2009年10月26日

Calculating on the ice

Mass loss from marginal glaciers along the Greenland and Atrantic ice sheets is known to be contributing to sea-level rise but the precise magnitude and mechanisms are unclear, making it difficult to predict its potential future contribution to sealevel.

Surface mass loss is certainly a factor.

Ice loss from faster glacier flow ― called dynamical thinning ― has been more difficult to pin down.

High-resolution satellite altimetry measurements now show that dynamical thinning is for more important and extensive than previously thought, especially at ocean margins.


### DataBace ###
nature Vol.461 837-1018 Issue no.7266 15 October 2009
Letter p.971 / Extensive dynamic thinning on the margins of the Greenland and Antarctic ice sheets / H D Pritchard et al.



Many glaciers along the margins of the Greenland and Antarctic ice sheets are accelerating and for this reason, contribute increasingly to globalsea-level rise.

Globally, ice losses contribute 〜1.8 mm yr^-1, but this could increase if the retreat of ice shelves and tidewater glaciers further enhances the loss of grounded ice or initiates the large-scale collapse of vulnerable parts of the ice sheets.

Ice loss as a result of accelerated flow, known as dynamic thinning, is so poorly understood that its potential contribution to sea level over the twenty-first century remains unpredictable.

Thinning on the ice-sheet scale has been monitoerd by using repeat satellite altimetry observations to track small changes in surface elevation, but previous sensors could not resolve most fast-flowing coastal glaciers.

Here we report the use of high-resolution ICES at (Ice, Cloud and land Elevation Satellite) laser altimery to map chabge along the entire grounded marguns of the Greenland and Antarctic ice seets.

To isolate the dybamic signal, we compare rates of elevation change from both fast-flowing and show-flowing ice with those expected from surface mass-balance fluctuations.

We find that dynanuc thinning of glaciers now resches all latitudes in Greebland, has intensified on key Antarctic grounding lines, has endured for decades after ice-shelf collapse, penetrates for into the interior of each ice sheet and is spreading as ice shelves thin by ocean-driven melt.

In Greebland, glaciers frowing faster than 100 m yr^-1 thinned at an average rate of 0.84 m yr^-1 , and in the Amundsen Sea embayment of Antarctica, tinning exxceedeed 9.0 m yr^-1 for some glaciers.

Our results show that the most profound changes in the ice sheets currently result from glacier dybamics at ocean margins.


(… to be continue…)
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