The rate at which Himalayan glaciers are melting has doubled in recent years, according to research which used spy satellite pictures from the Cold War.

Analysis from 40 years of satellite observations across India, China, Nepal and Bhutan shows loss of ice from the glaciers in the region – described as the Third Pole – has accelerated as the climate warms.

Some of the images used were captured by military satellites in the 1970s, which took thousands of pictures worldwide before ejecting film recovery capsules and parachuting them to Earth over the Pacific.

Loss

The scientists used the recently declassified spy satellite photographs to create 3D models and combined them with modern satellite imagery to produce a consistent set of data showing the changing elevation of glaciers over time.

They quantified the ice loss trends for 650 of the largest glaciers along a 2,000km (1,240 mile) sweep of the Himalayas from Spiti Lahaul, northern India to Bhutan, which account for around 55 percent of the region’s total amount of ice.

Ice loss from glaciers is important because it contributes to rising sea levels.

The glaciers high in the Himalayan mountains are also an important source of water for drinking, agriculture and hydropower for hundreds of millions of people downriver in south Asia.

Results from the comprehensive study published in the journal Science Advances indicate that the glaciers across the Himalayas have experienced significant loss in the thickness of ice over the past 40 years.

Rainfall

Ice was being lost twice as fast in the period 2000-2016 compared to 1975-2000, and the trend was consistent across the region – suggesting a warmer climate was the main driver, researchers said.

The thickness of the ice had shrunk by around 43cm a year on average in the 21st century, around twice the rate seen between 1975 and 2000.

Researchers estimated that less than three-quarters (72%) of the total amount of ice present in the region in 1975 was still there in 2016.

Lead author Joshua Maurer, from Columbia University’s Lamont-Doherty Earth Observatory said: “This is the clearest picture yet of how fast Himalayan glaciers are melting over this time interval, and why.”

While other factors such as changes in rainfall and increase in soot from industry landing on the ice and speeding up melting could be playing a role, the researchers said temperature was the overarching force.

Warming

Data from weather stations in the high mountains of Asia have recorded temperature increases of around 1C between 2000 and 2016.

The team compiled temperature data from the study period and calculated the amount of melting the warming conditions would be expected to produce – which matched the figures for what happened, the scientists said.

Mr Maurer said: “It looks just like what we would expect if warming were the dominant driver of ice loss.”

This Author

Emily Beament is Press Association environment correspondent.