Ice Sheet Loss Cut In Half
Posted by Jeff Id on August 27, 2010
Submitted by Doug L. Hoffman on Thu, 08/26/2010 – 13:22
Much concern has been raised by climate scientists regarding ice loss from the world’s two remaining continental ice sheets. Rapid loss of ice-mass from the glaciers of Greenland and Antarctica are cited as proof positive of global warming’s onslaught. The latest measurements involve the use of satellite gravimetry, estimating the mass of terrain beneath by detecting slight changes in gravity as a satellite passes overhead. But gravity measurements of ice-mass loss are complicated by glacial isostatic adjustments—compensation for the rise or fall of the underlying crustal material. A new article in Nature Geoscience describes an innovative approach employed to derive ice-mass changes from GRACE data. The report suggests significantly smaller overall ice-mass losses than previous estimates.
The storage of water or ice on land—the presence of large bodies of water or glacial ice sheets—affect the Earth’s gravitational field. This effect is detected by the NASA Gravity Recovery and Climate Experiment (GRACE) satellites. Twin satellites were launched in March 2002, to make detailed measurements of Earth’s gravity field. Since then, GRACE has been used to study tectonic features, estimate ground water volumes and calculate the amount of ice contained in the Greenland and Antarctica ice sheets. However, other factors can contribute to the GRACE measurements than just the volume of ice in an ice sheet. These factors include the response of Earth’s crust (the lithosphere) to past changes in ice load.
As the weight of covering ice varies, the underlying surface rock can be pushed down or rise up, buoyed by the magma that the crust floats on. This would obviously impact efforts to measure the height of terrain, including glaciers. Compensating for the rise and fall of bedrock is termed glacial isostatic adjustment, and it can have a significant impact on estimated ice-mass losses. Changes in the spatial distribution of the atmospheric and oceanic masses can also enter into the picture. Correctly assessing these different factors is the key to accurately calculating ice-sheet mass balance. Xiaoping Wu and colleagues have proposed a new method for untangling these factors from GRACE measurements. In a News and Views commentary on the work by Wu et al., David H. Bromwich and Julien P. Nicolas sum up the problem:
The atmospheric and oceanic contributions are commonly derived from global reanalyses or other global climate models that assimilate observations. However, the contribution from glacial isostatic adjustment is more difficult to evaluate because the Earth’s mantle is viscoelastic and therefore responds to changes in surface loading with a long delay. Indeed, the variations of the mass and extent of the ice sheets since the Last Glacial Maximum, about 20,000 years ago, continue to affect present-day changes in bedrock elevation. Assessments of the glacial isostatic adjustment typically rely on deglaciation models—which simulate the evolution of the ice sheets since the Last Glacial Maximum—together with assumptions about the viscosity profile of the mantle. Much is still unknown regarding the history of the ice sheets, and even less is known about the behaviour of the mantle in response to loading and unloading.
The method used by Wu et al., in “Simultaneous estimation of global present-day water transport and glacial isostatic adjustment,” estimates ice-mass changes and glacial isostatic adjustment simultaneously, instead of estimating the latter separately from deglaciation models as had been done before. The problem is expressed in terms of a single matrix equation, with the observed surface-height changes decomposed into their different contributions. The equation is then solved for ice-mass changes using matrix inversion. While the glacial isostatic adjustment that results is not directly generated by deglaciation models, the inversion method still requires a first-guess estimate to begin the calculations.
Ice loss in Greenland has been significantly overestimated.
In describing their work, Wu et al. state: “Here we combine gravity measurements and geodetic data of surface movement with a data-assimilating model of ocean bottom pressure to simultaneously estimate present-day water transport and glacial isostatic adjustment. We determine their separate contributions to movements in the geocentre, which occur in response to changes in the Earth’s mass distribution, with uncertainties below 0.1 mm yr−1.” They further describe their methodology as follows: