Declassified Cold War–era spy satellite film shows that the melting of hundreds of Himalayan glaciers has sped up in recent decades.
An analysis of 650 of the largest glaciers in the mountain range revealed that the total ice mass in 2000 was 87 percent of the 1975 mass. By 2016, the total ice mass had shrunk to only 72 percent of the 1975 total. The data show that the glaciers are receding twice as fast now as they were at the end of the 20th century, report Joshua Maurer, a glaciologist at Columbia University, and colleagues June 19 in Science Advances.
The primary cause for that acceleration, the researchers found, was warming: Temperatures in the region have increased by an average 1 degree Celsius from 2000 to 2016.
Meltwater from Himalayan glaciers are a source of freshwater to hundreds of millions of people each year. However, recent studies examining changes in glacier mass from 2000 to 2016 have shown that this store of freshwater is shrinking, threatening future water security in the region (SN Online: 5/29/19).
To project future glacier melt, scientists need to understand what has been driving the ice loss. In addition to warming, changes in precipitation and deposits of tiny pollutant particles called black carbon onto the surface of the ice have been implicated in speeding up melting. Such particles can darken the ices surface and reduce its albedo effect, or the reflection of incoming radiation from the sun back into space (SN Online: 5/19/14). As a result, the ice absorbs more heat and melts more quickly.
Some glaciers are melting faster than others, making it difficult to determine long-term trends for the whole region. So Maurer and his colleagues turned to the declassified spy data to get the big picture.
In the 1970s and 1980s, U.S. intelligence agencies used 20 KH-9 military satellites to collect reconnaissance data around the globe. The satellites took thousands of photographs, including of glaciers in the Himalayas, and then ejected the film capsules, which parachuted to Earth. After the images were declassified in 2011, scientists with the U.S. Geological Survey scanned them and made them publicly available.
Those photographs can be used to make stereoscopic images, in which two images of the same scene, taken from slightly different angles, are combined to create a 3-D image. Maurer and his colleagues devised a computer program to automate this time-consuming process and create three-dimensional digital snapshots of elevation across much of the Himalaya region. The team then did the same with data collected beginning in 1999 by a joint NASA-Japanese Ministry of Economy, Trade and Industry satellite called Terra.
By analyzing elevation changes over time, the researchers were then able to calculate loss of mass for each glacier. Using 1975 as a starting point, the team determined how much mass was lost by 2000, and then by 2016. The average rate of ice loss, they found, was about 0.43 meters of water per year per glacier from 2000 to 2016 — twice as fast as the rate calculated for the period from 1975 to 2000, about 0.22 meters of water per year.
An increase of between 0.4 degrees and 1.4 degrees Celsius fRead More – Source