Envisioning Ice Loss
After little net change in the 1990s, Greenland is now melting and shedding billions of tons of ice, according to NASA satellite observations. This trend especially concerns scientists because meltwater and ice emptying into the ocean raise global sea level. Currently, sea level is increasing at about 1.25 inches per decade, and researchers estimate Greenland is contributing about 15% of this rate. Greenland holds a great deal of ice; if all of it returned to the ocean, sea level would rise about 23 feet. (Such a loss would take many centuries to play out, even with substantially more warming than today.)
Why is Greenland losing ice? It appears linked in several ways to climate warming, which is strongest in the Arctic. First, surface melt of ice on Greenland has been increasing. Second, much of the meltwater drains to the base of glaciers and then lubricates the glaciers’ flow toward the sea. And finally, where the glaciers plunge into the ocean, warmer water appears to be eroding the glacial tongues that help hold flow back.
How do we know Greenland has been losing ice on balance? Data from two satellite missions have independently led to the same conclusion. ICESat has repeatedly measured the elevation profile of the Greenland Ice Sheet in great detail; changes over time, combined with estimates of ice compression and density, have allowed scientists to track changes in mass.Separately, the GRACE mission has provided a direct measure of mass change through time, through its unique “scale in the sky” capabilities. Before ICESat and GRACE, scientists used satellite radar altimetry and aircraft laser measurements to estimate Greenland’s ice mass, and estimated essentially no net change took place during the 1990s.)
This graphic is based on an estimated annual loss of 177 billion metric tons of ice from Greenland over the period 2004-2007.1 For each state, we then calculate, over its area footprint, the height of the water column required to match the annual mass lost, and then convert that height to a snowfall equivalent using the common ratio of 10 inches of snow per inch of water.
ICESat: Launched in January 2003, NASA’s Ice, Cloud and land Elevation Satellite (ICESat) carries laser altimeters capable of measuring the height of objects on and above Earth within an accuracy of roughly one inch, from an orbit over 400 miles high. The final laser out of three stopped working in October 2009; together, the lasers have completed just shy of two billion measurements, tracking small and large changes over time in the elevation of Arctic sea ice and of Greenland and Antarctic ice sheets, as well as cloud and aerosol heights, land topography, vegetation, and more.
GRACE: Launched in March 2002, NASA’s Gravity Recovery and Climate Experiment mission (GRACE) deploys two satellites that orbit the Earth in tandem. The pair measure the distance separating each other to an accuracy of 1% 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.
For additional information, see the Arctic Changes Collection.
- From S. B. Luthcke et al., NASA GSFC, Code 698, presentation of GRACE satellite data at the December 2009 annual conference of the American Geophysical Union. ↩