Alaska glaciers melt faster than thought
- Alaska glaciers melt faster than thought
By MATT VOLZ
Associated Press writer Sunday, September 24, 2006
JUNEAU, Alaska -- Less than 10 minutes after lifting off from the
airport, the helicopter entered the frozen world suspended above
Snowcapped mountains rose on either side as the small team of
scientists and students peered down at a jagged blue carpet of ice
below. The pilot turned up one arm of Mendenhall Glacier only to find
the way blocked by a wall of fog. The storm was moving in; the work
would have to be done quickly.
Hydrologist Eran Hood used a handheld global positioning system to
guide the pilot higher up the ice field on a clearer path. Circling
low, the scientists spotted what they were after: A tiny pyramid of
wire nearly invisible in the field of white.
In this lonely corner of an ice field larger than Rhode Island, the
packed snow crunching under their boots, the group set up shop. They
were about to find out just how much this part of the glacier had
melted over the summer and how fast it was moving.
Hood and physicist Matt Heavner, his colleague at the University of
Alaska Southeast, measured at least 10 feet of ice loss since May
there and at two other spots on the glacier.
Rain was beating down on the tourists at the glacier's terminus below.
The year's consistently bad weather has been dreary for the visitors,
but something of a reprieve for the melting Mendenhall Glacier.
"It's a good summer to be a glacier," Hood said.
There haven't been too many, judging by the rate at which Southeast
Alaska's rivers of ice are melting.
Most of the glaciers stretching from Yakutat Bay to the Stikine
Icefield, which goes into northwestern British Columbia, are thinning
at twice the rate that was previously estimated, according to a new
study co-authored by Hood's mentor, glaciologist Roman Motyka of the
University of Alaska Fairbanks' Geophysical Institute.
Comparing radar mapping data from a space shuttle mission six years
ago with air photos taken between 1948 and 1979, Motyka, UAF colleague
Chris Larsen and three other scientists pinpointed the extent of the
glaciers' volume change.
They found that 95 percent of Southeast Alaska's glaciers are
thinning. Some glacier surface elevations had dropped as much as 2,100
feet since 1948, such as the Muir Glacier in the popular Glacier Bay
National Park and Preserve.
With the more precise data, they figured the rate of thinning was
greatly underestimated from the last study done in 2002.
The scientists calculated that an average of 3.5 cubic miles of
glacier ice melts each year in the region due to a combination of
climate change and glacier dynamics. They say even that may be an
understatement of the actual rate of melting.
Mendenhall Glacier is a relatively small river of ice compared to the
rest of Southeast Alaska's extensive network, but it stands out. It is
Alaska's most visited glacier, drawing 367,000 people to the U.S.
Forest Service's visitor center last year.
The glacier is rapidly shrinking up the mountainside -- as rapidly as
glaciers can, anyway. Visitors who have observed the glacier see the
change themselves. Motyka estimated that the glacier's terminus will
pull out of Mendenhall Lake entirely within 10 years.
Hikers can trek up the side of the glacier along craggy rock that was
under a deep layer of ice just two years ago. They can poke around in
ice caves that weren't there at the beginning of the summer -- and
which will be gone by the season's end.
"We don't want to spend too much time underneath," Hood said in one
such cave, as water from the blue roof dripped all around. "These are
all pretty ephemeral."
Southeast Alaska's glaciers are very sensitive to climate change
because of their large surface areas at low elevations. In Juneau, the
winters have been getting warmer and rainier -- 6.8 degrees warmer
compared to 50 years ago, according to Laurie Craig, a naturalist for
the Tongass National Forest.
Those warmer temperatures can disrupt a glacier's surface mass
balance, the balance achieved between the melting period of summer and
accumulation period of winter.
For many Alaska glaciers at lower elevations, warmer temperatures are
causing the equilibrium line that separates the accumulation zone from
the melting zone to rise. Yakutat Glacier, for example, has lost
nearly all of its accumulation zone.
"This icefield will likely disappear completely under current
conditions," the authors of the new study wrote.
While climate change causes equilibrium shifts and thinning, it isn't
the only reason Alaska's tidewater glaciers are retreating from lakes
and the sea. The retreat may be triggered by warmer temperatures, but
then the dynamic cycle of a tidewater glacier takes over.
The speed of the glacier increases, drawing down the ice from above at
a faster rate and increasing calving below. In Southeast Alaska, the
ice loss at their terminus can cause tidewater glaciers to retreat
more than half a mile a year -- and that loss can't be directly
attributed to climate change, the scientists say.
"Once initiated, these calving losses are largely independent of
climate change and can be an order of magnitude greater than ice
losses driven solely by climate change," they wrote.
Then there are the anomalies. Five percent of the glaciers studied,
such as the Taku in the Juneau Ice Field, are expanding and thickening.
Many of these glaciers extend higher in elevation, giving them a
larger zone where snow can accumulate.
Glacier dynamics have the opposite effect with these glaciers. Their
accumulation zones are expanding and their melting zones are
shrinking. The result is a different kind of imbalance, one that
causes the glaciers to advance.
Motyka said scientists will have a better understanding of what has
happened to the glaciers since the 2000 space shuttle data once new
photos taken this summer are analyzed. With the last analysis showing
glaciers melting at twice the rate previously thought, he said he
expects more of the same.
"Presumably, things have accelerated," he said.