Taku Glacier Transient Snow Line Paper Published

This post examines in simpler terms and more images the paper published this week in The Cryosphere on “Utility of late summer transient snowline migration rate on Taku Glacier, Alaska”. The transient snowline (TSL) is the point of transition from snow to older glacier firn and ice. The TSL rises during the course of the summer melt season and at the end of the melt season is the equilibrium line altitude (ELA). This paper represents a concept that occurred to me while skiing and probing snow pack on the Taku Glacier in 1998 with the Juneau Icefield Research Program (JIRP), something I have spent six months doing over the years. There simply was not enough consistent satellite imagery to apply the model until recently, we also needed field data-ground truth-to quantify and verify the TSL model. This meant probing snowpack along a 5 km transect near the TSL during several summers, following my 1998 probing, Matt Beedle completed the probing in 2004 and 2005 with JIRP and Chris McNeil did so in 2010, 2011 and 2014. Below is the transient snowline in August 2014 on the Juneau Icefield. juneau Icefield tsl 8212014
Landsat Image: T=Taku, G=Gilkey, H=Herbert, M=Mendenhall and N=Norris. Black arrows indicate the snowline which was quite high at over 1000 m with a month left in the melt season.
The ELA is the point at which accumulation equals melting on temperate alpine glaciers this is where snow transitions to bare glacier ice. Mass balance for non-calving glaciers is the difference between snow accumulation on a glacier and snow and ice loss from the glacier. The easiest to observe and most useful estimate of mass balance without detailed measurement is the equilibrium line altitude (ELA). Today the TSL can be observed frequently in satellite imagery. There are two ways the TSL is useful in assessing mass balance. First the rise of TSL during the melt season provides an assessment of the rate of melting. Second the TSL rate of rise can be used near the end of the melt season to determine the ELA, when imagery at that point is not available due to cloud cover. This allows widespread assessment of melt rate on glaciers. On Taku Glacier which is fairly typical we found a very consistent gradient of snowpack change with elevation from year to year. This allows determination of melt rate simply from rate of TSL rise. We use Landsat Imagery of which there are typically only two-four useful images during the melt season, barely enough and more recently MODIS imagery from GINA, which is obtained daily for the entire globe and provides the most frequent point of observation. However, the resolution of MODIS makes it inaccurate on glaciers less than 1 km wide or 1 km long. Taku Glacier is 55 km long and 5 km wide at the ELA. As the melt season begins the snow cover extent is large on Taku Glacier. The key is how rapidly the TSL rises during the melt season. On the ground the JIRP measures the snow depths and snow melt during July and August on Taku Glacier. This program was led by Maynard Miller, U Idaho for more than 50 years and is currently under the direction of Jeff Kavanaugh U Alberta. The Taku Glacier mass balance measurements allows validation of the melt rate, note snowpit locations on map below. For example in 2004 the TSL was at 850 meters on July 15, first image below. At this time the snowpack was 1.6 meters at 1000 meters. On September 1 the snowline was at 1030 m, second image below. The TSL had risen at an average rate of 3.9 meters per day, all 1.6 m of snow had melted. Below are images from May 26, 2006, then July 29, 2006 and then Sept. 15 2006. Indicating the rise of the snowline.
The below images from May 26, July 29 and Sept. 15 2006 indicate the rise of the ELA during the course of the melt season, from 370 m to 800 m to 975 m. Snow depths at the the Sept. 15 ELA, where snowpack=0, was 2 m on July 22. Thus, we had 2 meters of snow melt at 975 m between July 22 and Sept. 15. In 2004 the melt rate was 0.036 meters per day and in 2006 0.038 meters per day. All of the TSL images above are from Landsat> For Sept. 14, 2009 (top), Sept. 20, 2010 (middle) and Sept. 11, 2011 (bottom) MODIS images are used, resolution not as good as with the Landsat images. Note the similarity in the end of the year snowline on Taku Glacier for those three years. . The next task is to apply the TSL to other glaciers and to carefully compare results from MODIS and Landsat. Through 2010 there were only four days with good coverage from both. Below is the Landsat imagery from Sept. 11, 2011, same as the MODIS date above. Noted is the TSL, in this case the ELA for Lemon Creek and Taku Glacier.

Field Glacier, Alaska Retreat, Lake expansion, tributary separation

field glacier change
Above is a paired Landsat image from 1984 left and 2013 right, indicating a 2300 m retreat in this period, below is further detailed examination.
The Field Glacier flows from the northwest side of the Juneau Icefield, and is named for Alaskan glaciologist and American Geographical Society leader William O. Field. Bill also helped initiate the Juneau Icefield Research Program, which Maynard Miller then ably managed for more than 50 years. The JIRP program is still thriving today. In 1981, as a part of JIRP, I had my first experience on this glacier. It was early August and there was new snowfall everyday that week. Jabe Blumenthal, Dan Byrne and myself undertook a ski journey to examine the geology on several of the exposed ridges and peaks, note the burgundy line and X’s on image above. This was truly a remote area. The glacier begins from the high ice region above 1800 meters, there are several icefalls near the snowline at 1350 meters, and then it descends the valley ending at 100 meters. The runoff descends the Lace River into Berners Bay. This post focuses on the significant changes occurring at the front of the Field Glacier. The development of a proglacial lake at the terminus is accelerating and spreading into the main southern tributary of the glacier. This lake is going to quickly expand and develop a second arm in that valley. The USGS map from 1948 imagery and the 1984 imagery indicate little change in the terminus position, with only a small lake at the terminus. . After 1984 the mass balance of the Juneau Icefield became more negative, this was apparent from the rise in the snowline elevation on all the glaciers and by the increasing losses and resultant thinning typified by the Lemon Creek Glacier (Miller and Pelto, 1999). The Field Glacier by 2006 had developed a proglacial lake at the terminus that averaged 1.6 km in length, with the east side being longer. There are several small incipient lakes forming at the margin of the glacier above the main lake, each lake indicated by black and orange arrow. In 2009 the lake had expanded to 2.0 km long and was beginning to incorporate the incipient lake on the west side of the main glacier tongue. By 2011 the main lake has nearly reached the southern tributary lakes. The lake has expanded to 2.6 km long, with the west side having caught up with the east side, and an area of 4.0 square kilometers. In addition the main lake has joined with the fringing lake on the south side of the south tributary. There is also a lake on the north side of this tributary. This lake should soon fill the valley of the south tributary and fully merge with the main, as yet unnamed lake at the terminus, maybe this should be Field Lake. Below in order is the 2006, 2009 and 2011 Landsat images. This glacier is experiencing retreat and lake expansion like several other glaciers in the icefield, Gilkey Glacier, Eagle Glacier, and Antler Glacier.

Eagle Glacier Retreat, Juneau Icefield Alaska

eagle glacier changeAbove is a paired Landsat image from 1984 left and 2013 right indicating the 1100 m retreat during this period of Eagle Glacier.My first visit to the Eagle Glacier was in 1982 with the, ongoing and important, Juneau Icefield Research Program, that summer I just skied on the glacier. In 1984 we put a test pit at 5000 feet near the crest of the Eagle Glacier to assess the snowpack depth. This was in late July and the snowpack depth both years was 4.3 meters, checking this depth in nearby crevasses yielded a range from 4-4.5 meters. In 1984 the snowline at the end of the summer melt season in early September was at 1050 meters. In the image below the glacier is outlined in green, the snowpit location is indicated by a star and the snowline that is needed for the glacier to be in equilibrium at 1025 meters is indicated.
Eagle Glacier has experienced a significant and sustained retreat since 1948. The first map image below is of the glacier in 1948, at this time the glacier ended at the south end of a yet to be formed glacier lake. By 1982 when I first saw the glacier and when it was mapped again by the USGS (second image) the glacier had retreated to the north end of this 1 kilometer long lake. In the sequence of images the red line is the 1948 terminus, the magenta line the 1982 terminus, the green line 2005 terminus and the orange line the 2011 terminus. From 1982 to the 2005 image used in Google Earth the glacier retreated 500 meters, 21 meters/year (next image). The bottom image is from a 2011 Landsat image in May and indicates the terminus position once again with an additional retreat in six years of 400 meters, 65 meters/year. Going back to the 1948 map the terminus in 2011 is located where the ice was 500-600 feet thick in 1948The more rapid retreat follows the pattern of more negative balances experienced by the glaciers of the Juneau Icefield (Pelto et al, 2008). The Equilibrium line altitude which marks the boundary between the accumulation and the ablation zone each year is a good marker of this. On Eagle Glacier to have an equilibrium the glacier needs to have an ELA of 1050 meters. At this elevation more than 60% of the glacier is in the accumulation zone. Satellite imagery allows identification of the ELA each year, seen below is the elevation in 1984, 1998, 1999 and each year since 2003. The number of years where the ELA is well above 1050 meters dominate leading to mass loss, thinning and glacier retreat. This follows the pattern of Lemon Creek Glacier that is monitored directly for mass balance, which has lost 26 meters of thickness on average since 1953.

Gilkey Glacier Ogive Spacing and Retreat

The Gilkey Glacier is a 32 km long outlet glacier flowing west from the Juneau Icefield. From 1948 to 1967 the Gilkey Glacier retreated 600 m and in 1961 a proglacial began to form. By 2005 Gilkey Glacier has retreated 3900 m from the 1948 terminus location. The glacier is currently terminating in this still growing lake, notice the new bergs and rifting at the glacier terminus. The retreat has been resulted from and in a thinning of in the lower reach of the glacier and the separation from Battle and Thiel Glacier. A major tributary to Gilkey Glacier, is Vaughan Lewis Glacier. At the base of the Vaughan Lewis Icefall where the Vaughan Lewis Glacier joins the larger Gilkey Glacier ogives form, as seen from above and below the icefall (Scott McGee). The ogives form annually and provide a means to assess annual velocity in this section of the glacier. Aerial photography of the ogives from the 1950’s combined with current satellite image provide the opportunity to assess ogive wavelength over a 50 year period, providing a long term velocity record for Gilkey Glacier. An ogive is a bulge-wave that forms annually due to a seasonal acceleration of the glacier through an icefall. The acceleration is enhanced in icefalls that are horizontally restricted. In most cases we do not have specific measurements of velocity through all season to ascertain the timing of the accelerated period, though typically spring would be the fastest. After formation the bulges move down glacier and a new bulge is formed the following year. The resulting train of ogives extending down glacier can be used to estimate the ice velocity by measuring the peak to peak separation between adjacent waves. Ogives can be visually identified as a series of arcuate wave crests and troughs pointing down glacier. Downglacier from this formation point the crests and troughs gradually flatten until the ogives are merely arcuate light and dark bands on the surface of the glacier. The dark bands are dense, blue and dusty ice that is compressed during summer, whereas the light bands are bubbly, white, air-filled ice that is compressed during winter.
In 1981 one of my tasks was to ski out through the top of the icefall inserting stakes in the crazily crevassed region to track summer velocity for the Juneau Icefield Research Program (JIRP). This has been completed often but not most years by JIRP. What we discovered was that velocity in 1981 had not changed from the 1960’s and 1970’s. Today we have frequent satellite imagery of the ogives to ascertain annual velocity that can be compared to the few aerial photographic records, in this case from 1056 and 1977. In several recent years Scott McGee of JIRP has specifically surveyed the distance between the first 11 ogive crest below the icefield. A comparison of the the ogives in 1956, 1977 and 2005 is possible by overlaying the images below. . The distance from the first to the 40th ogive has gone from 6.8 km in 1956 to 6.75 km in 1977 to 6.2 km in 2005. In 1956 and 1977 the first ten ogives spanned 1500 meters indicating an annual glacier velocity of 150 meters. From 2003-2007 the distance of the first ten ogives averaged 1440 m, or 144 meters per year. The change in velocity is quite small, compared to the large retreat of the glacier. One other key measure of the ogive surveying program is the surface elevation. A longitudinal profile containing 179 survey points was established at the base of the Icefall in 2001-2007. This profile begins in the trough immediately upglacier of the crest of the first wave ogive and continues downglacier nearly 1.8 kilometers to a point where the amplitude of the ogives becomes zero (Graphs and data from JIRP) During this six year time period, the surface has lowered an average of 17 meters – nearly 3 meters per year – along the longitudinal survey profile, with a maximum of 22 meters. This substantial thinning at the base of the icefall indicates reduced discharge through the icefall from the accumulation zone above. This will lead to further retreat and velocity reduction of Gilkey Glacier.