The northern coastline of Alaska midway between Point Barrow and
Prudhoe Bay is eroding by up to one-third the length of a football
field annually because of a "triple whammy" of declining sea ice,
warming seawater and increased wave activity, according to new
study led by the University of Colorado at Boulder.
The conditions have led to the steady retreat of 30 to 45 feet a
year of the 12-foot-high bluffs -- frozen blocks of silt and peat
containing 50 to 80 percent ice -- which are toppled into the
Beaufort Sea during the summer months by a combination of large
waves pounding the shoreline and warm seawater melting the base of
the bluffs, said CU-Boulder Associate Professor Robert Anderson, a
co-author on the study. Once the blocks have fallen, the coastal
seawater melts them in a matter of days, sweeping the silty
material out to sea.
Anderson, along with collaborators Cameron Wobus of Stratus
Consulting and Irina Overeem of CU's Institute of Arctic and Alpine
Research, or INSTAAR, each presented results from components of
their study at the annual meeting of the American Geophysical Union
in San Francisco held Dec. 14-18.
The problem is caused by several factors, including increased
erosion along the Alaskan coastline due to longer ice-free summer
conditions and warmer seawater bathing the coast, Anderson said.
The third potential factor is that the longer the sea ice is
detached from the coastline, the further out to sea the sea-ice
edge will be. This open-ocean distance between the sea ice and the
shore, known as the "fetch," increases both the energy of waves
crashing into the coast and the height to which warm seawater can
come into contact with the frozen bluffs, said Anderson.
"What we are seeing now is a triple whammy effect," said
Anderson. "Since the summer Arctic sea ice cover continues to
decline and Arctic air and sea temperatures continue to rise, we
really don't see any prospect for this process ending."
In addition to Wobus and Overeem, co-authors on the studies
include Gary Clow and Frank Urban of the U.S. Geological Survey in
Lakewood, Colo., and Tim Stanton of the Naval Postgraduate School
in Monterey, Calif.
The shoreline bluffs are made up of contiguous, polygon-shaped
blocks, primarily made of permafrost and each roughly 70 to 100
feet across, he said. Ice "wedges" created by seeping summer
surface water that annually freezes and thaws are driven deeper and
deeper into the cracks between individual blocks each year. The
blocks closest to the sea are undermined as warm seawater melts
their base, and eventually split apart from neighboring blocks and
topple during stormy conditions, said Anderson.
The researchers used a variety of instruments and methods in the
study to examine the dynamic transition between the land and the
sea, including time-lapse photography of shoreline erosion, global
positioning systems (GPS), meteorological measurements including
temperature and wind speed, and sediment analyses of the coastal
bluffs. Offshore measurements included sea-ice distribution, ocean
floor depth, sea-surface temperatures and wave dynamics, said
Anderson, also a fellow at INSTAAR.
The time-lapse images were taken with four tripod mounted "game
cameras" often used by hunters and wildlife biologists and which
were set up parallel to the shoreline. The cameras snapped pictures
every six hours during the 24-hour summer daylight months to track
the effects of the waves on the coastline, said Anderson.
"Once one of these blocks topples, the process continues on to
the next block," Anderson said. "These images are very powerful,
because they pick up activity during severe storms when we aren't
there to watch." The images also illustrate the steady melting
along the water's edge that helps to undermine the bluffs even in
the absence of storm activity.
The research team also deployed four submerged ocean buoys
attached to metal sleds with sensors to measure the wave activity
at different depths in the shallow coastal waters, comparing wave
power with the shoreline fetch. The team attached temperature
sensors to the buoy mooring lines to monitor seawater temperatures,
which have been warming in recent summers due to increased solar
radiation, he said.
When the sea ice is further from the shore, currents from the
Beaufort and Chukchi seas transport warmer water to the coastline,
said Anderson. While the temperature hovers around 45 degrees
during the summer months, the shallow coastal water warmed to as
much as 59 degrees during the 2007 field season -- the same year
the largest loss of summer Arctic sea was recorded, he said.
As the ice wedges cut down through the polygon blocks, the
surface soil above them -- which thaws each summer -- is pushed up
slightly, forming small ridges that eventually surround each
polygon, said Anderson. Small ponds form above individual polygons
during the summer months as the surface ice and snow melts,
providing habitat for migrating birds that feed and breed along the
Beaufort Sea coastline.
"This is an important habitat for birds and other wildlife,"
said Anderson. "One of the concerns we have is that some larger
ponds and lakes located slightly further inland may begin draining
into the sea as the shoreline continues to recede."
While there are no towns adjacent to the specific study area,
coastal erosion threatens abandoned military and petroleum
infrastructure, he said. Coastal erosion occurs at similar sites
elsewhere along Alaska's coastline. Bank stabilization measures
using sandbags, for example, have been undertaken at the Alaskan
town of Kaktovik on the Beaufort Sea in an attempt to slow the
problem.
According to a 2009 CU-Boulder study, Arctic sea ice during the
annual September minimum is now declining at a rate of 11.2 percent
per decade. Only 19 percent of the ice cover was more than two
years old -- the least ever recorded in the satellite record and
far below the 1981-2000 summer average of 48 percent.
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