[asa] Yet Again, Ice Melt is Worse than We Previously Thought

Published online today by *Nature*:

http://www.nature.com/nature/journal/vaop/ncurrent/full/nature08471.html

Extensive dynamic thinning on the margins of the Greenland and Antarctic ice
sheets
Many glaciers along the margins of the Greenland and Antarctic ice sheets
are accelerating and, for this reason, contribute increasingly to global
sea-level rise1,
<http://www.nature.com/nature/journal/vaop/ncurrent/full/nature08471.html#B1>2,
<http://www.nature.com/nature/journal/vaop/ncurrent/full/nature08471.html#B2>3,
<http://www.nature.com/nature/journal/vaop/ncurrent/full/nature08471.html#B3>4,
<http://www.nature.com/nature/journal/vaop/ncurrent/full/nature08471.html#B4>5,
<http://www.nature.com/nature/journal/vaop/ncurrent/full/nature08471.html#B5>6,
<http://www.nature.com/nature/journal/vaop/ncurrent/full/nature08471.html#B6>
7<http://www.nature.com/nature/journal/vaop/ncurrent/full/nature08471.html#B7>.
Globally, ice losses contribute [image: approx]1.8 mm yr-1 (ref.
8<http://www.nature.com/nature/journal/vaop/ncurrent/full/nature08471.html#B8>),
but this could increase if the retreat of ice shelves and tidewater glaciers
further enhances the loss of grounded
ice9<http://www.nature.com/nature/journal/vaop/ncurrent/full/nature08471.html#B9>or
initiates the large-scale collapse of vulnerable parts of the ice
sheets
10<http://www.nature.com/nature/journal/vaop/ncurrent/full/nature08471.html#B10>.
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
unpredictable11<http://www.nature.com/nature/journal/vaop/ncurrent/full/nature08471.html#B11>.
Thinning on the ice-sheet scale has been monitored by using repeat satellite
altimetry observations to track small changes in surface elevation, but
previous sensors could not resolve most fast-flowing coastal
glaciers12<http://www.nature.com/nature/journal/vaop/ncurrent/full/nature08471.html#B12>.
Here we report the use of high-resolution ICESat (Ice, Cloud and land
Elevation Satellite) laser altimetry to map change along the entire grounded
margins of the Greenland and Antarctic ice sheets. To isolate the dynamic
signal, we compare rates of elevation change from both fast-flowing and
slow-flowing ice with those expected from surface mass-balance fluctuations.
We find that dynamic thinning of glaciers now reaches all latitudes in
Greenland, has intensified on key Antarctic grounding lines, has endured for
decades after ice-shelf collapse, penetrates far into the interior of each
ice sheet and is spreading as ice shelves thin by ocean-driven melt. In
Greenland, glaciers flowing 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,
thinning exceeded 9.0 m yr-1 for some glaciers. Our results show that the
most profound changes in the ice sheets currently result from glacier
dynamics at ocean margins.

This is really, really bad. It's not just the warming air temperature but
the water and the flowing of the water in both the Greenland and Antarctic
ice sheets that is producing a positive feedback loop. Water begats more
melting which begats more water, etc. etc.

The Associated Press quoted scientists not part of this study to assess its
gravity:

As scientists watch ice shelves retreat or just plain collapse, some thought
> the problem could slow or be temporary. The latest measurements eliminate
> "the most optimistic view," said Penn State University professor Richard
> Alley, who wasn't part of the study.
>
> The research found that 81 of the 111 Greenland glaciers surveyed are
> thinning at an accelerating, self-feeding pace.
>
> The key problem is not heat in the air, but the water near the ice sheets,
> Pritchard said. The water is not just warmer but its circulation is also
> adding to the melt.
>
> "It is alarming," said Jason Box of Ohio State University, who also wasn't
> part of the study.
>
> Worsening data, including this report, keep proving "that we're
> underestimating" how sensitive the ice sheets are to changes, he said.
>

Rich Blinne
Member ASA

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Received on Wed Sep 23 18:30:05 2009