Ice cliffs on the surfaces of glaciers in the Himalayas are hotspots for melting, even in areas where rocky debris or chilly temperatures should slow it down, new research indicates.

Scientists used satellite imagery, data from weather stations and mathematical modeling to investigate how much ice cliffs contributed to melting across four debris-covered glaciers in Nepal. They found that, despite covering only 0.6% of the glacial area, ice cliffs were responsible for an estimated 15% of the mass lost over one melt season. The team reported the findings on Feb. 19 in Geophysical Research Letters. 

"The fact that our simulations showed relatively high contributions of ice cliffs to glacier melt in zones of the glacier where glacier ice is normally melting less ... indicates that ice cliffs might become more important for glacier melt," said Pascal Buri, a postdoctoral researcher at the Swiss Federal Institute for Forest, Snow and Landscape Research WSL and first author of the paper. As the climate warms, he said, "The impact of ice cliffs in terms of melt contribution will most likely increase at the scale of single glaciers, but also at a regional scale."

Recent studies have predicted that climate change has caused melting on Himalayan glaciers to accelerate since 2000, and that at least one-third of the glaciers will melt by the end of the century.

A significant portion of the glacier ice in High Mountain Asia — a region that spans five mountain ranges covering parts of Afghanistan, China, India and a number of other countries — is covered in a blanket of rocks. This debris is commonly found on icy projections from the base of the glacier called tongues. It insulates the ice underneath, preventing it from melting as quickly as bare glacier ice, Buri says.

Surprisingly, though, debris-covered glaciers are melting at similar rates as "clean" glaciers. Scientists think that areas of exposed ponds and ice cliffs, which are too steep to hold rocky material, could be the culprits. Previous research has indicated that ice cliffs are melting much more than the surrounding debris-covered ice. 

"But their actual contribution to glacier mass loss has never been quantified beyond subsets of a glacier, so far," Buri said.

He and his colleagues used a mathematical model that they developed for previous research to simulate melting on 366 ice cliffs they detected across four glaciers in Nepal's Langtang Valley. The team examined satellite images and elevation data from April 2014, when the melt season began, to identify the topography of each cliff and the surrounding area. They also drew on hourly records of temperature, wind speed, relative humidity and other weather-related measurements collected from the area between May and October 2014.

"With this input data, the initial cliff shapes and the hourly weather data throughout the melt season, we could run the model for one entire melt season [and] simulate how the cliffs melted and changed over this time and how much ice they lost," Buri said. "Cliffs can shrink and flatten out and disappear, or grow in size or [get] steeper."

He and his team next calculated how much glacier ice melted from the glaciers overall between 2006 and 2015, based on previous estimates of glacier surface elevation changes and upwellings of ice from the lower regions of the glaciers. This allowed the researchers to compare the amount of melting that the ice cliffs endured in 2014 to the average yearly loss of glacier ice across the area. 

The team found that, although glacier tongues exposed only 2.1% of their surface area to the atmosphere as ice cliffs, these ice cliffs accounted for 17% of the tongues' annual ice loss.

Across the whole region Buri and his team investigated, ice cliffs covered less than 1% of the glacier area but caused 15% of the ice loss.

The cliffs also caused ice to melt in areas with environmental conditions that would normally suppress it, Buri said. These included parts of the glaciers at high elevations — where the air temperature is lower — and in areas where the rocky debris was generally abundant. This means that in the lowest parts of glacier tongues, where rocky debris is thickest, ice cliffs account for a larger share of the total glacier melt, Buri said.

Buri suspects that the model actually slightly underestimated the contribution of ice cliffs. Because researchers aren't sure exactly how ice cliffs form, they couldn't take into account the "birth" of new ice cliffs during the melting season. Additionally, the researchers may not have been able to detect all of the region's ice cliffs from the satellite images, and melting may have begun before or continued after the months they examined. 

Buri and his team next plan to investigate whether their findings apply to glaciers in other parts of High Mountain Asia and beyond.

This is important because as climate change causes glaciers to shrink, more rocky debris will become exposed and slide down to cover part of the remaining ice. 

"Since glacial debris-cover will increase with a warming climate globally," Buri said, "a better understanding of debris-type glaciers and hopefully robust predictions of how they evolve and affect water resources might be crucial for estimating regional and global estimates of sea level rise and socio-economic effects."

The study, "Supraglacial ice cliffs can substantially increase the mass loss of debris‐covered glaciers," published Feb. 19 in Geophysical Research Letters, was authored by Pascal Buri, University of Alaska Fairbanks and Swiss Federal Institute for Forest, Snow and Landscape Research WSL; Evan S. Miles, Swiss Federal Institute for Forest, Snow and Landscape Research WSL; Francesca Pellicciotti, Swiss Federal Institute for Forest, Snow and Landscape Research WSL and Northumbria University; Jakob F. Steiner, Utrecht University and International Centre for Integrated Mountain Development; and Silvan Ragettli, ETH Zürich.