Mendenhall Glacier continues thinning trend

Posted: Sunday, March 06, 2011

Visitors at this week’s lecture at the Mendenhall Glacier Visitor Center learned about something going on right in their own backyard. Researchers from the University of Alaska Southeast gave some insight into the work they’ve done over the year on the glacier, and how the ice that makes it up continues to decrease.

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“Glacier ecosystems are unique in that they’re not particularly well-studied,” said environmental science and geography professor Eran Hood. “Additionally, glacier ecosystems are changing very rapidly, so it’s important to understand how these ecosystems play.”

Instructor Michael Hekkers started the evening with some insight into how Mendenhall’s state has changed. There was a big change, indeed, as the terminus lost 540 feet between 2009 and 2010. This is more than twice the yearly average of ice loss between 1997 and 2009. Hekkers and Hood explained that some of this can be explained through climate and environmental changes in temperature and precipitation, which researchers are still studying.

Such climate influences may also affect the glacier’s calving. UAS student David Sauer did a case study, during which he found the year’s largest calving event took place Aug. 18. This calving was preceded by increased temperatures, up to 65 degrees Fahrenheit, plus 2.5 inches of rain at the Northstar camp.

Sauer then explained the significance of calving such as this, such as the straining effect of the terminus’ buoyancy, increasing the release of accumulated precipitation and melt. The draining to the base also increases glacier sliding. The terminus does become more buoyant as the lake level rises.

He also said terminus retreat has opened up the deepest portion of the lake over the last decade, causing more instability and calving of the terminus. The three presenters agreed the glacier will eventually retreat out of the lake, but it’s not certain when that will happen. Future calving studies will be done before this happens.

Sauer said the deepest point was measured at 77 meters in 2008, but is slowly decreasing due to increased sedimentation.

In terms of climate effects at Mendenhall, Hekkers showed the numbers. The mean annual temperature on the glacier from 1949 to 2009 was 3.2 degrees Fahrenheit, with the average slightly rising over the winter months and dropping during the summer, which he said is an important time to retain accumulated snow for the glacier to grow.

“A significant change in winter temperature means less snowfall for the glacier,” he said.

Hood went on to describe the significance of glacier water to the local ecosystem, as 95 percent of Southeast Alaska’s glaciers are thinning, a phenomenon he said is worldwide.

“One thing we know from a variety of studies is that we’re losing glacier water very rapidly in Southeast Alaska,” he said.

Southeast glaciers are also a large contributor to the Gulf of Alaska. The region gives 42 percent of its runoff to the gulf, a large part of that from glaciers. The Gulf of Alaska itself gets 47 percent of its discharge from such runoffs. Hood said it’s important to study how climate changes will affect that percentage and how much the gulf water itself will change.

He said marine and terrestrial ecosystems are very tightly coupled, and the importance of glacier runoff is its strong impact on water’s physical properties. For example, glacier stream temperatures go down in the summer. Colder water can affect fish habitats and other marine habitats. Such ecosystems that are accustomed to certain temperatures and timing are affected when glacier runoff changes over time.

Glacier thinning can also make room for more vegetation, as Hood pointed out has happened in Lynn Canal.

Hood added that a future project could involve a climate and modeling study of Juneau’s ice field.

Hekkers also presented findings from geology professor Cathy Connor, who discovered newly exposed sediments and wood. Carbon dating places some of these sediments around 2,000 years old.

The sediments lie on the previously eroded Taku terrane bedrock and represent late Holocene channels and a quiet water pond or floodplain. Larger sediments were carried by rivers, while smaller streams deposited the finer sediments.

The wood washed in from nearby upland forests, as no rooted wood was found. The wood was deposited there more than 2,000 years ago and was covered by the advancing glacier sometime after 20 A.D.

• Contact reporter Jonathan Grass at 523-2276 or at

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