Over the last decade or so, scientists have tracked a significant loss of ice from the Greenland Ice Sheet (GIS). While some of that loss has been as a direct result of surface melting, most of it presently appears to be a result of warmer ocean waters melting the ice tongues that stretch out into fjords. Essentially, the warmer water melts the bottom of the glacier and makes it more likely to break up, and as the ice tongue breaks up, the glacier behind the tongue starts to move faster, dumping yet more ice into the ocean.
There has been a significant amount of study of the GIS, and multiple independent lines of evidence have shown that Greenland’s glaciers are thinning and thus losing mass. These include satellite radar altimetry, the GRACE gravity mapping satellites, and both airborne and satellite laser altimetry. Now a peer-reviewed paper published in March shows that another analysis of GRACE and new GPS data has found that mass loss has spread from the warmer southeast coast to the comparably cooler northwest coast, significantly increasing the amount of Greenland coastline affected by mass loss.
The paper looks at two things – how the location of mass loss has changed over the course of the GRACE mission, and how the GRACE data compares to GPS data. The GRACE satellites measure how much the force of gravity affects two satellites differently as they orbit the Earth, and so the mass below the satellites can be calculated. When the GRACE satellites show a change in the force of gravity from one year to another, that means that the amount of mass below the satellite has changed. And with some knowledge of the area, the source of that change in mass can be determined. In the case of Greenland, it’s the thinning and melting of glaciers that drain the GIS. The movie below, part of the paper’s supplementary material, shows how the mass distribution on Greenland has changed since January 2003.
Many of us are familiar with the basic concept of rebound from boats or ships – take the mass out of a boat and it rises higher in the water as result. The same basic idea applies to the Earth’s crust too – move mass from one place to another and the elevation of the first place will rise while the elevation of the other will fall. In the case of Greenland, when millions of tons of ice leave the crust and enter the water via melting and calving icebergs, there is less mass weighing down the land and so it rises. Because GRACE measures mass changes, the GRACE data can be used to calculate an approximate amount of change in Greenland’s elevation as a result of the changes in mass. The paper compares this calculated uplift to observed GPS data taken from three different locations. After adjusting the GRACE data to account for a much larger spatial scale than the GPS measurements, the results show that the calculated GRACE uplift estimates are very similar to the GPS measurements and show the same overall structure.
The left image compares the GPS uplift measurements (adjusted using different corrections derived from the Jet Propulsion Laboratory and MIT) at a location with relatively stable mass to that of GRACE, the center image shows the same comparisons at a GPS station on the southeast coast of Greenland (the area that has seen the most mass loss), and the right image shows the same comparison on the northwest coast where mass losses have recently appeared.
Finally, this paper presents another independent source of evidence that Greenland is melting. The images below show radar altimetry, laser altimetry, and GRACE-derived uplift data from left to right. All show reductions in mass or glacial thinning in the same coastal areas of Greenland.
While the scales are difficult to read, purple and blue in the radar altimetry image (right) show the greatest reduction in ice elevation, red shows the greatest reduction in ice elevation in the laser altimetry image (center), and purple shows the fastest rate of isostatic uplift in the GRACE image (left).
According to the paper’s conclusions, we can expect yet another independent source in a few years – when all 51 permanent GPS stations scattered around Greenland have collected enough data to determine statistically significant trends in the uplift, and thus loss of ice mass, all around Greenland. The top image shows the Greenland GPS Network, or GNET.
Greenland GPS Network (GNET)
Geophysical Research Letters
IPCC AR4 WG1 Figure 4.17
Thanks to Dr. John Wahr of CIRES for a review copy of this paper and for the GRACE video