The Himalayan landscape in 1975 on the border between eastern Nepal and Sikkim, IndiaJosh Maurer/LDEO By Adam VaughanThe speed at which glaciers in the Himalayas are losing ice has doubled since the turn of the century as the region warms, an analysis of declassified spy film has revealed. The rapid melting of the region’s glaciers has been grimly illustrated recently on the north side of Everest, where receding snow and ice has begun exposing bodies of the hundreds of people who have died climbing the peak. Glacier loss in the region could play havoc with water supplies for millions of people, crops and hydro-electric dams, as well as contribute to glacier lakes flooding. But researchers charting losses in the region have been reliant on recent modern satellite records and only know ice mass losses well for the past two decades. Advertisement Read more: Climate change means nearly all glaciers in the Alps may disappear Now a US team has used 42 images of the mountain range taken by US military satellites during the cold war to turn the clock back further in time. The spy film, which was ejected from the satellites in cartridges and parachuted down for collection mid-air by cargo planes, was declassified in the last decade and made publicly available online. As the images were taken at different angles, Josh Maurer of Columbia University and colleagues could use software to work out the depth of glaciers. Combining the results with data from modern depth-sensing satellites, they found on average 0.25 metres of ice thickness were lost between 1975 and 2002, a rate that doubled to half a metre between 2000 and 2016. The results shows the Himalayas have lost a quarter of their ice mass since 1975. “Going back this far back in time for the entire region is great,” says Walter Immerzeel of Utrecht University, who was not involved in the work. Monsoon-driven climate The complex monsoon-driven climate of the region makes it hard to say precisely what factors are to blame for the ice melt. But the fact that the ice losses are seen consistently throughout the region and on different types of glaciers suggests climate change is the main driver. “We see a close correlation between rising temperatures and ice loss accelerating,” says Maurer. Jonathan Bamber of Bristol University, who did not take part in the study, says that while the current rate of loss is not a big surprise given the observed temperature rises, the research was important because it showed how glaciers respond to regional warming. While the analysis looked at 650 of the largest glaciers across a 2,000km line in the Himalayas, roughly 55 per cent of the region’s ice mass, it did not look at the Karakoram and Kunlun mountain ranges to the north-east, where glaciers are stable or even growing in places. When Himalayan glaciers will vanish depends on how much the world warms. But the new research is in line with models suggesting that with the highest expected temperature rises, 64 per cent of the region’s ice will be lost. The UN climate science panel had to apologise in 2010 after it said Himalaya’s glaciers would very likely disappear by 2035, a claim whose ultimate source was an Indian scientist’s comment in a 1999 New Scientist article. Journal reference: Science Advances, DOI: eaav7266 More on these topics: climate change glaciers
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