The state of Himalayan Glaciers
by Owen King
Nearly 30,000 of High Mountain Asia’s glaciers are found in the Himalaya (Frey et al., 2014), the arc of the worlds tallest mountains that stretches from Western India to South East China. The glaciers of the Himalaya are the source of rivers such as the Ganges, the Indus and the Brahmaputra. Recent research has shown that most glaciers in the Himalaya are now shrinking (Brun et al., 2018), likely because of long term increases in temperature in the region (Maurer et al., 2019). The contribution of glacier meltwater to river flow cannot be sustained over long periods of glacier recession, thus the sustainability of river flow in south east Asia will soon be at risk.
Thousands of new glacial lakes have formed in response to the melting of the region’s glaciers over the last few decades (Nie et al., 2017; Zhang et al., 2015), many of which remain in contact with the glaciers that have formed them. Until now, the impact of widespread lake development at the front of Himalayan glaciers has not been studied in detail.
Jialongco glacial lake (Poiqu basin, Central Himalaya) occupying the former extent of its host glacier, which now sits above the lake level. Photo credit: Owen King.
Galungco glacial lake (Poiqu basin, Central Himalaya) occupying the former extent of its host glacier. Photo credit: Jan Bouke Pronk.
Spying on glacier recession since the 1970s
To examine glacier behaviour over a similar period to that of lake formation, we have used declassified US Hexagon KH-9 spy satellite images, which captured the state of Himalayan glaciers as early as 1973 (King et al. 2019). Combined with data from the Shuttle Radar Topographic Mission (SRTM) in 2000 and data from modern day satellites (WorldView, Geoeye), we created three dimensional models (Digital Elevation Models- DEMs) of the surface of Himalayan glaciers at different points in time over the past 5 decades. By examining the differences in the surface elevation of glaciers at different points in time, we have been able to measure how quickly glaciers across the region have thinned over our study period. We found that glaciers flowing into lakes have thinned at around twice the rate (up to 4 m/year since 2000) of glaciers flowing on land since the 1970s.
Using these surface elevation change data, we calculated the geodetic mass balance of each glacier we studied. Calculation of a glaciers mass balance shows whether snow and ice accumulation (and therefore glacier advance) or ice melt (and therefore glacier recession) is predominant over a certain time period. We found that the mass balance of glaciers flowing into lakes was more negative than those flowing on land. Since 2000, the rate of mass loss of Himalayan glaciers has increased, and the gap between lake and land terminating glacier mass loss rates has widened.
We also looked at how the position of the front of the same glaciers changed since the 1970s. Again, glaciers that are now flowing into lakes retreated at more than twice the rate (26 m/year on average from 2000 to 2018) than glaciers flowing on land (~9 m/year from 2000 to 2018). Since the 1970s, glaciers that are now flowing into lakes reduced in length by a mean value of 13%, but by as much as 49% in some individual cases. Glaciers flowing on land reduced in length by a mean value of 9%.
What does the future hold for Himalayan glaciers?
The glacierised areas of High Mountain Asia are warming at a higher rate than the global average (Kraaijenbrink et al., 2018). Even under the most conservative IPCC RCP2.6 scenario, a 1.5°C increase of global temperatures will cause 2.1°C of warming in High Mountain Asia (Kraaijenbrink et al., 2018). Such a rise in air temperatures will ensure continued glacier retreat in the region. Whilst precipitation may increase in the Himalaya in coming decades (Krishnan et al. 2019), snowfall is not predicted to increase to counter glacier melt.
The continued expansion of current glacial lakes, as well as the formation of more new glacial lakes, will only amplify ice loss levels above that caused by temperature increases. The existence of a glacial lake at the front of a glacier causes enhanced melting because of the contact of ice with warmer lake water, and because the lake promotes calving- the detachment of large chunks of ice from the front of a glacier flowing into water.
Glacial lakes can form directly in front of retreating glaciers, in so-called overdeepenings (large bedrock hollows) formed by a glacier, or even on top of flat, slow-flowing glaciers. Further work on how widespread suitable conditions for further glacial lake development are across the region are key to our understanding of the future behaviour of glaciers in High Mountain Asia.
- King, O., Bhattacharya, A., Bhambri, R. et al. Glacial lakes exacerbate Himalayan glacier mass loss.. Sci Rep 9, 18145 (2019) doi:10.1038/s41598-019-53733-x
- Frey, H., Machguth, H., Huss, M., Huggel, C., Bajracharya, S., Bolch, T., Kulkarni, A., Linsbauer, A., Salzmann, N., and Stoffel, M. Estimating the volume of glaciers in the Himalayan–Karakoram region using different methods. Cryosphere, 8, 2313–2333, (2014).
- Brun, F., Berthier, E., Wagnon, P., Kääb, A. & Treichler, D. A spatially resolved estimate of High Mountain Asia glacier mass balances from 2000 to 2016. Nat. Geosci. 10, 668–673 (2017).
- Maurer, J., Schaefer, J. M., Rupper, S. & Corley, A. Acceleration of ice loss across the Himalayas over the past 40 years. Sci. Adv., 5 (2019).
- Nie, Y. et al. A regional-scale assessment of Himalayan glacial lake changes using satellite observations from 1990–2015. Remote Sense. Environ. 189, 1–13 (2017).
- Zhang, G. et al. An inventory of glacial lakes in the Third Pole region and their changes in response to global warming. Global and Planetary Change 131, 148–157 (2015).
- Kraaijenbrink, P.D.A., Bierkens, M.F.P., Lutz, A.F. & Immerzeel, W.W. Impact of a global temperature rise of 1.5 degrees Celsius on Asia’s glaciers. Nature, 549, 257-260 (2017).