Gravity-measuring satellites and GPS confirm Greenland ice melting, affecting more of Greenland Ice Sheet

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.

Image Credits:
Greenland GPS Network (GNET)
Geophysical Research Letters
Nature
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

3 comments on “Gravity-measuring satellites and GPS confirm Greenland ice melting, affecting more of Greenland Ice Sheet

  1. Brian,

    Dumb question time from the guy over at the humanities building:

    We in the East had an especially snowy, cold winter this year. I know El Nino’s presence is an influence, but (here comes the dumb question) could Greenland/Arctic ice mass loss be causing lowering of Atlantic Ocean temps (I know the change might be minute, but even that can be climatically significant) also be having some effect? If so, wouldn’t this be only a temporary situation and as GH continues and accelerates this would end?

  2. What I’ve read is that a weather pattern known as the Arctic Oscillation was largely responsible for the cold and wet weather back east this past winter, rather than El Nino. What basically happens is a high pressure system over the Arctic drives cold weather further south, and when the cold air hits warm air holding lots of water, you get LOTS of snow. Here’s a link that shows the “cold/negative” (this past winter) vs. “warm/positive” phases.

    Scientists love creating indices to track things, and the Arctic Oscillation is no different – the more negative the AO Index is, the colder the eastern US is, and as you can see from the image below, the AO Index was right about as negative as it’s ever been in the history of the index.

    So no, I don’t think that ice melt from Greenland had anything to do with this year’s odd weather in your neck of the woods. It’s also unlikely that the melt will have any significant effect on ocean temperatures beyond very locally (ie very close to the point at which the ice is melting) – it will have a major effect on ocean salinity, however, and a paper I read in the last year indicated that reduced salinity could slow the Gulf Stream several tens of percent, enough to cool Europe a little (and make it a little more like Montana, southern Alaska, and northern British Columbia, which have similar latitudes). Keep in mind that prevailing winds are from west to east and from the equator toward the poles, so Greenland’s melt will have much less direct impact on the US than it will on Europe.

    I’ll point out something that, if you look closely, doesn’t necessarily fit what I told you above about the AO. If you looked closely at the first link, you’ll notice that usually Western Europe is drier than usual in a cold phase AO, and the UK was most definitely NOT drier this year. I haven’t seen any good explanations for why, but I can propose a possible explanation – it could be a side-effect of the collision of cold and dry air with warm, wet air from over the Gulf Stream. Warm air holds more water vapor than cold air, and when the warm air is suddenly cooled, all that water vapors condenses and precipitates out. When the new air temperature is below freezing, that means snow. And the greatest snowfall amounts always happen when the temperature is just below freezing, because that’s when the air can hold the most water vapor to start with.

  3. Im currently carrying out a study looking at Greenland melting and te resultant change on a GPS satellites orbit. The question is as follows; Imagine that the Greenland ice mass melts & is distributed around the global ocean evenly. Roughly estimate the resulting maximum possible change in the predicted position of a GPS satellite over one orbit, due to the changed gravitational force. Show and discuss your calculations clearly, & state, with reasons, the error bars of your answer. If any light could be shed into this topic and which areas should be concentrated on, that would be much appreciated.

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