The rate glaciers are melting in the Himalayas has doubled in just 20 years, according to a study which examined 40 years of satellite data.

Glaciers have been losing more than a vertical foot and a half of ice each year since 2000. This equates to 8 billion tons of water being released – or the equivalent of 3.2 million Olympic-size swimming pools.

The rate of melting is more than double that which took place between 1975 and 2000, according to the study published in Science Advances. It is the latest indication climate change is threatening the water supplies for 800 million people living downstream.

“This is the clearest picture yet of how fast Himalayan glaciers are melting over this time interval, and why,” said lead author Joshua Maurer, a PhD candidate at Columbia University.

These glaciers currently harbour 600 billion tons of ice and they may have lost as much as one quarter of their mass over the past four decades. On Mount Everest long-lost corpses of climbers are even starting to emerge from the ice.

Many of the 20th century observations were from declassified photographs taken by US spy satellites (Everest, pictured) (KH-9 Hexagon program)

At this rate of decline, two-thirds of the Himalayas’ ice cover – referred to as Earth’s ‘Third Pole’ – will be gone by 2100.

Rising temperatures are to blame, according to scientists who estimate that from 2000 to 2016 temperatures were 1C warmer than between 1975 and 2000.

Scientists scanned satellite images from 650 glaciers spanning 2,000 kilometres from west to east. Many of the 20th century observations were from declassified photographs taken by US spy satellites. Researchers used these to create 3D models that could show the changing elevations of the glacier over time.

Between 1975 and 2000 glaciers lost on average 0.25 metres of ice. In 2000 this loss increased to around half a metre every year and at lower levels the decline is up to five metres a year.

Glacier collapse shows climate impact Show all 20 1 /20 Glacier collapse shows climate impact Glacier collapse shows climate impact An iceberg floats in a fjord near the town of Tasiilaq Reuters Glacier collapse shows climate impact Meltwater pools on top of the Helheim glacier near Tasiilaq Reuters Glacier collapse shows climate impact Safety officer Brian Rougeux works with student Febin Magar to assemble a radar dome while working in a science camp on the side of the Helheim glacier Reuters Glacier collapse shows climate impact Airplane Mechanic, David Fuller, left, works with a local worker to move a Nasa Gulfstream III during a pre-flight inspection before a flight to support the Oceans Melting Greenland research mission Reuters Glacier collapse shows climate impact Meltwater pools on top of the Helheim glacier Reuters Glacier collapse shows climate impact Earth Science Flight Programs Director at Nasa, Eric Ianson, looks out at the Greenland ice sheet Reuters Glacier collapse shows climate impact Glacial ice is seen from the window during the Nasa flight Reuters Glacier collapse shows climate impact Oceanographer David Holland's science camp on the side of the Helheim glacier Reuters Glacier collapse shows climate impact An iceberg floats in a fjord near the town of Tasiilaq Reuters Glacier collapse shows climate impact A large crevasse forms near the calving front of the Helheim glacier Reuters Glacier collapse shows climate impact Safety officer Brian Rougeux uses a drill to install antennas for scientific instruments that will be left on top of the Helheim glacier Reuters Glacier collapse shows climate impact Tabular icebergs float in the Sermilik Fjord after a large calving event at the Helheim glacier Reuters Glacier collapse shows climate impact Radar Engineer, Ron Muellerschoen, monitors data collection inside a NASA Gulfstream III flying above Greenland to measure loss to the country's ice sheet Reuters Glacier collapse shows climate impact GPS tracking equipment is left on top of the Helheim glacier REUTERS Glacier collapse shows climate impact Sunshine lights up the Helheim glacier Reuters Glacier collapse shows climate impact A glacial terminus above the east coast of Greenland REUTERS Glacier collapse shows climate impact Student Febin Magar watches as leftover wood burns in a research camp Reuters Glacier collapse shows climate impact Tabular icebergs float in the Sermilik Fjord after a large calving event Reuters Glacier collapse shows climate impact Oceanographer David Holland repairs a broken GPS module at his research camp Reuters Glacier collapse shows climate impact An iceberg floats in a fjord near the town of Tasiilaq Reuters

Another driving factor is Asian nations burning more and more fossil fuels, sending soot into the sky. This eventually lands on the surface of the glacier where it absorbs solar energy and accelerates melting, researchers belive.

The Himalayas are not melting as fast as the Alps, but the general progression is similar.

Eight-hundred million people depend on seasonal runoff from Himalayan glaciers for irrigation, hydropower and water. There is currently more run-off during warm seasons but scientists say within the next few decades this will decrease as the glaciers lose mass. This will eventually lead to water shortages.

The study shows that “even glaciers in the highest mountains of the world are responding to global air temperature increases driven by the combustion of fossil fuels,” said Joseph Shea, a glacial geographer at the University of Northern British Columbia who was not involved in the study.

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“In the long term, this will lead to changes in the timing and magnitude of streamflow in a heavily populated region.”

“It shows how endangered [the Himalayas] are if climate change continues at the same pace in the coming decades,” said Etienne Berthier, a glaciologist at France’s Laboratory for Studies in Geophysics and Spatial Oceanography, who also was not involved in the study.

The study follows research from last month that found glaciers in the high-mountain regions around the Himalayas were shrinking 1.6 times faster between 2000 and 2016 than the period between 1951 to 2007.

“A lot of the climate projections suggest the melt will continue to accelerate,” lead researcher Dr Hamish Pritchard, a glaciologist from the British Antarctic Survey (BAS), told The Independent at the time.