Melt-rate of Himalayan glaciers has doubled

A new study on central Himalayan glaciers in India, China, Nepal and Bhutan finds that this region has been losing ice during the 21st century at twice the rate it did during the previous 25 years. This trend was likely driven by increasing air temperatures, the study, published in Science Advances, finds.

Why it matters: This region is home to so much ice that it's sometimes referred to as Earth's "Third Pole." Runoff from ice melt nourishes some of the most populous nations on Earth, and the fate of these glaciers are thus intertwined with the ability of this region to sustain high population growth and avert conflict over increasingly stressed water supplies.

This study provides some of the clearest evidence yet that air temperature is a main control knob for glacial melt in the Himalayas, which was not previously known.

“Even glaciers in the highest mountains of the world are responding to global air temperature increases driven by the combustion of fossil fuels.”

What they did: The study analyzed 650 Himalayan glaciers along a 1,243-mile transect of the mountain range, representing 55% of the region's total ice volume.

• Researchers used a combination of repeat NASA satellite and declassified Cold War-era spy satellite photographs to construct a set of three-dimensional digital elevation models for each glacier.
• By examining the differences between the historical reconstruction and measurements of contemporary glacier surfaces derived from modern satellites, the researchers were able to detect glacier mass changes over time.

What they found: Of the total ice mass that existed in the study area in 1975, 87% of it still was present in 2000. Only 72% was present just 16 years later.

• According to study lead author Josh Maurer of Columbia University, the main section of the Himalayas lost an average of about 4 billion tons of ice per year between 1975 and 2000.
• During the period from 2000 to 2016, however, the glaciers lost an average of about 8 billion tons of ice per year.

Context: A separate study, published last month in the journal Nature, found that the meltwater coming off the glaciers high in the Himalayas is 1.6 times greater than the rate needed to keep the glaciers in balance with snowfall.

• Previous studies have come to differing conclusions about how Himalayan glaciers have been faring in a warming world, or presented a limited geographical view.
• A report released in February found that the Himalayas could lose two-thirds of its glaciers by 2100 if greenhouse gas emissions are not significantly curtailed.

But, but, but: The study notes that there are uncertainties involved in what is driving increased ice loss in the 1,243-mile region.

• While temperatures have increased, there are also areas where precipitation has decreased, which could be shrinking these glaciers.
• In addition, the role of soot deposition from cars and coal-fired power plants in South Asia is little understood, but could be accelerating ice loss as well.
• Some Himalayan glaciers outside the region studied show countervailing trends, with glaciers remaining relatively stable or growing slightly.