Rongbuk glacier retreat – From a Glaciers Perspective

Mount Everest region snowline identified on Dec. 11, 2019 Landsat image (yellow dots). Green dots indicate the terminus and pink arrows flow direction of specific glaciers: T=Trakarding, DN=Drogpa Nagtsang, M=Melung, BK=Bhote Koshi, S=Shalong, Y=Yanong, G=Gyabarg, YN=Yanong North, GY=Gyachung, J=Jiuda, R=Rongbuk, ER=East Rongbuk, I=Imja, L=Lhotse, M=Marala, K=Khumbu, N=Ngozumpa.

The winter monsoon for the Himalaya is a dry cold period with limited new snow accumulation. The Landsat image of Dec. 11, 2019 highlights the snowline at 5500-6200 m on glaciers around Mount Everest. This high of an elevation indicates the accumulation area of the glaciers is too small to sustain the current ablation areas. This is explored in detail below. King et al (2019) found during the 2000-2015 period Himalaya mass balance losses of debris-covered and clean-ice glaciers to be substantially the same, with mass balance loss for lake-terminating glaciers being significantly higher. The overall mean was −0.39 m/year. Maurer et al (2019) found a doubling of the average rate of loss across the Himalaya during 2000–2016 relative to the 1975–2000 interval. King et al (2017) observed the mass balance of 32 glaciers in the Mount Everest area from 2000-2015 finding a mean mass balance of all glaciers was −0.52 m/year, increasing to -0.7 m/year for lake terminating glaciers. Brun et al (2017) identify a mean balance of -0.33 m/year for 2000-2016 in Eastern Nepal, similar to King et al (2019) and not the highest loss rate in the region. Dehecq et al (2018) examined velocity changes across High Mountain Asia from the 2000-2017 period identifying a widespread slow down in the region. The key take away is warming temperatures lead to mass balance losses, which leads to a velocity slow down, and both will generate ongoing retreat.

For an alpine glacier to be in equilibrium requires at least 50% of its area to be in the accumulation zone, this is the accumulation area ratio (AAR). On Dec. 11, 2019 the snowline indicates where the accumulation zone begins. The elevation ranges from 5500 m on Melung Glacier to 6200 m East Rongbuk Jiuda and Gyabarg Glacier. The area above the snowline, AAR, is less than 30% of the total glacier on: Trakarding, Drogpa Nagtsang, Melung, Bhote Koshi, Shalong, Yanong, Gyabarg, Jiuda, Rongbuk, East Rongbuk, Imja, Lhotse, Marala Glacier. Gyachung and North Yanong Glacier have an AAR between 30 and 40%. Khumbu and Ngozumpa Glacier have a high mean elevation and an AAR of close to 50%.

In 2015 and again in 2018 high winter snowlines indicated the same process in the Mount Everest region. See below the rise from Nov. 2017 to Feb. 2018 to similar elevation as seen in Dec. 2019. The high snowlines indicate an accumulation area that is too small to maintain these glaciers, which drives continued retreat, such as reported at Drogpa Nagtsang and Yanong Glaciers.

Dec. 11, 2019 snowline:

6200 m =East Rongbuk, Jiuda, and Gyabarg

6100 m = Gyachung,

5900 m= Rongbuk, Imja, Lhotse

5800 m=Trakarding, Melung, Yanong North

5700 m= Ngozumpa, Drogpa N., Yanong, Shalong, Bhute Khosi

5600 m= Marala, Khumbu

5500 m= Melung

Landsat images from Nov. 17 2017 and Feb. 10 2018 indicate a rise in the snowline, purple dots, on glaciers east of Mount Everest, indicating ablation even in winter from the terminus to the snowline. Rongbuk Glacier=R, East Rongbuk Glacier=ER Far East Rongbuk Glacier=F, Kada Glacier=K, Barun Glacier=B, Imja Glacier=I and Kangshung Glacier=KX.

The Rongbuk Glacier is famous as the climbing gateway to the north side of Mount Everest (Qomolangma). In recent years the great climber and mountain photographer David Breashears has highlighted the change of this glacier over a the 1921-2007 period using repeat images of the glacier. The collection is part of Breashears Glacier Works Project, that is chronicling with repeat images the changes in a number of major Himalayan glaciers, the results of which have been in several well traveled exhibits. Here we examine a change on the surface of the main Rongbuk glacier over the last 15 years, the development of a substantial supraglacial lake. Ren et al (2006) noted the retreat of glaciers on the north side of Qomolangma as 5-10 meters/year. Ye et al (2009) noted a loss in glacier of 15 square kilometers for the glacier draining the north side of Qomolangma, and further noted the rapid expansion of the supraglacial lake from 0.05 in 1974 to 0.71 square kilometers in 2008. . The red arrows in the Landsat images from 1992, 2001 and 2012 indicate the extent of the supraglacial lake in 2012 , first three images below. In the two Google Earth images from 2003 and 2011, last two images in sequence, the lake extent is marked with point A and B. In 1992 the lower section of the Rongbuk Glacier was heavily debris covered with a isolated melt ponds amidst the rugged debris covered surface. By 2001 some of the melt ponds have coalesced, but a well defined lake is not present. The key to lake expansion is the downwasting of the higher mounds of debris covered ice on the ice tongue. A second key is that the stream exiting the East Rongbuk Glacier now in part feeds this lake system, this could not happen until the glacier surface was low enough for this stream to be able to access the surface. By 2012 the lake has expanded to a length of 1.9 kilometers, and is still expanding in area. A higher resolution view using Google Earth images indicates that most of the lake formation occurred between 2003 and 2011. The terminus is so covered by debris that examining retreat of the front of this glacier is not as important as the thinning of the ice in the lower 8 km of the glacier, which is downwasting rapidly and will melt away. Notice the meltwater stream entering the lake system from the lower right in the 2011 image.

Upglacier of the this lake at the main junction of the Rongbuk Glacier is another area of lakes that could coalesce into the same type of feature. This indicates that this glacier will have to retreat a long ways to reestablish equilibrium. The glaciers behavior reflects the same trends though the specific symptom, surface lake formation is different, as Lumding Glacier, Ngozumpa Glacier and Imja Glacier