THEGREENGROK    Planetary Watch

Update: The Slow Creep of Ice and Science

by Bill Chameides | September 15th, 2010
posted by Erica Rowell (Editor)

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The ice sheets of Greenland (shown here) and West Antarctic may not be melting as rapidly as previous studies had concluded, but the bad news is that even so, both ice sheets are losing mass. (NASA/Expedition 17 crew)

New estimate slows the rate of melting of the Greenland and Antarctic ice sheets.

When it comes to climate change impacts, sea-level rise tends to be front and center in most people’s minds. Over the last 50 years, it’s estimated that sea levels have risen by about 1.6 millimeters a year and that sea-level rise is accelerating (e.g., between 2004 and 2007 sea level rose by 2.4 millimeters a year).

What’s Causing Sea Levels to Rise?

There are two major contributors to that increase: expansion of ocean water in response to higher temperatures (thermal expansion) and increases in the amount of water from the melting of glaciers, ice caps and ice sheets.

Until recently, the largest contributor to sea-level rise has been thermal expansion. But over the last 10 years or so there is disagreement whether that balance has shifted as melting rates from land ice (glaciers and ice sheets) have accelerated. (See here and here.) More importantly, because we’re not certain how the melting rates will be affected by rising temperatures, there is a great deal of uncertainty in how much sea level will increase during the 21st century.

One of the ways of reducing that uncertainty is to carefully measure the rate of the ice sheets’ melting and try to understand what is determining that rate. Our ability to do this has been significantly enhanced with the 2002 launch of the Gravity Recovery and Climate Experiment (GRACE) into orbit. Before GRACE, estimates of ice loss depended upon cobbling together individual measurements of surface features. That all changed with GRACE which uses gravity measurements to effectively weigh the entire mass of ice. (For more information on GRACE see previous post and/or animations in sidebar.)

How GRACE works

Used with permission by the Center for Space Research at the University of Texas at Austin.

Technologically Advanced Methods for Measuring Melting Rates Not Without Challenges

But like all measurements, even GRACE has its complications. The biggest one being how to correct for the so-called post-glacial rebound.

As I wrote in an earlier post on ice mass and GRACE, post-glacial rebound, also termed glacial isostatic adjustment, refers to the movement in the Earth’s surface in response to the retreat of ice sheets since the last ice age.

It sounds complicated, but it’s quite simple. An ice sheet weighs a lot and so it exerts a lot of downward force on the ground below which causes the ground to sink down. When the ice sheet melts, that downward force goes away and the ground rebounds. But since the process involves the Earth’s crust and mantle, it happens very, very slowly.

This ongoing change in the Earth’s surface complicates the process of developing an accurate estimate of ice-sheet mass because the mass of the underlying rock must be subtracted out in order to infer the ice sheet’s mass at any one point. Any error in the estimate of the rock mass will get propagated as an error in the estimate of the ice mass. To accurately know the mass of rock, one must know the rate of this rebound.

Most analyses (see here, here and here) use models for post-glacial rebound to correct for the rock mass rebound, but at the same time these models are known to be poorly constrained and typically have large margins of error. In fact the preliminary study I wrote about last fall (same post linked to above) found that rates of ice-mass loss based on these models may increase the rate of ice-sheet mass loss by up to 30 percent.

Previous best estimates of the rate of ice loss using GRACE data and corrections for post-glacial rebound/glacial isostatic adjustment were in the range of 230 ± 33 gigatons of water per year for Greenland and 132 ± 26 gigatons of water per year for West Antarctic.

New Research Shows Ice Still Retreating but at Rates Slower Than Previously Thought

Now Xiaoping Wu, of the Jet Propulsion Laboratory at the California Institute of Technology, and colleagues have used a unique methodology to get at melting by simultaneously solving for mass loss and post-glacial rebound. Their methodology, as reported in a new paper in the journal Nature Geoscience, attributes more of the changing mass signal to post-glacial rebound than previous GRACE studies. Wu et al suggest that the rate of loss between 2002 and 2008 is about 50 percent slower than previous estimates (104+-23 gigatons/year and 64+-32 gigatons/year for Greenland and West Antarctic, respectively).

Wu et al go on to state that their downward revision of ice-mass contributions to sea-level rise is within the uncertainty range of recent sea-level rise budgets for similar time periods.

If you’re concerned about sea-level rise, this recent paper by Wu and colleagues is an example of what appears to be a bit of good news with some bad news lurking beneath the surface. The good news: the ice sheets of Greenland and West Antarctic may not be melting as rapidly as previous studies had concluded. The bad news: even so, both ice sheets are losing mass.

While this recommended downward revision in melting rates shows that we have a lot to learn about the climate system, there is an interesting counterpoint. It wasn’t too long ago that whether or not the Greenland and West Anta
rctic ice sheets were growing or shrinking was a major issue of contention in the climate change debate (see here, here and here). Now the shrinking ice sheets are so much of an accepted fact that Wu et al place little emphasis in their discussion on how their work confirms that the ice sheets are in fact losing mass.

While it may not always be apparent, scientific understanding is creeping forward. When it comes to climate change, rates of change and how much change are often questions that are hard to answer, but the direction of the change is pretty clear.

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1 Comment

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  1. Mauri Pelto
    Sep 19, 2010

    As part of the Arctic Report Card Hanna, Box and Huybrechts (2008) summarized the mass balance state of the Greenland Ice Sheet. The mass balance has been assessed using multiple measures only one of which is GRACE. Reading this report card will indicate that the bulk of the studies are closer to the Wu et al., (2010) results discussed above. Ten years we could not assess the mass balance of the ice sheet. Today we are still developing this skill. As usual it is best to rely on multiple data sets. We have laser altimetry, surface mass balance models and GRACE. A couple of sentences from the aforementioned authors …” A recent survey concludes that the GrIS is currently losing ~100 Gt yr-1 (Shepherd and Wingham 2007). However, there remains considerable discrepancy among these pioneering observational estimates. ” and “Airborne and satellite laser-altimetry data analyses indicate a volume loss of about 60 km3 yr-1 in the 1993/4 – 1998/9 period, that increased to about 80 km3 yr-1 in 1997-2003 (Krabill et al. 2004, Thomas et al. 2006). Various recent analyses of gravimetric (GRACE) satellite data suggest greater mass (volume) losses in the 101-226 Gt yr-1 (111-248 km3 yr-1) range within the recent few years, that is, 2002-2006 (Luthcke et al. 2006, Velicogna and Wahr 2006). ” The time periods and methods are not identical but do indicate that the low 100 Gt yr-1 has been a frequent result.

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