The La Plata Mountains as seen from above the
              author’s home.


Durango Bill’s
Glaciology


The Taku Glacier

What it is and what is going to happen to it


   The Taku Glacier is the largest glacier draining the Juneau Icefield. The Juneau Icefield covers about 1,500 sq. miles of Alaska and British Columbia, and has dozens of major glaciers and hundreds of small glaciers. All of these have been retreating with the exception of the Taku. (And its distributary – the Hole-in-the-Wall Glacier) It appears that this anomaly of the Taku Glacier is about to end as the Taku appears to be going into retreat. The retreat is expected to accelerate rapidly with the retreat rate likely to become >= 500 feet per year within a few decades.

The following sections will examine:
1) The Taku Glacier and its source – the Juneau Icefield
2) A model for tidewater glaciers and how this applies to the Taku Glacier
3) The history of the Taku Glacier
4) What is happening now to the Taku Glacier
5) Why the Taku Glacier is going to start retreating at >= 500 ft. per year



1) The Taku Glacier and its source – the Juneau Icefield


A Landsat satellite view of the Juneau Icefield

   The picture above is a Sept. 1984 Landsat satellite view of the Juneau Icefield. There are about 38 glaciers that drain ice from icefield.
http://www.alaska.org/assets/content/related_items_pdfs/mendenhall_glacier/mendenhall.glacier.pdf

   These 38 glaciers include the Taku and Juneau’s major tourist attraction, the Mendenhall. All of these glaciers have been retreating except the Taku. The Taku has been advancing even though it shares the same snow and ice source as the other glaciers. The Taku alone has been advancing – up to now. A closer look at the Taku will begin to explain this anomaly.


A topographic map of the Juneau Icefield

   The picture above is a detailed topographic map of the Juneau Icefield (including the Taku and Mendenhall Glaciers). The map was generated via http://www.mytopo.com/

   A topographic map provides more information than just a plain satellite view. The topographic map tells us that most of the Taku Glacier’s surface area is less than 4,000 feet above sea level. The elevations on a topographic map are more apparent if we look at a color-coded elevation map.


A color coded map of the Juneau Icefield

    The source for the above picture is at:
http://en-ca.topographic-map.com/places/Juneau-Icefield-692123/

   The color-coded map shows that the average elevation of the Taku Glacier is lower than most of the rest of the icefield. At this lower average elevation, precipitation is still mostly snow, but it’s a borderline situation as the slightest increase in temperature will change this precipitation from mostly snow to mostly rain. Global warming is providing this incremental increase in temperature.

   If we take a little diversion from the Taku Glacier, it is rather obvious what global warming is doing to one of Juneau’s main tourist attractions – the Mendenhall Glacier. The two pictures below are from the video at https://vimeo.com/69802285


A 1958 view of the Mendenhall Glacier

   The picture above shows the Mendenhall Glacier as of 1958 as seen from near the current Mendenhall Glacier Visitor Center.


A 2012 view of the Mendenhall Glacier from the same
          location.

    The picture above was taken from the same location and shows the Mendenhall Glacier as of 2012.

   It seems probable that at least the lower portion of the Mendenhall Glacier will not be visible from the visitor center in another 20 years. It is important to remember that the Mendenhall and Taku Glaciers have the same source area – The Juneau Icefield.

   We will return to the subject of the low elevation of the Taku Glacier and the consequences of global warming in the “5) Why the Taku Glacier is going to start retreating at >= 500 ft. per year” section.

   A second major feature of the Taku Glacier is that it is a “tidewater glacier”. Tidewater glaciers terminate at sea level or in terminal moraines that protect them from the ocean. The Taku Glacier is the only glacier that starts in the Juneau Icefield and terminates at sea level. It is this “tidewater glacier” feature that is responsible for the Taku’s advance over the last century.



2) A model for tidewater glaciers
 and how this applies to the Taku Glacier



A diagram that shows the major features of an arbitrary
          glacier

   The picture above depicts the major features of a glacier. The source for the picture can be found at http://slideplayer.com/slide/8509495/

   The following equations apply to all glaciers. If a glacier is going to remain stable, then:

Snowfall on the glacier = melting + calving

(From https://en.wikipedia.org/wiki/Ice_calving :   Ice calving, also known as glacier calving or iceberg calving is the breaking of ice chunks from the edge of a glacier)

   If we remove the restriction “If a glacier is going to remain stable”, then we get a more complete equation that tells us whether a glacier will grow or not.

Snowfall on the glacier = melting + calving + glacier growth

or alternately by rearranging terms

Glacier growth = snowfall on the glacier - melting - calving

(Note that “glacier growth” can be negative)

   If snowfall is greater than melting + calving, then the glacier will grow. If snowfall is less than melting + calving, then the glacier will shrink.

   Calving is an important part of the mass balance/budget of a tidewater glacier. Thus we must look at the Tidewater Glacier Cycle which is superimposed on glacial changes from ordinary climate changes.

The tidewater glacier cycle

Cyan = glacier
Blue = water
Brown = terminal moraine (push moraine)
Gray = bedrock

   The diagram above is from https://www.nature.com/articles/s41467-017-00095-5 , and illustrates a tidewater glacier cycle. In the initial part of the cycle, a tidewater glacier is stable in a “retracted” state. When a glacier is stable, snowfall = melting + calving, and the “glacier growth” is zero.

    However glaciers carry rocks and debris, and deposit the rocks & sediment in a terminal moraine. As the terminal moraine of a tidewater glacier becomes larger, it begins to restrict warm water access to the terminus of the glacier. This decreases melting & calving, and eventually cuts off the calving part of the equation. Even if snowfall decreases slightly, the glacier begins to grow because the melting & calving parts of the equation decrease faster than the snowfall component. From 1890 to about 2014 the Taku Glacier progressed from the “Retracted” to the “Extended” stages as illustrated above. In terms of distance for the Taku, this advance was about 4.5 miles.

    As of 2017, it appears that the Taku Glacier is at the “306 yr” stage of the tidewater glacier cycle. In terms of what happens next, it should be noted that the retreat part of the cycle happens very rapidly. We will cover the “what happens next” in the “5) Why the Taku Glacier is going to start retreating at >= 500 ft. per year” section. But first, we will look at “3) The history of the Taku Glacier” which includes the “Retracted” to “Extended” part of the tidewater glacier cycle.



3) The history of the Taku Glacier

   The known history of the Taku Glacier dates back to the 1700s and is a result of excellent field work done in the 1940s by Donald Lawrence who was a professor of botany at the University of Minnesota, Minneapolis. Dr. Lawrence analyzed vegetation above and below old trimlines and moraines throughout the Juneau Icefield area. Using this information he could he could discover the extent of the Taku Glacier (and other glaciers) and when they were leaving records of their presence at various locations.


A diagram showing history of the Taku Glacier that
                was published in the American Geographical Society's
                Geographical Review

(Click on the above diagram to see a larger version that includes a description of the features. - recommended)

   The diagram above was part of Dr. Lawrence’s paper that was published by the American Geographical Society’s Geographical Review.
https://www.uas.alaska.edu/arts_sciences/naturalsciences/envs/faculty_staff/pubs/Lawrence_glacier_SE_AK_solar_activity1950.pdf

   The dotted lines show where the terminus of the Taku Glacier was as of various dates, and most important, the Taku’s maximum extent. In the middle 1700s the Taku Glacier reached Taku Point – thus blocking the Taku River - which created a lake upstream from Taku Point.

   When the lake overflowed, the river scoured (removed all previous vegetation) from parts of Taku Point. New vegetation began to grow after this scouring about 1755-1757.

From Dr. Lawrence’s paper:
“At Taku Point (Figs. 6, 9) a first-generation forest that began to grow about 1755-1757 stands immediately below the trimline, and a very old forest perched on rotten logs and surely undisturbed since 1390 or earlier stands above the trimline.”
   Thus the maximum extent of the Taku Glacier had to occur slightly before this date. The year 1750 is commonly used for this maximum extent. (Note the “or earlier” could be as much as 8,000 years)

Note: There has been some controversy as to whether the Taku Glacier made it to Taku Point on the southeast side of Taku Inlet.

A close-up view of some rock ledges at Taku Point

   The picture above is a close-up Google Earth view of some of the rock ledges at Taku Point. There are some taller trees on higher ground in the upper right corner. The rock ledges are sparsely covered with smaller vegetation. Even in a rain forest, vegetation does not grow well without adequate soil. It is the author's conclusion that something scraped the soil off the rock ledges within the last few hundred years. The author sides with Lawrence on this one.


A diagram showing the historical positions of the Taku
          Glacier

   The diagram above is from Fig. 89 of the USGS publication “Part 3 - Descriptions of Alaska’s 14 Glacierized Geographic Regions”. https://pubs.usgs.gov/pp/p1386k/pdf/04_1386K_coastmts.pdf In turn the USGS publication adopted it from the published work of Post and Motyka (1995). The lines drawn on the Taku and Hole-in-the-Wall Glaciers show where the respective terminuses were as of various dates since 1750.

Note: If you use Google Earth, the 1750 terminal moraine can be easily seen north of the current terminal lobe of the Hole-in-the-Wall Glacier.


   The first European to visit the Taku Glacier was the explorer Vancouver who sailed up Taku Inlet in Aug. 1794. At that time the area that is currently occupied by the last couple of miles of the Taku Glacier was a fjord. Vancouver’s description of the area as quoted from Roman Motyka’s “Taku Glacier Advance – Preliminary Analysis” ( http://pubs.dggsalaskagov.us/webpubs/dggs/pdf/text/pdf1989_012.pdf ) was:
He reported large numbers of floating icebergs in Taku lnlet especially at the entrance, through which "passage was with difficulty effected". At a point about 21 km (13 mi) up the inlet (the approximate vicinity of Taku Point) he describes "the shores spread to the east and west, and formed a basin about a league broad, and 2 leagues across, in a N.W. and S.E. direction, with a small island lying nearly at its north-east extremity" (1 league = 3 miles = 4.83 km), and further describes "immense bodies of ice, that reached perpendicularly to the surface of the water in the basin". This description fits the tidal basin that existed in front of the Taku Glacier terminus when it was first charted in 1890 by the USC&GS (fig. 2) and would place the terminus well up-fjord but still fronting in tidewater in 1794 as icebergs were apparently quite common.

   There was no further useful information about the Taku Glacier area until the U. S. Coast and Geodetic Service created the first map in 1890.

The USCGS map of 1890

(The picture above is a copy of the USC&GS 1890 map as shown in Motyka’s paper)

   In addition to the Taku Glacier’s retreat since 1750, there are two items of interest. The first is the 300+ foot depth of the fjord immediately downstream from the Taku's 1890 terminus. The second is “Hole-in-the-Wall Lake” near the top edge of the diagram. The subsequent advance and thickening of the Taku Glacier would spread ice across this 1.2 mile long lake and over the low ridge east of it to form the Hole-in-the-Wall Glacier. The west end of the lake was probably bounded by a lateral moraine from the Taku Glacier.

    When the results of Lawrence, Vancouver, and the USCGS (and subsequent observations of the Taku) are combined, we get a picture of what has been happening to the Taku over the last 300 years.

A graph showing the location of the Taku's terminus since
          1750

   The solid line in the above chart is from a paper by Nolan, Motyka, Echelmeyer, and Trabant that was published in the Journal of Glaciology ( https://www.igsoc.org/journal/41/139/igs_journal_vol41_issue139_pg541-553.pdf ). It shows the known positions of the Taku Glacier’s terminus starting with 1750, while the dashed line is a best estimate of where the terminus was in-between Vancouver’s trip and the USCGS’s map. From the Taku’s maximum extent at Taku Point in 1750, it retreated some 10 to 11 km (to near or slightly downstream from the location where the Hole-in-the-Wall Glacier currently branches off), and then (anomalously compared to most other glaciers) the Taku advanced until “about” 2014.


   The first known photographs of the Taku Glacier date from 1891. Several 1891 photos are known to exist, but the photo below is perhaps the best.

The terminus of the Taku Glacier as seen in 1891


   The photo above is from the Eastman collection.
http://collections.eastman.org/objects/107828/taku-glacier?ctx=f411cab2-014f-49a9-b3c4-0d80faedcb05&idx=5

   The Taku Glacier presents a steep calving face which is typical of tidewater glaciers terminating in deep water. There was probably a submerged terminal moraine well below the water’s surface, but it will be decades before it becomes large enough to be visible. The mountains in the center and right background are Goat Ridge. The present location where the Hole-in-the-Wall Glacier splits off the Taku is just off the right edge of the photo.

   At the time of the photo, the fjord was about 300 feet deep at the photographer's location. The same area now is covered by ice that extends about 1,100 feet above sea level.

   Shortly after the above photograph was taken, the tourist “cruise boat” industry took off. The following two photos show a couple of the early steamships bringing sightseers to the “wonders of Alaska”.

The steamship "Victorian" in front of the Taku
          Glacier in 1900

   The steamship “Victorian” is in front of the Taku Glacier in 1900.
Picture source:
https://commons.wikimedia.org/wiki/File:Steamship_VICTORIAN_making_a_tourist_stop_at_Taku_Glacier,_Alaska,_ca_1900_(HESTER_305).jpeg

   The view looks NE with the Brassier Hills on the right side of the photo and the extreme south end of Goat Ridge in the distance behind the Brassier Hills. Currently, the Taku overflows a low ridge in-between these mountains to form the Hole-in-the-Wall Glacier. 



The steamship Spokane in front of the Taku Glacier in
          1907

    The original source of the above 1907 picture of the steamship Spokane in front of the Taku Glacier can be seen at:
 https://commons.wikimedia.org/wiki/File:Steamship_SPOKANE_at_Taku_Glacier,_Taku_Inlet,_Alaska,_1907_(NOWELL_78).jpeg

 
A 1929 aerial view of the Taku Glacier

The original source for the above 1929 aerial view is:
https://nsidc.org/cryosphere/glaciers/gallery/tidewater.html

    In the above aerial photo, note the deep water-filled fjord in front of the glacier. (Work by Motyka gives a depth of 300 feet.) That deep fjord is still there except it is currently filled with the Taku Glacier’s ice. In a few short decades, it’s all going to be water again.


History of the Taku’s distributary – The Hole-in-the-Wall Glacier

    By the 1930s, the thickening of the Taku Glacier started to push an arm eastward over the old Hole-in-the-Wall Lake. The photographs below show what happened next.

1934 view of wherte the Hole-in-the-Wall Glacier will
          form

   The photograph above is courtesy of the Glacier Photograph Collection at the National Snow & Ice Data Center. http://nsidc.org/data/glacier_photo/search/

   The picture above shows the location of the Hole-in-the-Wall Glacier in 1934 before the Taku Glacier thickened enough to overflow the low ridge between the Brassier Hills (lower left) and Goat Ridge (upper right). In subsequent years, the Taku Glacier (upper left) would overflow the low ridge (right-center) onto the low flats (lower right) – thus forming the Hole-in-the-Wall Glacier.

   There is still a very small portion of the former Hole-in-the-Wall Lake remaining between the glacier and the crest of the low ridge.

   The lack of trees on the low ridge indicates the Taku had overflowed the area sometime in the previous 200 years, and had scraped off all the soil that trees would need to grow.


A Google Earth approximation of what the same area looked
          like in July 2010

     The picture above is a Google Earth approximation of what the same Hole-in-the-Wall Glacier area looked like in July 2010. The large crevasses are formed where the glacier overrides the old low ridge. Note the trimline that is beginning to form on the left side of the glacier.

   The following 3 photographs were included as part of Dr. Donald Lawrence's paper that was published by the American Geographical Society.
https://www.uas.alaska.edu/arts_sciences/naturalsciences/envs/faculty_staff/pubs/Lawrence_glacier_SE_AK_solar_activity1950.pdf


A 1934 view of the Hole-in-the-Wall Glacier

   The photo above was taken in 1934 and shows the incipient Hole-in-the-Wall Glacier just before it pushed over the low ridge. The Taku Glacier flows from right to left in the background.

A 1941 view of the Hole-in-the-Wall Glacier

   The 1941 photograph above shows the Hole-in-the-Wall Glacier just as it is beginning to overflow the low ridge. Presumably the dog in the foreground is one of the famous sled dogs from the Taku Glacier Lodge. The photographer's location is on the west side of the Taku River, nearly 2 miles SW of the Taku Glacier Lodge  (and probably slightly inside of the current terminus of the glacier), and looks NW toward the glacier.

A 1949 view of the Hole-in-the-Wall Glacier

   By 1949 the glacier had thickened enough to completely overflow the low ridge. the Hole-in-the-Wall Glacier would continue to expand for another 60 years.




The Norris and Taku Glaciers as of 1948

   The photograph above is courtesy of the Glacier Photograph Collection at the National Snow & Ice Data Center. http://nsidc.org/data/glacier_photo/search/

   The picture above is an aerial view of the Norris Glacier (left) and the Taku Glacier (right) as of 1948. Up until the 1940s, the terminal (push) moraine in front of the Taku had not grown large enough to appear above sea level; but as of 1948 a muddy silt flat can be seen in front of the center of the Taku Glacier. There is a small area of calving remaining on the right side of the Taku, and this may be the basis of other sources that state that calving continued to 1953. ( For example: https://www.geographie.uni-freiburg.de/publikationen/abstracts/fgh50-en )

    By 1948 (at least one source uses a 1953 date - see above) the terminal “push” moraine had grown to the point where there was no ocean water contact with the Taku Glacier. The “snowfall on the glacier = melting + calving” portion of the equation was still out of balance as “calving” had disappeared from the equation. The Taku would continue to grow for decades to come.


The recent record of the position of the terminus of the
          Taku Glacier

   The picture above shows the position of the Terminus of the Taku Glacier from 1948 to 2014. The original picture can be seen at:
http://juneauicefield.com/blog/2016/8/15/taku-glacier-anomaly-of-the-juneau-icefield

    As of 1948 the Taku Glacier was still advancing rapidly. The advance slowed after 1973, but still continued up to 2014. As of 2014 it looked like the Taku Glacier had reached the “Extended” stage of the “tidewater glacier cycle” diagram.


A diagram showing the mass balance of the Taku Glacier

   The picture above is a PrintScreen image from the Juneau Icefield Research Program’s blog. http://juneauicefield.com/blog/2017/8/31/the-mass-balance-student-research-project

   The picture shows a historical record of the mass balance (total ice volume) of the Taku Glacier as well as the Lemon Creek Glacier (another glacier at the southern end of the Juneau Icefield) as measured by researchers for the Juneau Icefield Research Project.

   The mass balance (total ice volume) of the Taku increased from the the first measurements by the JIRP in the 1940s until 1988. This was the period when the Taku’s growing terminal moraine protected it from the melting and calving that had been taking place earlier due to the glacier’s contact with “warm” ocean water.

   Additionally, the moraine was a partial barrier to the glacier’s forward advance, and thus ice piled up and thickened behind the moraine. The backup and thickening extended for miles upstream, and this thickening caused the Taku to overflow a low ridge to form the Hole-in-the-Wall Glacier.

   After 1988, global warming started to catch up with the Taku. The mass balance (total ice volume) began to shrink. The pressure from the previously accumulated thickness of the ice continued to push the terminus forward even while the middle portion of the glacier began to thin. The process is similar to what happens when you pour syrup on a stack of pancakes. Even after you stop pouring syrup, what you have already poured continues to spread out on your plate while the portion on top of your pancakes thins.

   From 1988 on, the total volume of the Taku Glacier began to decrease even though the terminus continued to advance a little more. The thickness in the middle part of glacier started to decrease from the combined forces of “spreading out” plus global warming.

   The rate that the Taku’s ice can spread out decreases as the thickness in the middle part of the glacier decreases. Thus the ice supply to try to push the terminus forward will tend to decrease.

   Unlike syrup, ice is subject to melting and eventually disappears. Unfortunately for the Taku, the rate of melting continues to increase since the surface area of the glacier subject to melting has increased as well as global warming has increased the melt rate per unit area.  Sooner or later, the increase in melting from global warming will overwhelm all parts of the glacier.


A longitudinal (lengthwise) cross section of the Taku
          glacier

Source for the above diagram is the 1995 paper by Nolan, Motkya, Echelmeyer, and Trabant.
https://www.igsoc.org/journal/41/139/igs_journal_vol41_issue139_pg541-553.pdf
Please see the source article for information about the sampling points.

   The diagram above shows a longitudinal (lengthwise) cross section of the Taku Glacier starting at Taku Point on the left end and extending some 60 km. (37 miles) upstream to Matthes Divide on the right end. The Hole-in-the-Wall Glacier splits off just to the right of the Brassiere Hills label. The top line shows the 1990 surface elevation of the glacier (in meters) above sea level. The bottom line shows the elevation (in meters) of the base of the glacier.

   There are several items of interest. First, the base of the glacier is below sea level up to the 40 km. (25 miles) mark. When (not “if”) the Taku Glacier melts, it’s lower portion will be replaced by a 20-mile long lake. (It is assumed that the Taku’s terminal moraine remains intact and/or it is augmented by silt transported in by the Taku River. Thus it will be a lake instead of a fjord.)

   A similar process of a melting glacier forming a long lake is already underway at New Zealand’s Tasman Glacier. (The Tasman Glacier has been retreating at 400 feet per year for the last 30 years.)

   Also, the glacier has scoured out another 300 feet of sediments in addition to the 300-foot deep fjord that existed in front of the 1890 terminus. The 300-foot deep area downstream from the 1890 terminus is now ice down to 600 feet below sea level. It’s going to be a very deep lake.

   The separation of the two lines in the above diagram shows how thick the Taku Glacier is. At a little upstream from the “Goat” measuring point, the Taku Glacier was some 1477 meters (4,800+ feet) thick as of 1990 making it the thickest known glacier outside the polar icecaps. As the lower end of the Taku starts melting, this thick portion will thin rapidly as ice will ooze rapidly downstream to try to fill in the void. (Eventually this thick portion and everything downstream will end up floating on the growing lake.)

   The equilibrium line marks the 1990 position that separated the “accumulation zone” (The right side where more snow falls than melts) from the ablation zone (The left side where more ice melts than snow falls). Since 1990 this equilibrium line has moved higher (moved to the right) due to global warming, and will continue to move higher in the future.


By 2015 the advance of the Taku Glacier had come to a stop.
Juneau Empire
“Taku Glacier's advance stagnates”
http://juneauempire.com/outdoors/2015-09-04/taku-glaciers-advance-stagnates


   In June 2017 the author took a sightseeing flight (Wings Airways) over the Taku Glacier, and its distributary, the Hole-in-the-Wall Glacier. The author took videos of the Taku and Hole-in-the-Wall Glaciers and posted the result to YouTube.
https://www.youtube.com/watch?v=O5v_Bf3cbHI&feature=youtu.be

   The relevant portion of the video begins at 13:07 into the video. As of 2017 it looks like the Taku Glacier has reached the “306 yr” stage of the tidewater glacier cycle diagram. It appears that the retreat of the Taku Glacier has begun.


4) What is happening now to the Taku Glacier

   The following pictures are from the author’s sightseeing flight over the Taku and Hole-in-the-Wall Glaciers in June 2017. Please see the author’s video at https://www.youtube.com/watch?v=O5v_Bf3cbHI&feature=youtu.be – especially the part beginning at 13:07 for more views.

   The Hole-in-the-Wall Glacier is part of the Taku Glacier’s distributary system. (please see the pictures in the “1) The Taku Glacier and its source – the Juneau Icefield” section ) The Hole-in-the-Wall Glacier formed in the 1940s when the Taku thickened enough to flow over a low ridge. Whatever the Taku Glacier does (advance or retreat), the Hole-in-the-Wall Glacier will do the same.


The terminus of the Hole-in-the-Wall Glacier

   The picture above is a PrintScreen image from the author’s video, and shows the June 2017 position of the terminus of the Hole-in-the Wall Glacier. The terminus is retreating from the trees. We can compare the above picture with 2010 and 2006 views as seen via Google Earth.

A view of the same area in April 2010 via Google Earth

   The picture above is a Google Earth view of the same area but as of April 2010. The trees are the same as in the view from the 2017 video. (The Google Earth view has a lot of extra mud, but otherwise the view is approximately the same.)


The terminus of the Hole-in-the-Wall Glacier in July
          2010

   The picture above is a Google Earth view of the same area but as of July 2010. (The bright area is a carryover from April 2010 as it wasn't included in the July 2010 image.)

   In 2010 the glacier was threatening to obliterate the trees. But in 2017, the trees are still standing and the terminus is further away. (In the July 2010 Google Earth view, the glacier has retreated a little bit from the trees, but not as far as shown in the June 2017 view.)


The terminus of the Hole-in-the-Wall Glacier in May 2006

    The picture above is a Google Earth view of the terminus of the Hole-in-the-Wall Glacier as of May 2006. The terminus of the glacier is nearly (but not quite) the same distance from the trees as the 2017 picture; but more important, the most advanced terminal moraine was in place before 2006. We can thus deduce the following chronology.
1) The glacier advanced sometime before 2006, and almost reached the trees.
2) The glacier retreated by the time the 2006 photograph was taken.
3) The glacier advanced up to 2010, but didn't quite reach the pre-2006 position.
4) The glacier has retreated from 2010 to 2017.
      Thus the glacier's advance has been at least stalled over the last 17+ years. Meanwhile, thinning is taking place (see below) on the Taku's surface The thinning will make it difficult for the glacier to stage any more advances, while at the same time global warming marches onward. The pre-2006 terminal moraine will probably not be challenged again.


A trimline above the current surface of the Taku Glacier

   The picture above is a PrintScreen frame from the author’s video which can be seen at https://www.youtube.com/watch?v=O5v_Bf3cbHI&feature=youtu.be The sightseeing flight is over the Taku Glacier. Taku Inlet can be seen in the upper right. The mountain in the left foreground is the Brassiere Hills ridge between the Hole-in-the-Wall Glacier and the Taku Glacier.

   When a glacier is growing, it obliterates any vegetation in its path. When a growing glacier thickens, it obliterates any vegetation in its path along the side walls of its valley.

   On the left side of the picture there is a sliver of rock between the glacier and the trees. When the Taku Glacier was growing and thickening it “trimmed” all vegetation that was in its path. The line marked by trees on top and rock underneath is a “trimline” It is a record of how thick the glacier was when it was at its thickest.

   The bare rock is a zone where the glacier has removed everything except the solid rock. In order to see the bare rock area, the Taku Glacier had to thin. The area in the photograph is more than 2 miles upstream from the glacier’s terminus. The Taku Glacier  is melting and thinning 2 miles upstream from its terminus. This is telling us what will happen downstream at the terminus a few years in the future. The Taku Glacier has started its retreat.


A closeup view of the trimline

   The trimline is perhaps the best available clue for letting us know whether the Taku is in an advancing or retreating mode. The picture above is digitally enhanced portion of a frame from the original video.

    This thinning of the glacier is extremely important. The thinning is more than a few inches. It’s not easy to get an accurate measurement of the amount of thinning as viewed from an airplane, but the trees in the background should give some idea of the scale.


The July 2010 terminus of the Taku Glacier

   The picture above is a Google Earth view of the terminus of the Taku Glacier as of July 2010.


The June 2017 position of the terminus of the Taku
          Glacier

   The picture above shows approximately the same area of the terminus of the Taku Glacier, but as of June 2017.

There are two items of interest.

1)  The terminus of the Taku Glacier has retreated from 2010 to 2017. The maximum extent was probably greatest about 2014 (+/-) The July 2010 Google Earth view doesn’t show any retreat yet. By June 2017 there is a noticeable gap between the glacier and the untouched vegetation.

2)  In the 2017 picture there is a small innocuous-looking melt-water pond next to the glacier in the foreground of the picture. This small pond (and what will follow) is a source of water that can tunnel its way underneath and start melting the glacier from the bottom up. Water is its densest at about 39 deg. F. Water at 39 deg. F. will sink below water at any other temperature; and of course, water at any temperature will sink below ice if it has any possible way of doing so. 39 deg. F. water is not warm enough to encourage swimmers, but it is warm enough to melt ice.

    Glaciers are full of cracks and crevices, and water will find any available opening to sink downward as water is denser than ice. In the tidewater glacier cycle diagram, the “2006 yr” diagram shows a “Void opening”. The innocuous-looking meltwater pond in the 2017 view is the start of a “Void opening”, and marks the start of the collapse of the Taku Glacier. And the innocuous-looking meltwater pond (or something similar to follow) may very well be the start of what will become a 20-mile (+/-) long lake.



5) Why the Taku Glacier is going to start
retreating at >= 500 ft. per year


   In part 2), we looked at the tidewater glacier cycle. Even if nothing else changed, a tidewater glacier retreats rapidly once retreat sets in. This is because the lake (or ocean) in front of the glacier melts both the front end and underneath part of the glacier.

   In turn, if we look at the basic equation for glacial balance (Snowfall = melting + calving), once a tidewater glacier starts melting back, it will start calving rapidly. Thus the ablation side of the equation becomes larger, and the glacier retreats.

   Global warming will also hit the “snowfall” side of the equation. We saw earlier that large portions of the Taku Glacier are at relatively low elevations. As global warming continues to progress, the precipitation over large portions of the glacier will change from predominately snow to predominately rain.

    We can apply all of these changes to the complete equation for a glacier's budget.

The complete equation is:

Glacier growth = snowfall on the glacier - melting - calving

   The changes that will be taking place are:

Snowfall on the glacier - Decreasing as global warming changes snowfall to rainfall
Melting - Increasing as air temperatures warm and a lake melts the ice
Calving - Increasing as the front end of the glacier calves into the lake

   Any one of the above will force "glacier growth" to become negative. The combination of all three will force "glacier growth" to become strongly negative.

   In regard to the estimate that the Taku Glacier will retreat at 500 (or more) feet per year, we only have to look at what has happened in the past.

A diagram showing the location of the terminus of the
          Taku Glacier

    The diagram above is the same diagram that we looked at earlier when were looking at where the terminus of the Taku Glacier has been over the last 300 years. The lowest part of the graph tells us how fast the Taku was advancing or retreating during this 300 year interval. If we look at the portion from 1750 to 1794 (From Lawrence's trimline analysis to Vancouver's visit), the Taku retreated at some 170 meters (550+ feet) per year. The climate has warmed significantly since the 1700s, and will continue to warm in the future. The 500 feet per year retreat rate may be overly conservative.



   The first few years of glacial retreat for the Taku will be relatively slow – less than 100 feet of retreat per year. Once a lake starts to appear behind the terminal moraine, the retreat rate will pick up. In 20 to at most 30 years, this retreat will accelerate to 500 or more feet per year as serious calving begins.

   100 years ago there was a deep fjord suitable for ocean-going ships in front of the Taku. This was subsequently filled in with ice. A few decades from now the fjord will become a deep lake with a rapidly calving (and retreating) glacier at the other end.

   200 years from now, if you travel to visit the Taku Glacier, you will instead find a large lake (about 20 miles long). There will still be much smaller glaciers in the higher mountains, but most of what is now the Juneau Icefield will have become lakes and newborn forests.



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