Polar ice loss rates may need to be lowered, say scientists

Ice sheets at Earth’s poles might be losing ice only half as fast as the high-end of previous estimates, according to scientists from NASA and the Netherlands. Their results were published in the Aug 15 issue of Nature GeoScience.

NASA’s Grace satellites, the Gravity Recovery and Climate Experiment, were used to in effect to weigh the polar land ice and determine the loss from year-to-year.  Grace land ice measurements were taken for Greenland, West Antarctica, and Alaska, from 2002 to 2008, by measuring tiny changes in the satellites orbit from the pull of the Earth.  As land ice melts, the Earth’s crust rebounds up in response to the removed weight of the ice. Combined with measurements from NASA’s ICESat (Ice, Cloud,and land Elevation Satellite), which measures the elevation of ice, the researchers determined the changes to the volume of ice.

The revised estimates for loss of ice in Greenland, for example,  are 104 +- 23 gigatons of ice loss per year, compared to prior estimates as high as 171 +- 4 gigatons per year.  The researchers write that this difference can be explained by better estimates of uplift also caused by the retreat of glaciers that once covered large areas of Earth, which include the upper U.S., Northern Europe, Canada, Chile, and Argentina 20,000 years ago.

Jay Zwally, Chief Cryospheric Scientist at NASA’s Goddard Space Flight Center, commented by email on the ice loss study.

“It is good.  Except their value for Greenland in not new, and their half applies to their selection of the biggest numbers published.  It is about the same as others, published,” wrote Zwally.

Richard Alley gave EarthSky his comments. Dr. Alley is Evan Pugh Professor in the Department of Geosciences and also at the Earth and Environmental Systems Institute of The Pennsylvania State University.

“The long story: We have known for a long time that sea level is high in warm times because ice melts, and low in cold times because ice grows, so we expect warming to raise sea level.  But, the path from here to there could have curves.  In particular, we also know that a lot of Antarctica and central Greenland is too cold to melt without a lot of warming, and that warmer air can carry more moisture to give snowfall, so it is possible that a little warming might cause a little growth in Antarctica and central Greenland, at least for a while, even if too much warming eventually causes shrinkage of the ice sheets and sea-level rise.

In 2001, the IPCC report looking at sea-level rise discussed the great uncertainties with the ice sheets.  Warming would raise sea level by increasing melting of small glaciers in the mountains, and by warming ocean water and causing it to expand, but the best estimate was that the ice sheets would offset a little of this change over the next century by growing a little, with increasing snowfall in central regions exceeding increasing melting in coastal Greenland, and with little change in ice flow.

In 2007, the IPCC noted that the best estimates gave shrinkage of the ice sheets, that this shrinkage wasn’t huge, but that this shrinkage came at least in part from changes in ice flow that had not been predicted,   The IPCC thus could not provide useful estimates of total future sea-level rise; instead, estimates were provided for everything except future rapid dynamical changes in ice flow, which was left for future work.

The estimates that the ice sheets were shrinking came from several sources. One approach is to measure or model the snowfall rate, the melt rate, and the rate at which ice flows to the sea to make icebergs.  If the melt and icebergs exceed the flow, the ice sheet is shrinking.

Another approach is to measure changes in surface elevation of the ice sheets, using lasers or radars on aircraft or satellites, correct for any changes in snow density (snow is squeezed to ice by the weight of more snow, so changes in snowfall can affect the amount of air that is under the snow surface, and this must be corrected for) and for the small effect of rising or falling bedrock, and see whether ice is being added or subtracted.

Other approaches use sophisticated geodesy.  The most commonly discussed is to “weigh” the ice sheets using the GRACE gravity satellites, and see whether the ice sheets are gaining or losing mass.  This is affected a good bit by changes in bedrock. (Rock being denser than ice, a little change in the rock has more effect on gravity than on elevation.)  If the ice was bigger in the past, the weight of the ice pushed the surface of the Earth down; this caused the hot deep rock down in the asthenosphere to move away to beneath surrounding regions.  Loss of ice (such as at the end of the ice age) then allows the deep rock to flow back.  Think of a water bed with maple syrup–when you get up, the dent left by your seat takes a while to fill.  People also use the rate of change of the length of the day, and the direction the North Pole points. Melting ice near the poles and putting it into the oceans (which are on average closer to the equator) is like an ice skater extending her arms to slow a spin.  If a spinning ice skater stuck out one arm at a funny angle, the spin would become lopsided; Greenland (and Alaska) are not right at the pole, so changing them changes the wobble in the Earth’s spin.

The new paper advances the science of the geodetic approaches, bringing more data together in one analysis to get a better picture of what is going on.

… the main IPCC conclusions–mass loss well ahead of previous expectations–are confirmed. The mass loss is in places of known flow acceleration, so our understanding of causes of changes is not affected either, nor is our ability to predict future changes.

For those of us trying to get things just right, and to understand all the changes in the Earth system, this is an important paper, and will generate a lot of additional work and careful consideration.  For anyone in the public who has been informed by the IPCC, this changes nothing–the estimates of mass change are not overturned, this falls within the range of prior estimates, it is consistent with the statements in the IPCC, and does not (at least yet) change projections of sea-level rise.”

September 8, 2010

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