GLIMS Miniworkshop: Erratum2! 100 m thinning/year!

[Jeff Kargel]

Dear Colleagues,

I must pass on this second erratum to the GLIMS Miniworkshop summary.  How
intriguing!  This is from Gordon Hamilton, responding to my errant
characterization of Bill Sneed's great presentation of the Maine group's
work on Svalbard glaciers:

Kargel and Raup wrote (and error was Kargel's):

> Said they saw 100 m thinning based on ASTER DEMs 2004-2005.
> 
> I THINK THIS WAS OVER AN EXPANSE OF DECADES LEADING UP TO 2004-5.

Actually, believe it or not, the 100 m elevation difference was between July
2004 and July 2005, based on differencing ASTER-derived DEMs. Sounds like a
lot, but we surveyed similar elevation changes on the same glaciers in the
field this summer, so we don't doubt these changes are real. These glaciers
are changing big time (5 km retreat in one year, 100 m vertical thinning in
one year, and acceleration from 5 km/yr in 2004 to 14 km/yr in 2005!). We
will be talking about these observations at AGU and in Palm Springs.
========================

Some further description by Hamilton:


Both glaciers terminate in tidewater. Our first interpretation is that there
was a small amount of thinning (either caused by anomalous summer melting or
warm ocean water) that floated the glaciers off their pinning points and
caused them to retreat. The change in buttressing then caused the glaciers
to accelerate, and this in turn pulled ice out of the system and caused the
rapid thinning. The same thing was observed on Jakobshavn Isbrae in West
Greenland a year or so prior to the events we witnessed in SE Greenland.

The abstract is posted below, or can be found on the web at
http://www.agu.org/cgi-bin/SFgate/SFgate?&listenv=table&multiple=1&range=1&d
irectget=1&application=fm05&database=%2Fdata%2Fepubs%2Fwais%2Findexes%2Ffm05
%2Ffm05&maxhits=200&=%22C41A-03%22


HR:  08:30h
AN: C41A-03
TI: Rapid Changes of Large Tidewater Glaciers in SE Greenland
AU: * Stearns, L A
EM: leigh.stearns at maine.edu
AF: Climate Change Institute, University of Maine, 5790 BGSC, Orono, ME
04469 United States
AU: Hamilton, G S
EM: gordon.hamilton at maine.edu
AF: Climate Change Institute, University of Maine, 5790 BGSC, Orono, ME
04469 United States
AB: New field and satellite remote sensing measurements show that
Kangerdlugssuaq Glacier and Helheim Glacier, two fast-flowing tidewater
glaciers in South-East Greenland, accelerated 40-300% between 2001 and 2005
and retreated 3-5 km since July 2003. Together, the catchment basins of
these two glaciers encompass ~10% of the area of the Greenland ice sheet.
Previous studies observed rates of surface lowering on the main trunks of
both glaciers that were too large to be caused by enhanced surface melting
or decreased snow fall alone. One hypothesis to explain the thinning rates
is a change in ice dynamics. We use repeat satellite imagery and published
reports to reconstruct the last ~decade of flow histories for both glaciers
and compare the results with velocities derived from field GPS surveys in
the summer 2005. Helheim Glacier was flowing at ~8 km/yr in 1995 and 2001.
In 2005, flow speeds were ~11.7 km/yr, a ~40% increase. The acceleration of
Kangerdlugssuaq Glacier was mo
re substantial. Portions of the main trunk that were flowing at ~5 km/yr in
1988, 1996 and 2001 were flowing at ~14 km/yr in summer 2005, an almost
threefold increase. The accelerations in flow speeds were accompanied by
other changes, including the rapid retreat of calving fronts that had
maintained quasi-stable positions for the previous ~40 years, and a lowering
of the ice surface by about 100 m, leaving stranded ice on adjacent ridges.
The rapid thinning, acceleration and retreat of these two relatively nearby
glaciers suggests a common triggering mechanism, such as enhanced surface
melting due to regional climate warming. The current flow speeds, ~11 - 14
km/yr at the terminus, are too fast to be caused solely by internal
deformation of the ice, implying that an increase in basal sliding forced by
additional meltwater production is the probable cause of the velocity
increases. The new observations and the hypothesized cause highlight the
sensitivity of large outlet glaci
ers to local climate changes, and have implications for ice sheet mass
balance and sea level rise predictions.
DE: 0720 Glaciers
DE: 0726 Ice sheets
DE: 0758 Remote sensing
DE: 0776 Glaciology (1621, 1827, 1863)
SC: Cryosphere [C]
MN: Fall Meeting 2005