime University of Colorado Boulder physics professor John Wahr, who helped lead a study of the ice between 2003 and 2010. "The strength of GRACE is it sees all the mass in the system, even though its resolution is not high enough to allow us to determine separate contributions from each individual glacier."