Rising global mean temperature has led to the retreat of the majority of the world’s glaciers (Lemke et al., 2007). Glaciers in the Mount Everest (Sagamartha) region of Nepal are receding at an average rate of 10–59 m a–1 (Bajracharya and Mool, 2009). The Imja glacier, located just southeast of Mount Everest (fig.1), in the Khumbu Range of Eastern Nepal’s Himalaya, retreated at 41 meters/year from 1961-2000 and 74 meters/year from 2001-2006 (Bajracharya and Mool, 2009).
Its heavy recession resulted in the formation of a glacial lake at the foot of the glacier in the 1960’s. Since then, the Lake Imja has expanded from 0.03 km2 to 1.35 km2 at a rate of 0.026 km2a-1, developing into one of the largest glacial lakes in the Himalayas. Glacial lakes can be very dangerous, as they can trigger an outburst flood.
Glacier recession also causes sea level to rise, and can help accelerate climate change through a number of glacier-climate feedback processes (Lemke et al., 2007). Glacial change also impacts river flows and landscape evolution.
It is therefore important to understand the speed of glacier retreat and to predict their evolution. In this study, mass balance and thickness of the Imja glacier is predicted, in order to estimate its likely longevity.
Glacier mass balance tells us about the change in mass of a glacier over a specified time period, due to an imbalance between accumulation and ablation. The geodetic approach is used to calculate the mass balance of the Imja glacier, which computes the difference between digital elevation models (DEMs) from satellite imagery several years apart, producing a glacier surface elevation change. The sensors contain three cameras – one facing forwards, one directly downwards and one backwards. Stereoscopy is then used to compute the elevation for every image pixel. One DEM was derived from satellite imagery acquired by the Advanced Land Observing Satellite (ALOS) Panchromatic Remote-sensing Instrument for Stereo Mapping (PRISM) in April 2009. The other DEM is from satellite imagery acquired by the Shuttle Radar Topography Mission (SRTM) in February 2000. The glacier volume change is calculated from the difference between the two DEMs, which is converted to a mass change. A mass balance in metres water equivalent (m.w.e.) of negative 1 m.w.e. means that every year a 1 m depth of water, across the whole glacier surface, is lost.
Along with an estimate of the glacier’s maximum thickness, and by making several assumptions, the mass balance can be used to estimate the lifetime of the Imja glacier, before it completely disappears:
Glacier longevity = Current maximum thickness ÷ Glacier mass loss
The maximum glacier thickness is used because this gives us the longevity of the thickest part of the glacier, which is likely to last the longest amount of time before the glacier completely disappears. The maximum ice thickness is estimated using the perfect plasticity approach (Nye, 1951), which relates the thickness and surface slope to a yield stress.
The geodetic approach produced a summed elevation change of -2.90x105 m between February 2000 and April 2009 for the Imja glacier. A negative elevation change for Imja Glacier was also found by Thakuri et al. (2016) (fig.2) and by King et al. (2016) (fig.3).
Our summed elevation change equates to a glacier mass balance of -1.31 m.w.e.a-1. This essentially means that every year a 1 m depth of water, across the whole glacier surface, is lost. This agrees with Bolch et al. (2011), Nuimura et al. (2012) and Gardelle et al. (2013), who showed the Imja Glacier experiencing a mean of –1.45 m.w.e.a-1 during 2002–2007, of –0.93 m.w.e.a-1 during 2000–2008, and of –0.70 m.w.e.a-1 during 1999–2011.
Figure 2. Glacier elevation change of Imja Glacier for 2001–14, with the area mean (inset box). Mean elevation change is plotted as a function of elevation in the panel on the right. Taken from Thakuri et al. (2016).
With a mass balance of -1.31 ma-1 and a maximum glacier thickness of 329 m, a simple calculation suggests that the Imja Glacier will completely disappear in 251 years (year-2260), if it were to continue to lose mass at the same rate.
The process used to calculate the lifetime of the Imja Glacier was greatly simplified and contains some technical uncertainties. However, large uncertainties arise from glaciological and climatological factors influencing the future dynamics and evolution of the glacier, which are not considered in our estimation of the glacier lifetime.
Firstly, global mean temperatures are set to continue to rise. This will continue to cause greater ablation of glaciers, increasing the rate of glacier retreat. Differing emission and climate scenarios means future global temperatures are unknown, so the effect of rising temperature on glacier mass loss contains are large amount of uncertainty. Natural forcings, such as volcanic eruptions and changes in incoming solar radiation, also add uncertainty to glacier mass changes, particularly in the lower latitudes (Huss et al., 2009).
As the Imja Glacier losses mass, the dynamical processes that control its mass balance are likely to change. Differences in the rate of thinning across the glacier can result in changes to the glacier slope, which can lead to changes in the glacier speed.
Glacial lakes enhance glacier melt and favours mass loss through calving, so the continued growth of the Imja Lake may increase the retreat rate of the glacier. Basnett et al. (2013) found that debris covered glaciers in the Sikkim Himalaya with proglacial lakes have greater retreat than glaciers without proglacial lakes
Changes to the amount of debris cover as the glacier retreats will also affect the mass balance over time. If the debris cover is thin, it tends to enhance glacier melting, but if the debris cover is thick enough, it tends to reduce melting by insulating the glacier. Takeuchi et al. (2000) found that in the Khumbu Glacier, debris cover less than 5 cm increases ablation, whilst debris cover greater than 5 cm reduces ablation. Debris cover is likely to increase with increased melting due to warmer temperatures, so this is likely to increase the glacier mass loss with time.
Other changes to accumulation and ablation zones of the glacier may also have an impact on its mass loss, such as changes to the frequency and/or impact of avalanches.
Overall, it is likely that the glacier mass loss will increase with time, resulting in our lifetime estimation for the Imja Glacier of 251 years to be an overestimate. Thakuri et al. (2016) found that the loss rate of the Imja glacier has increased over time, from 0.04 km2a-1 for 1962-1992, to 0.11 km2a-1 for 1992-2013. This is confirmed by Bolch et al. (2011) and by Nuimura et al. (2012).
BAJRACHARYA, S. R. & MOOL, P. 2009. Glaciers, glacial lakes and glacial lake outburst floods in the Mount Everest region, Nepal. Annals of Glaciology, 50, 81-86.
BASNETT, S., KULKARNI, A. V. & BOLCH, T. 2013. The influence of debris cover and glacial lakes on the recession of glaciers in Sikkim Himalaya, India. Journal of Glaciology, 59, 1035-1046.
BOLCH, T., PIECZONKA, T. & BENN, D. 2011. Multi-decadal mass loss of glaciers in the Everest area (Nepal Himalaya) derived from stereo imagery. The Cryosphere, 5, 349-358.
HUSS, M., FUNK, M. & OHMURA, A. 2009. Strong Alpine glacier melt in the 1940s due to enhanced solar radiation. Geophysical Research Letters, 36.
LEMKE, P., REN, J., ALLEY, R. B., ALLISON, I., CARRASCO, J., FLATO, G., FUJII, Y., KASER, G., MOTE, P. & THOMAS, R. H. 2007. Observations: changes in snow, ice and frozen ground.
LI, H., NG, F., LI, Z., QIN, D. & CHENG, G. 2012. An extended “perfect‐plasticity” method for estimating ice thickness along the flow line of mountain glaciers. Journal of Geophysical Research: Earth Surface, 117.
NUIMURA, T., FUJITA, K., YAMAGUCHI, S. & SHARMA, R. R. 2012. Elevation changes of glaciers revealed by multitemporal digital elevation models calibrated by GPS survey in the Khumbu region, Nepal Himalaya, 1992–2008. Journal of Glaciology, 58, 648-656.
NYE, J. The flow of glaciers and ice-sheets as a problem in plasticity. Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, 1951. The Royal Society, 554-572.
King, O., Quincey, D.J., Carrivick, J.L. and Rowan, A.V., 2016. Spatial variability in mass change of glaciers in the Everest region, central Himalaya, between 2000 and 2015. The Cryosphere Discussions, pp.1-35.
RAPER, S. C. & BRAITHWAITE, R. J. 2006. Low sea level rise projections from mountain glaciers and icecaps under global warming. Nature, 439, 311-313.
TAKEUCHI, Y., KAYASTHA, R. B. & NAKAWO, M. 2000. Characteristics of ablation and heat balance in debris-free and debris-covered areas on Khumbu Glacier, Nepal Himalayas, in the pre-monsoon season. IAHS PUBLICATION, 53-62.
WANG, D. & KÄÄB, A. 2015. Modeling glacier elevation change from DEM time series. Remote Sensing, 7, 10117-10142.
Figure 1. Location of the Imja glacier. Its glacial lake, Lake Imja, can be seen in the bottom-left of the image. Taken from Google Earth Pro.
Its heavy recession resulted in the formation of a glacial lake at the foot of the glacier in the 1960’s. Since then, the Lake Imja has expanded from 0.03 km2 to 1.35 km2 at a rate of 0.026 km2a-1, developing into one of the largest glacial lakes in the Himalayas. Glacial lakes can be very dangerous, as they can trigger an outburst flood.
Glacier recession also causes sea level to rise, and can help accelerate climate change through a number of glacier-climate feedback processes (Lemke et al., 2007). Glacial change also impacts river flows and landscape evolution.
It is therefore important to understand the speed of glacier retreat and to predict their evolution. In this study, mass balance and thickness of the Imja glacier is predicted, in order to estimate its likely longevity.
Methods
Glacier mass balance tells us about the change in mass of a glacier over a specified time period, due to an imbalance between accumulation and ablation. The geodetic approach is used to calculate the mass balance of the Imja glacier, which computes the difference between digital elevation models (DEMs) from satellite imagery several years apart, producing a glacier surface elevation change. The sensors contain three cameras – one facing forwards, one directly downwards and one backwards. Stereoscopy is then used to compute the elevation for every image pixel. One DEM was derived from satellite imagery acquired by the Advanced Land Observing Satellite (ALOS) Panchromatic Remote-sensing Instrument for Stereo Mapping (PRISM) in April 2009. The other DEM is from satellite imagery acquired by the Shuttle Radar Topography Mission (SRTM) in February 2000. The glacier volume change is calculated from the difference between the two DEMs, which is converted to a mass change. A mass balance in metres water equivalent (m.w.e.) of negative 1 m.w.e. means that every year a 1 m depth of water, across the whole glacier surface, is lost.
Along with an estimate of the glacier’s maximum thickness, and by making several assumptions, the mass balance can be used to estimate the lifetime of the Imja glacier, before it completely disappears:
Glacier longevity = Current maximum thickness ÷ Glacier mass loss
The maximum glacier thickness is used because this gives us the longevity of the thickest part of the glacier, which is likely to last the longest amount of time before the glacier completely disappears. The maximum ice thickness is estimated using the perfect plasticity approach (Nye, 1951), which relates the thickness and surface slope to a yield stress.
Results and Discussion
The geodetic approach produced a summed elevation change of -2.90x105 m between February 2000 and April 2009 for the Imja glacier. A negative elevation change for Imja Glacier was also found by Thakuri et al. (2016) (fig.2) and by King et al. (2016) (fig.3).
Our summed elevation change equates to a glacier mass balance of -1.31 m.w.e.a-1. This essentially means that every year a 1 m depth of water, across the whole glacier surface, is lost. This agrees with Bolch et al. (2011), Nuimura et al. (2012) and Gardelle et al. (2013), who showed the Imja Glacier experiencing a mean of –1.45 m.w.e.a-1 during 2002–2007, of –0.93 m.w.e.a-1 during 2000–2008, and of –0.70 m.w.e.a-1 during 1999–2011.
Figure 3. Glacier surface elevation change over the study area between 2000 and 2014/15. Also shown is a summary of off-glacier terrain differences. Areas of no data show the ASTER GDEM underlay. Taken from King et al. (2016).
With a mass balance of -1.31 ma-1 and a maximum glacier thickness of 329 m, a simple calculation suggests that the Imja Glacier will completely disappear in 251 years (year-2260), if it were to continue to lose mass at the same rate.
|
Summed elevation change (m)
|
-2.90x105
|
|
Volume change (m3)
|
-2.61x108
|
|
Mass change (tonnes)
|
-2.35x108
|
|
Mass change per year (tonnes per year)
|
-2.56x107
|
|
Mass balance (metres water equivalent)
|
-1.31
|
|
|
|
|
Maximum thickness (m)
|
329
|
|
Glacier lifetime (years)
|
251
|
Uncertainties in the Evolution Forecast
The process used to calculate the lifetime of the Imja Glacier was greatly simplified and contains some technical uncertainties. However, large uncertainties arise from glaciological and climatological factors influencing the future dynamics and evolution of the glacier, which are not considered in our estimation of the glacier lifetime.
Firstly, global mean temperatures are set to continue to rise. This will continue to cause greater ablation of glaciers, increasing the rate of glacier retreat. Differing emission and climate scenarios means future global temperatures are unknown, so the effect of rising temperature on glacier mass loss contains are large amount of uncertainty. Natural forcings, such as volcanic eruptions and changes in incoming solar radiation, also add uncertainty to glacier mass changes, particularly in the lower latitudes (Huss et al., 2009).
As the Imja Glacier losses mass, the dynamical processes that control its mass balance are likely to change. Differences in the rate of thinning across the glacier can result in changes to the glacier slope, which can lead to changes in the glacier speed.
Glacial lakes enhance glacier melt and favours mass loss through calving, so the continued growth of the Imja Lake may increase the retreat rate of the glacier. Basnett et al. (2013) found that debris covered glaciers in the Sikkim Himalaya with proglacial lakes have greater retreat than glaciers without proglacial lakes
Changes to the amount of debris cover as the glacier retreats will also affect the mass balance over time. If the debris cover is thin, it tends to enhance glacier melting, but if the debris cover is thick enough, it tends to reduce melting by insulating the glacier. Takeuchi et al. (2000) found that in the Khumbu Glacier, debris cover less than 5 cm increases ablation, whilst debris cover greater than 5 cm reduces ablation. Debris cover is likely to increase with increased melting due to warmer temperatures, so this is likely to increase the glacier mass loss with time.
Other changes to accumulation and ablation zones of the glacier may also have an impact on its mass loss, such as changes to the frequency and/or impact of avalanches.
Overall, it is likely that the glacier mass loss will increase with time, resulting in our lifetime estimation for the Imja Glacier of 251 years to be an overestimate. Thakuri et al. (2016) found that the loss rate of the Imja glacier has increased over time, from 0.04 km2a-1 for 1962-1992, to 0.11 km2a-1 for 1992-2013. This is confirmed by Bolch et al. (2011) and by Nuimura et al. (2012).
References
BAJRACHARYA, S. R. & MOOL, P. 2009. Glaciers, glacial lakes and glacial lake outburst floods in the Mount Everest region, Nepal. Annals of Glaciology, 50, 81-86.
BASNETT, S., KULKARNI, A. V. & BOLCH, T. 2013. The influence of debris cover and glacial lakes on the recession of glaciers in Sikkim Himalaya, India. Journal of Glaciology, 59, 1035-1046.
BOLCH, T., PIECZONKA, T. & BENN, D. 2011. Multi-decadal mass loss of glaciers in the Everest area (Nepal Himalaya) derived from stereo imagery. The Cryosphere, 5, 349-358.
HUSS, M., FUNK, M. & OHMURA, A. 2009. Strong Alpine glacier melt in the 1940s due to enhanced solar radiation. Geophysical Research Letters, 36.
LEMKE, P., REN, J., ALLEY, R. B., ALLISON, I., CARRASCO, J., FLATO, G., FUJII, Y., KASER, G., MOTE, P. & THOMAS, R. H. 2007. Observations: changes in snow, ice and frozen ground.
LI, H., NG, F., LI, Z., QIN, D. & CHENG, G. 2012. An extended “perfect‐plasticity” method for estimating ice thickness along the flow line of mountain glaciers. Journal of Geophysical Research: Earth Surface, 117.
NUIMURA, T., FUJITA, K., YAMAGUCHI, S. & SHARMA, R. R. 2012. Elevation changes of glaciers revealed by multitemporal digital elevation models calibrated by GPS survey in the Khumbu region, Nepal Himalaya, 1992–2008. Journal of Glaciology, 58, 648-656.
NYE, J. The flow of glaciers and ice-sheets as a problem in plasticity. Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, 1951. The Royal Society, 554-572.
King, O., Quincey, D.J., Carrivick, J.L. and Rowan, A.V., 2016. Spatial variability in mass change of glaciers in the Everest region, central Himalaya, between 2000 and 2015. The Cryosphere Discussions, pp.1-35.
RAPER, S. C. & BRAITHWAITE, R. J. 2006. Low sea level rise projections from mountain glaciers and icecaps under global warming. Nature, 439, 311-313.
TAKEUCHI, Y., KAYASTHA, R. B. & NAKAWO, M. 2000. Characteristics of ablation and heat balance in debris-free and debris-covered areas on Khumbu Glacier, Nepal Himalayas, in the pre-monsoon season. IAHS PUBLICATION, 53-62.
WANG, D. & KÄÄB, A. 2015. Modeling glacier elevation change from DEM time series. Remote Sensing, 7, 10117-10142.
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