Josh Maurer / LDEO
The glaciers of the world are melting faster than before, but it will take decades for changes to occur in an ice-age tempo.
To look into the past, researchers turn to a once secret source: spy satellite images from the 1970s and 1980s that have now been released. "The latest images can be downloaded for free from the USGS website, and users can use them," says Josh Maurer, Ph.D. student at Columbia University's Lamont-Doherty Earth Observatory.
Maurer is the lead author of a study using satellite imagery to show that Himalayan glaciers have melted twice as fast in the past 20 years as they did in the 1980s and 1990s. The work was published Wednesday in the journal Science Advances .
The spy satellite images are from KH-9 Hexagon military satellites launched during the Cold War to help the US view the Iron Curtain, says Summer Rupper, co-author of the study. Each satellite was about the size of a school bus and bore miles of film. Packed in buckets with parachutes, the film was later ejected into the upper atmosphere and pulled from the air by pilots of the Air Force over the Pacific. Most Hexagon images were released in 2011 as a continuation of a decree issued in 1995 by President Bill Clinton to publish spy satellite footage that was "scientifically or ecologically useful."
Maurer's study compares the spy satellite images, mostly from the mid-1970s With recent images of ASTER, an instrument connected to a NASA satellite co-developed by the US and Japan and launched in 1999.
There is a history of researchers using declassified surveillance images. Some scientists have used spy satellite data to study Arctic ice coverage, Antarctic streams, meteor tracks, and smaller glacier studies. According to Maurer, his team has found an efficient way to transform satellite images into 3D elevation models across a large region.
"With the help of spy satellites we can traverse the entire Himalayan range for a much longer period of time, [and measure] hundreds of glaciers of all types and sizes," says Rupper, an adjunct professor of geography at the University of Utah.
The Himalayas, home of Mount Everest, is home to tens of thousands of glaciers. The authors of the study considered 650 of them over a distance of 1,240 miles. They found that the Himalayan glaciers lost an average of 10 inches of ice per year between 1975 and 2000. With rising global average temperatures, the average loss rate doubled from 2000 to 2016 to 20 inches of ice per year.
] The glaciologist Etienne Berthier from the French national research agency CNRS, who was not involved in the study, said by email that the study had used the same analytical method throughout the Himalayas "[made] rate very convincing."  The Himalayas contain many different types of glaciers – such as Such as those that are covered by debris or near water – in many different environments. The researchers were surprised to find that the melting rate was constant across all glaciers studied. "In the east, rainfall in the Himalayas falls in the middle of summer [driven by monsoon winds] while in the west, most of the snow [in the winter] falls over a western storm lane," says Rupper. "So they do have two very different settings for these glaciers, but we still see a relatively even mass change from east to west."
That the Himalayan glaciers melt faster, signals unpredictability in the coming years. These glaciers provide mountain communities with fresh water and feed rivers that rely on billions of people in South Asia.
Sonam Futi Sherpa, a graduate student at Arizona State University, has written an article on how glaciers in the Everest region are changing with precipitation and storms. She says, "It is important to have long-term monitoring not only in Nepal, where she comes from, but also in Bhutan, Tibet, in other places, for two main reasons: to find out about future water availability and possible catastrophic events how to anticipate floods and landslides.
Deborah Balk of the City University of New York, who formerly worked on a panel of the National Research Council for Himalayan Glaciers and Climate Change, said by email that "understanding the loss of Glacial ice is particularly important in South Asia where the consequences of climate change are already unfolding "- consequences such as extreme heat in India, sea level rise and salinization in Bangladesh and regional flooding.
Over the next 80 years, according to a study of the Hindu Kush in 2019 The Himalayan region, where up to two-thirds of Himalayan glaciers are melting due to climate change
Pien Huang is Reflect America Fellow of the NPR and helps bring more different voices to the air and the Internet.