The rate of glacier retreat in the Himalayas has doubled since the late 20th century, a new study finds.
By comparing modern-day satellite data to declassified film taken by US spies during the cold war, researchers have built the most complete picture yet of how ice levels in the world’s “third pole” have changed over the past 40 years.
The results show that, from 2000-16, the Himalayas lost an average of 8bn tonnes of ice per year. From 1975-2000, ice loss averaged 4bn tonnes of ice per year.
A recent uptick in temperature rise across the Himalayas is the most likely cause of this surge in glacier ice melt, the lead author tells Carbon Brief.
The findings provide “an indication of the sensitivity of glaciers across the Himalayas to changes in temperature now and in the future”, another scientist tells Carbon Brief.
The Himalayas is the world’s highest mountain range. The region’s tall peaks are the source of rivers that are relied upon by almost two billion people for freshwater across countries including India, China and Pakistan.
The Himalayas and its surrounding glaciers are acutely threatened by climate change. The Himalayas and Hindu Kush region has lost around 15% of its ice since the 1970s and is likely to lose at least a third of its total mass by 2100, even if global temperatures are limited to 1.5C above pre-industrial levels, according to a recent report.
Published in Science Advances, the new study is the first to create a comprehensive picture of recent glacier shrinkage across 2,000km of the mountain range. The study analyses changes in “650 of the largest glaciers”, which make up 55% of the ice volume in the region.
To study changes in the Himalayas, the authors made use of declassified film of the region taken by US intelligence agencies from 1973 to 1980.
Agencies used military satellites equipped with a “telescopic camera” to acquire “thousands of photographs worldwide”, the authors write in their research paper. “After which film recovery capsules were ejected from the satellites and parachuted back to Earth over the Pacific Ocean.”
They combined these files with modern imaging data taken by NASA’s Terra satellite.
The researchers then analysed both sets of images using modelling to create a 3D rendering of the ice surface for every year across the Himalayas. (The same technique has been used to study craters on the surface of Mars.)
The findings reveal that the rate of ice loss in the Himalayas has doubled since the period of 1975 to 2000, explains study lead author Josh Maurer, a graduate student in earth and environmental sciences at Columbia University. He tells Carbon Brief:
“We show that the central portion of the Himalayas lost around 4bn tonnes of ice per year, on average, between 1975 and 2000. During 2000 to 2016, the glaciers melted approximately twice as fast compared to the 1975 to 2000 interval, losing 8bn tonnes of ice per year. For a sense of scale, 8bn tonnes of ice is enough to fill 3.2m Olympic-sized swimming pools every year.”
The figure below, taken from the paper, includes a map of the glaciers included in the study (top) and two charts showing the average rate of ice mass loss per year for each glacier from 1975-2000 (middle) and 2000-16 (bottom).
(It is worth noting that the study did not include the glaciers in Karakoram and Kunlun, where levels of ice are “anomalously” stable or increasing, according to the researchers. Carbon Brief has previously reported on the surging of Karakoram’s glaciers.)
On the chart, circle sizes are proportional to glacier size and colour is used to indicate whether the glacier is debris-covered (black), debris-free (blue) or terminating in a glacial lake (red). (The presence of debris can affect the rate at which glaciers melt.)
On the charts, any number below zero indicates an overall decrease in the “mass balance” of a glacier. The mass balance is the difference between how much snow a glacier receives and the amount of its ice that melts away.
The charts indicate that the majority of glaciers across the Himalayan region have seen an acceleration in ice loss in response to a warming climate, Maurer says:
“The magnitude of ice loss we observe falls within the range of what we would expect if atmospheric warming were the dominant cause, based on [factors including] the rather homogeneous pattern of ice loss across such a large and climatically complex region.”
To further understand the influence of climate change on glacier loss, the researchers calculated the amount of energy that would be required to melt the quantity of ice that disappeared from 2000-16.
They found that a temperature increase of 0.4-1.4C in 2000-16, when compared to 1975-2000, would be required to melt the amount of ice lost in the Himalayas.
This “approximately agrees with the magnitude of warming observed by meteorological stations located throughout the High Mountains of Asia”, the research paper says.
The authors also compared the rate of ice loss in the Himalayas to the global average and to that in the European Alps. In their research paper, they say:
“We find that mass balance in the Himalayas is less negative compared to the Alps and the global average, despite close proximity to a known hot spot of increasing ‘black carbon’ [soot] emissions with rapid growth and accompanying combustion of fossil fuels and biomass in South Asia.”
There are several reasons why ice in Himalayas is melting more slowly than ice in the Alps, explains Prof Jonathan Bamber, a glaciologist at the University of Bristol, who wasn’t involved in the study. He tells Carbon Brief:
“One reason is that part of the mass loss in the Alps is due to glaciers in that region receding at the end of the Little Ice Age in around 1900. This affected the Alps but less likely the Himalayas. Second, the Himalayas are strongly influenced by the dynamics of the Asian monsoon and that has a complex relation to temperature changes.”
The Little Ice Age (LIA) was a period of colder than average temperatures and glacier advance between the 16th and mid-19th centuries. An increase in solar activity and a lull in volcanic eruptions are thought to have caused the LIA to end during the 19th century.
The end of the LIA saw the retreat of glaciers worldwide. The extent of glaciers can take “decades to centuries” to respond to changes in mass balance, which could explain why the event is still affecting the rate of glacier retreat in the Alps, according to a study published in 2014.
Overall, the findings “provide an indication of the sensitivity of glaciers across the Himalayas to changes in temperature now and in the future”, Bamber says:
“The results look pretty robust. There is a lot of variability from glacier to glacier but they have sampled a large number across a 2000km-long sector of the Himalayas, which adds robustness to the analysis.”
The study is “hugely significant because it provides the first robust and quantitative evidence …across the Himalayan range”, says Prof Duncan Quincey, a glaciologist from the University of Leeds, who also wasn’t on the research team. He tells Carbon Brief:
“What is particularly interesting for me is that the increasing air temperatures and accelerating mass loss over recent decades revealed by this study mirrors our own data on ice temperatures, which also suggest the glaciers are warming internally – and that large volumes of ice may already be close to the melting point.”
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