A new study shows that the melting of Himalayan glaciers caused by rising temperatures has accelerated dramatically since the start of the 21st century.
The analysis, spanning 40 years of satellite observations across India, China, Nepal and Bhutan, indicates that glaciers have been losing the equivalent of more than a vertical foot and half of ice each year since 2000 — double the amount of melting that took place from 1975 to 2000.
The study is the latest and perhaps most convincing indication that climate change is eating the Himalayas’ glaciers, potentially threatening water supplies for hundreds of millions of people downstream across much of Asia.
Clearest picture yet
“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 Ph.D. candidate at Columbia University’s Lamont-Doherty Earth Observatory. According to Maurer, the glaciers may have lost as much as a quarter of their enormous mass over the last four decades.
The Himalayas are sometimes called the earth’s “Third Pole” because they harbour some 600 billion tons of ice. While many other recent studies have suggested that the glaciers are wasting, those observations have been somewhat fragmented, zeroing in on shorter time periods, or only individual glaciers or certain regions.
Rising temperatures to blame
The new study, published last week in the journal Science Advances, synthesizes data from across the region, stretching from early satellite observations to the present. The synthesis indicates that the melting is consistent in time and space, and that rising temperatures are to blame. Temperatures vary from place to place, but from 2000 to 2016 they have averaged 1 degree Centigrade higher than those from 1975 to 2000.
Maurer and his colleagues analyzed repeat satellite images of some 650 glaciers spanning 2,000 kilometres from west to east. They found that from 1975 to 2000, glaciers across the region lost an average of about 0.25 meters of ice each year in the face of slight warming. Following a more pronounced warming trend starting in the 1990s, starting in 2000 the loss accelerated to about half a meter annually. Recent yearly losses have averaged about 8 billion tons of water, or the equivalent 3.2 million Olympic-size swimming pools, says Maurer.
Water shortages, floods to come
Some 800 million people depend in part on seasonal runoff from Himalayan glaciers for irrigation, hydropower and drinking water. The accelerated melting appears so far to be swelling runoff during warm seasons, but scientists project that this will taper off within decades as the glaciers lose mass. This, they say, will eventually lead to water shortages.
A separate study published this May estimates that yearly runoff is now about 1.6 times greater than if the glaciers were replenished at the same rate they were melting. As a result, in many high-mountain drainages, meltwater lakes are building rapidly behind natural dams of rocky debris; these are threatening downstream communities with potentially destructive and deadly outburst floods. Even on Mount Everest, long-lost corpses of climbers who failed to return are emerging from melting ice and snow along trails.
Entire planet affected by emissions
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. “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.