Bridge Glacier and the Life-Cycle of a Calving Glacier

1 Comment

By Matt Chernos

[Note:  This is a guest post by Matt Chernos, a Master of Science student at the University of British Columbia.  Read more on Matt’s blog Bridging The Gap.  Thanks, Matt!]

Bridge Glacier in the summer of 2011.  Photo by L. MacKenzie

Bridge Glacier is located in the Pacific Ranges of southwestern British Columbia, Canada, between the Coast and Chilcotin Mountains. The glacier is an outlet of the Lillooet Icefield, about 175 km north of Vancouver. Beyond being the source of my MSc thesis, Bridge Glacier is important because it is the source of the Bridge River, and the Bridge River Hydroelectric Project, which supplies British Columbia with 6-8% of its electrical supply, leading some to call it a giant “melting battery“.

Bridge Glacier is also an interesting study because it sits in a lake, known unofficially as “Bridge Lake”, allowing calving to be a significant source of ice loss from the glacier. This wasn’t always the case though. Until the late 1980s, Bridge Lake was relatively small and shallow. Since 1991 though, Bridge Glacier has retreated substantially, and has allowed the lake to triple in size (up to 6 km^2 in 2012). As we have already seen on this blog, and throughout scientific literature, glaciers that flow into large bodies of water retreat somewhat less directly than the traditional response of warmer temperatures driving retreat. The question is, how much does the lake actually affect Bridge Glacier’s retreat?

One easy way to figure out glacier retreat rates is to check Landsat Imagery (available easily through their aptly named “LandsatLook Viewer“). Satellites capture images roughly every 16 or so days. Unfortunately, if you’re studying an area that is pretty cloudy, 80% of your images will be blobs of cloud (which can look really cool, but doesn’t really help reconstruct glacial histories). Images for Bridge Glacier go all the way back to 1972, with a few gaps in the data due to clouds and low temporal resolutions in the 70 and early 80s. By collecting an image from late September every year, I was able to compare two sequential images and measure how far the glacier retreated.  Putting all the years together, here is an animation of Bridge Glacier 1972-2012.

Bridge Glacier 1972-2012. If the animation isn’t running on your browser, click the picture.

And again putting into a “science form” in a graph (below):

(a) Bridge Glacier cumulative retreat (b) Winter Precipitation Anomaly (snowfall) (c) Summer Temperature Anomaly (d) Mean Annual Flow Anomaly (Bridge River Flow). All figures explained in the text.

To get a better handle on the graph, (a) is the cumulative retreat of Bridge Glacier from its position in 1972. Cumulative values are often seen because individual years have a lot of variability, which can distort what is otherwise a really clear trend of retreat.  (b) is the winter precipitation anomaly (November-April inclusive). Anomalies are often used in climate science to clearly illustrate whether any given year was above or below average. For this graph, average winter precipitation (snowfall as water equivalent to eliminate snow density discrepancies) was taken for 1972-2012. (c) is the summer temperature anomaly (May-October). Again, positive (red) values indicate hotter than normal summers. (d) is the mean annual flow of Bridge River at the edge of the glacier. Mean Annual Flow (MAF) is a measure of how much water passes through the Bridge River, and is an excellent indicator of how much melt there is in any given year.

The first thing that jumps out to me in this graph, is that the retreat rate is ‘step-like’.  Until 1991, the glacier is retreating pretty consistently, but that summer, things become more irregular. 1991 and 1992 have fairly large retreats, but then the glacier stays pretty constant for a couple years until it retreats again dramatically in a couple bursts. In fact, most of the retreat since 1991 occurs in a couple years of large bursts 0f dramatic retreats.

One of the largest retreats occurred in 2005 between July 27 and August 15:

July 27, 2005

August 12, 2005

September 5, 2005

Hopefully this photographic evidence is convincing enough that this ‘step-like’ or ‘staircase’ retreat is a product of calving as the glacier terminus is destabilized by the lake that it is floating in. The question that remains, though, is how much of this can we blame on Bridge Lake, and how much of it is a product of a changing climate.

To do this, we can run a numerical experiment: How would Bridge Glacier have retreated if Bridge Lake wasn’t there? 

We’ve already discussed how a glacier’s change in length is a product of changes in temperature, buffered by its individual climatic sensitivity, and the period it takes for this change in temperature to fully affect the terminus, known as the response time.  If we calculate these variables for Bridge Glacier, and combine it with temperature data, we can predict how the glacier would have retreated had it not been for the lake.

Modeled retreat for Bridge Glacier. The LOESS line is the modeled retreat rate smoothed to even out the statistical ‘noise’ of temperature data.

What this shows is that until 1991, climate projected retreat is very close to what we actually observed at Bridge Glacier. However, from 1991 onwards, once Bridge enters the ‘staircase’ stage, it retreats a lot faster than projected. If we go back to the animation of Bridge Glacier’s retreat, we can see that 1991 is, not coincidentally, when we start to see icebergs in the lake. All of this suggests, once again, that once the glacier begins to calve icebergs, its retreat becomes less a product of warm temperatures, and more of calving events.

If calving is in fact driving the accelerated retreat of Bridge Glacier, as this model suggests, it isn’t all bad news. Eventually, the glacier will retreat far enough back, and high enough, that it will no longer spill into the lake. Once the glacier reaches this point, it should, once again retreat more in line with the gentle climate-controlled retreat (albeit still retreat!) projected in the model. Once it does so, it will have ended its calving phase, and will once again respond more directly to climatic trends.

Links

A more technical write-up of Bridge Glacier recession and relations to climate and calving

About Matt Chernos

Matt’s Blog:  Bridging The Gap

 

 

Glaciar Juncal Norte, Chile: Repeat Photography (1959-2011)

1 Comment

By Matt Beedle

In 2011 members of Chile Glacier Quest set out for Glaciar Juncal Norte  to repeat a 1959 photo taken by Ulrich Lorber.  This expedition was made possible through an American Alpine Club Nikwax Alpine Bellwether Grant (AAC NABG).  Read the report on this adventure here.

Glacial Juncal Norte is one of the largest and best studied glaciers of the Aconcagua river basin of central Chile.

The 1959 image is used here with the permission of Ulrich Lorber.  The 2011 image was taken by the AAC NABG Expedition to Glaciar Juncal Norte and was made available to GlacierChange.org by expedition member Kurt Sanderson.

The debris-covered toe of Glaciar Juncal Norte presents a bit of a challenge in visually detecting recent change.  Debris cover has increased since 1959 and it appears that there has been some recession.  Modest thinning is apparent on the right side of the tongue, where the glacier surface appears to have sunk in relation to the lateral moraine.

Work by Francisca Bown and others (2008) found Glaciar Juncal Norte to have receded 464 meters from 1955 to 2006.  However, recession of Juncal Norte (4 m per year) is small in comparison with other glaciers in Chile such as Glaciar Juncal Sur which receded at a rate of 50 m per year over the same time period.  GPS surveying of the surface elevation of Glaciar Juncal Norte showed an average rate of thinning of ~0.6 m per year from 1955 to 2003.

Learn more about glacier change in Chile and research by Chilean scientists at the Laboratorio de Glaciologia (in English here).

The field work of Chile Glacier Quest continues with new scientific endeavors.  Follow their 2013 efforts on Cerro Plomo here.

Information on Juncal Norte and repeat photographs will have a permanent home in the GlacierChange.org Scrapbook.

It’s a pleasure to expand the content of GlacierChange.org to the Southern Hemisphere.  Thank you, Kurt!

Llewellyn Glacier, British Columbia Repeat Photograph: 1909 – 2007

No Comments

By Matt Beedle

In 2007 Jens Petersen and Brent Campbell enjoyed a canoe trip on Atlin Lake, a journey that included some exploration of the Llewellyn Glacier terminus and surroundings.  “I’m absolutely fascinated with Llewellyn Glacier and the entire Atlin area now,” writes Jens.  He submitted his 2007 photo along with the ca. 1909 photo, which illustrates about 100 years of change of the Llewellyn terminus.

The 2007 photo is used here with the permission of Jens Petersen.  The ca. 1909 image is credited to C. R. Bourne and is held by Library and Archives Canada.

This perspective looks almost directly south from a low ridge at the southern end of Llewellyn Inlet of Atlin Lake.  A trail leads from the campsite on Llewellyn Inlet over this ridge to the glacier forefield and on to the terminal lake and glacier terminus.  Learn more about the area via the Atlin Provincial Park website.

In 1909 the surface of Llewellyn Glacier was well up the bedrock ridge/mountain on the left in the image above.  The elevation of this surface position – marked clearly by the current trimline – is approximately 900 m, while the present day lake surface is at an elevation of a bit below 700 m.  This thinning of some 200 m over the past century is dramatic, but not uncommon for the terminus of a large glacier in the Coast Mountains of British Columbia and Alaska.  This thinning, and ultimate recession away from this bedrock ridge was the trigger of a “disappearance of a glacial river” in 2011.

Thank you for submitting your photography to GlacierChange.org, Jens!

Links

More on Llewellyn Glacier at GlacierChange.org

“Disappearance of a Glacial River” at GlacierChange.org

Atlin Provincial Park

Discover Atlin, British Columbia

Mendenhall Glacier, AK Continues to Retreat: Photos and Satellite Imagery

2 Comments

By  Matt Beedle

For the past three years my dad, Jack Beedle, has made an annual journey to a spot on Thunder Mountain to photograph Mendenhall Glacier.  It’s a good 2-hour climb up to this location; a significant effort for one shot (Thanks, Dad!).  We first visited this location – overlooking the Mendenhall Valley of Juneau, Alaska – in 1993 when I was in high school.  The first repeat photograph my dad took yielded this photo pair, which I find quite stunning, especially in that the perspective provides a unique view of volume change in addition to area/length change:

Mendenhall Glacier as seen from Thunder Mountain.  That’s me in the 1993 image.  For a bit of a sense of scale:  the distance from Nugget Falls (waterfall at lower right)  to the terminus in the 2010 image is slightly more than 1 kilometer (0.62 miles).

Mendenhall Glacier has continued to retreat since 2010, but as the time between images is is only a few years as opposed to the better part of two decades, the results are not nearly as striking.  This is how the change of Mendenhall Glacier appears from this vantage point since 2010:

Mendenhall Glacier photographed from Thunder Mountain in September of 2010, 2011, and 2012. Photos by Jack Beedle.

The retreat from 2010 to 2011 is apparent, but it’s difficult to make out much change in the past year from this distance.  Within the next decade it’s likely that Mendenhall Glacier will retreat to a point where it is no longer in contact with Mendenhall Lake.  When this happens the calving process – by which glaciers can lose large amounts of mass – will end, and the retreat of Mendenhall Glacier should slow.  Are we nearing that point?  Did retreat of Mendenhall Glacier slow this past year?  To try to answer these questions I did a quick investigation with satellite imagery from 1993, 2010, 2011, and 2012.

These two images are from Landsat satellite imagery. I used what are called false-color composites that show vegetation as red, and make the glacier ice a bit easier to see. The unfortunate black striping in the 2012 image is from a failure of what’s called the Scan Line Corrector on the Landsat 7 satellite.  The outlines for Mendenhall glacier in 1993, 2010, 2011, and 2012 are shown in progressively lighter shades of blue.

In 1993 Mendenhall Glacier extended to Nugget Falls, where a popular trail ends today, and many people play in the sand where the ice stood not too long ago.  Since then it has retreated dramatically, especially where in contact with the lake where calving results in the loss of a great deal of mass.  Note that the retreat along the bedrock peninsula that juts out into the lake, where the glacier is grounded and there is no calving, is much less.  To measure the rate of retreat since 1993 I took the average across the entire terminus, including the portion that calves into Mendenhall Lake and the portion that is grounded on the bedrock peninsula.

Before I tell you the numbers I need to post a little disclaimer.  Landsat satellite imagery, with a resolution of 30 meters (the width of each pixel represents 30 meters on the ground), is far from ideal to determine annual glacier length change.  My intent here is to get a rough estimate of change to lend some context to the repeat photography.

With that said, here’s (roughly) how fast Mendenhall Glacier has been receding since 1993 when my dad and I first hiked Thunder Mountain:

  • 1993 – 2010:  Total retreat of 538 meters (m), or 32 m per year
  • 2010 – 2011:  56 m
  • 2011 – 2012:  40 m
  • 1993 – 2012:  Total retreat of 660 meters (m), or 35 m per year

[NOTE:  Values reported here are rounded to the nearest meter and thus total retreat does not jive with the annual rates.]

From these measurements it appears that recent retreat of Mendenhall Glacier (2010 – 2012) has been greater than the average rates since 1993.  Although it’s not readily apparent from the repeat photographs, the retreat of Mendenhall Glacier has continued in 2012, and at what appears to be a rate that is a bit faster than the average since 1993.

I too visited this site on Thunder Mountain this past summer with the intent of getting a shot of me standing in the same place as in 1993.  Unfortunately I didn’t have the right lens in my camera kit that day to get the same perspective that was captured in 1993.  I hope to return to Thunder Mountain with my dad in the summer of 2013 to capture the ’20th anniversary’ shot of our initial journey to this spot.

Me with cousins Jana and Shannon at the Thunder Mountain repeat-photograph site. August 2012.

Standing on Thunder Mountain, 1993 image in hand, enjoying the stunning view of Mendenhall Glacier, I was struck by the incredible magnitude of change.  Not really saddened, but more in awe of the incredible dynamism of the natural world around us.  I’m captivated by these rapidly changing, intensely beautiful masses of ice and snow, and hope that I can bring a bit of this experience to you through images and a bit of analysis.

Links:

More on Mendenhall Glacier at GlacierChange.org

Facts and figures on Mendenhall Glacier change from University of Alaska Southeast

Elleanor Boyce’s Masters Thesis on the retreat of Mendenhall Glacier

Stunning time lapse of Mendenhall Glacier retreat by James Balog’s Extreme Ice Survey

Peer-reviewed literature:

Motyka et al.  2003.  Twentieth Century thinning of Mendenhall Glacier, Alaska, and its relationship to climate, lake calving, and glacier run-off, Global and Planetary Change, 35, 93-112.

 

Our Days in Atlin, BC (Blogging From the Field #53)

1 Comment

Post and photos by Kristin Timm

[This post is part of a series of real-time communication from participants in the 2012 Juneau Icefield Research Program.  The program begins June 23rd and concludes August 18th.]

Atlin, British Columbia is a small Canadian community of about 300 people. It is located in Northern BC, but you actually cannot get to it from anyplace else in BC. You get to the town by driving south from the Yukon Territory (or by hiking over the Juneau Icefield :) ). The town has a really rich and interesting history, and at one time this area had about 10,000 people. In the late 1800’s it was the site of a gold rush, and people continue to mine placer gold here today. Placer is the small pieces of gold that are essentially “sorted” out from layers of old river sediments. The placer deposits can be as small as mere specks or nuggets of several ounces, or about the size of an acorn. A local miner showed us his operation, and he actually uses plastic outdoor carpeting to “catch” the smallest flecks of gold.

Old buildings from the past century are all over the area.

Old buildings meet the eclectic and artistic side of the community to make Atlin a very unique and beautiful community.

JIRP actually owns one of these interesting old buildings. We stay at and base our operations out of the old Atlin hospital (yes, it is a kind of creepy building) while we are in town. According to a historical society plaque, the hospital closed in the 50’s because there were not enough patients. It now has a lecture room, library/office, storage, and bedrooms for the JIRP faculty and staff. The Atlin boat harbor is out in front, and we spent several clear evenings sleeping on the dock under the stars.

The old Atlin hospital is now the base of operations for JIRP, while in Atlin.

Our time in Atlin was really quite relaxing. In between laundry, showers, and naps in the grass, we worked on our talks for our big community presentation. Traditionally, JIRP students have presented their research projects to the Atlin community each year after getting off the icefield, and this year was no different. A few of us also spent most of the day baking about 200 cookies for the event. The event was advertised locally with a sign and posters, and an interview with our director, Jeff was aired that morning on CBC radio.

A local sign advertises for our evening program.

It was a really lovely evening with cookies, coffee, an overview of JIRP and the status of the icefield by our director Jeff, five minute presentations by each of the 14 JIRP students, and a closing historical perspective from Toby. I was really impressed that out of about 300 residents, about 60 people showed up for the presentations, and it was an enthusiastic audience that had great questions and stuck around afterword to meet some of the students.

Annika Ord presents about her biological research in Paradise Valley, a valley just off the icefield that is believed to have been a refuge for biological diversity during the last glacial maxiumum.

Matt Osman provides an introduction to the process of isotope fractionation during his presentation.

Zeke Brechtel came to JIRP with a set of instruments used to measure windflow over the Taku Glacier, and will later model his results.

Atlin, also known as Camp-30, was a great place to spend a few days and acclimatize to being back in “civilization” before moving on to the big city of Juneau. The people of the small town were friendly and kind, and displayed a unique enthusiasm for seeing the “glacier kids” in town again, as they do each summer. Thank you Atlin for continuing to support the Juneau Icefield Research Program, and thank you for your hospitality. Thank you also to both stores and Jenz Place for helping us overcome our post-icefield ice cream cravings!

Links

Archive of all “Blogging From the Field:  JIRP 2012″ Posts

JIRP on Facebook

JIRP on Twitter

www.juneauicefield.com

crevassezone.org