A Two-Century-Long Advance Reversed by Climate Change

Taku Glacier in 2016 and 2019 Sentinel 2 images. The Hole in the Wall Tributary (HW) is upper right, Taku Glacier main terminus (MT). Yellow line is the 2016 terminus location. The arrows denote locations where thinning is apparent as the area of bare recently exposed bedrock has expanded. A closeup is below. Pink and brown areas between blue ice and yellow line in 2019 indicates retreat.

The Taku Glacier is the largest outlet glacier of the Juneau Icefield in Alaska. Taku Glacier began to advance in the mid-19th century, and this continued throughout the 20th century. At first observation in the 19th century, the glacier was calving in deep water in a fjord. It advanced 5.3 kilometers between 1890 and 1948 moving out of the fjord into the Taku River Valley (See maps below (Pelto and Miller, 1990). At this time calving ceased resulting in positive mass balance without the calving losses. The glacier continued to advance 2.0 km from 1948-2013 (Pelto, 2017). The advance was paralleled by its distributary terminus, Hole in the Wall Glacier. This advance is part of the tidewater glacier cycle (Post and Motyka, 1995), updated model by Brinkerhoff et al (2017). At the minimum extent after a period of retreat the calving front typically ends at a point of constriction in fjord width or depth that limits calving. With time, sedimentation in front of the glacier reduces water depth and calving rate, allowing the glacier to begin to advance. In the case of the Taku Glacier, after a century of advance, the glacier had developed a substantial proglacial outwash and moraine complex that had filled in the fjord, and the glacier was no longer calving. Images below, from 1961 and 1981, illustrate this. This allowed the advance to continue through the rest of the 20th century and into the 21st century. The slowing of the advance in the latter half of the 20th century has been attributed to the impedance of the terminus outwash plain shoal (Post and Motyka, 1995Pelto and Miller, 1990). There is a concave feature near the terminus with an increase in crevassing where the push impacts flow dynamics as seen at black arrow in 1975 and 1998 images below. In 1980’s the Taku Glacier’s accumulation area ratio was still strong enough for Pelto and Miller (1990) to conclude that the Taku Glacier would continue to advance for the remaining decade of the 20th century, which it did.

Beginning in 1946, the Juneau Icefield Research Program began annual mass balance measurements that is the longest in North America. In conjunction with JIRP and its first director Maynard Miller, we compiled and published an annual mass balance record in 1990. From 1990 to the present, in conjunction with JIRP and Chris McNeil, we have continued to compile and publish this annual mass balance record (Pelto et al 2013).  Much of the remarkable data record of JIRP has this month been made accessible to the public, particularly through the efforts of Seth Campbell, JIRP director; Scott McGee, survey team director; and Chris McNeil, mass balance liaison with USGS.

The ELA in 2018 and 2019 in Landsat images, purple dots indicate the record high snowlines for the 1946-2019 period for each year.

Taku Glacier is one of the thickest known alpine temperate glaciers. It has a maximum measured depth of 1,480 meters, and its base is below sea level for 40-45 km above the terminus (Nolan et al 1995). Moytka et al (2006) found that the glacier base was more than 50 m below sea level within 1 km of the terminus and had deepened substantially since 1984. This suggests a very long calving retreat could occur. The glacier had a dominantly positive mass balance of +0.42 m/year from 1946-1988 and a dominantly negative balance since 1989 of  -0.34 m/year (Pelto et al 2013). This has resulted in the cessation of the long term thickening of the glacier. On Taku Glacier, the annual ELA (end of summer snowline altitude) has risen 85 m from the 1946-1988 period to the 1989-2019 period. During the 70+ year annual record the ELA had never exceeded 1,225 m until 2018, when it reached 1,425 m (Pelto, 2018).  In 2019, the ELA again has reached a new maximum of 1,450 m (see above images). Contrast the amount of the glacier above the snowline in 2018 and 2019 to other recent years that had more ordinary negative balances (see Landsat images below).

In 2008 and 2012, JIRP was at the terminus, creating the map below. There was no change at the east and west side of the margin since 2008, with 55 to 115 m of advance closer to the center. The glacier did not advance significantly after 2013 and did not retreat appreciably until 2018. The Taku Glacier cannot escape the result of three decades of mass losses, with the two most negative years of the record being 2018 and 2019. The result of the run of negative mass balances is the end of a 150+ year advance and the beginning of retreat. Sentinel images from 2016 and 2019 of the two main termini Hole in the Wall Glacier (right) and Taku Glacier (left). The yellow arrows indicate thinning and the expansion of a bare rock trimline along the margin of the glacier. The Hole in the Wall terminus has retreated more significantly with an average retreat of about 100 m.  The Taku main terminus has retreated more than 30 m along most of the front.

The retreat is driven by negative balances, mainly by increased surface melt. The equilibrium flow of the Taku Glacier near the long term ELA for the 1950-2005 period was noted by Pelto et al (2008). This occurred during a period of glacier thickening, average profile velocity was 0.5 md-1  (Pelto et al 2008). Since 1988 the glacier has not been thickening near the snowline as mass balance has declined slightly (Pelto et al 2013). The remarkable velocity consistency measured by JIRP surveyors led by Scott McGee each year at profile 4 has continued. It is below this profile that surface ablation has reduced the volume of ice headed to the terminus.

All other outlet glaciers of the Juneau Icefield have been retreating, and are thus consistent with the dominantly negative alpine glacier mass balance that has been observed globally (Pelto 2017).  Now, Taku Glacier joins the group unable to withstand the continued warming temperatures. Of the 250 glaciers I have personally worked on, it is the last one to retreat. That makes the score: climate change 250, alpine glaciers 0.

1890 United States Coast Guard map indicating deep water in the fjord in front of Taku Glacier.
Map of terminus change from Lawrence (1950)
Taku Glacier aerial photograph from US Navy in 1948. Still minor calving on right (east side
Taku Glacier in 1961, photograph indicating calving had ended
This is a view across the glacier accumulation area that until 2018 had always been snow covered at the end of summer (Credit: Pelto).

To see more photos of Taku Glacier, check out the Mauri Pelto’s original post on From a Glacier’s Perspective, a blog published by the American Geophysical Union.

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