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The 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鈥檚 glaciers has been grimly illustrated recently on Everest, where receding snow and ice . Glacier loss in the region could play havoc with water supplies for millions of people, crops and hydroelectric dams, as well as contribute to flooding from glacier lakes.

But researchers charting losses in the region聽have been reliant on recent modern satellite records and only know ice mass losses well for the past two decades.

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. 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 past decade and .

As the images were taken at different angles, Josh Maurer at Columbia University in New York and his 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 per year between 1975 and 2002, a rate that doubled to half a metre per year between 2000 and 2016.

The results show the Himalayas have lost a quarter of their ice mass since 1975. 鈥淕oing back this far back in time for the entire region is great,鈥� says Walter Immerzeel of Utrecht University in the Netherlands, who wasn鈥檛 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 glacier, suggests climate change is the main driver. 鈥淲e see a close correlation between rising temperatures and ice loss accelerating,鈥� says Maurer.

Jonathan Bamber of Bristol University, UK, who didn鈥檛 take part in the study, says that while the current聽rate of loss isn鈥檛 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 2000-kilometre line in the Himalayas, roughly 55 per cent of the region鈥檚 ice mass, it didn鈥檛 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聽depends on how much the world warms. But the new research is in line with models suggesting that with the highest expected temperature rises, . The UN climate science panel had to apologise in 2010 after it said Himalaya鈥檚聽glaciers would very likely disappear by 2035, a claim whose ultimate source was an Indian scientist鈥檚 comment聽in a聽1999 快猫短视频 article.