Warmer winters in the Himalayas are triggering avalanches, finds a study of tree rings

Climate in the mighty yet fragile snow-covered Himalayas has been changing rapidly. But, monitoring the trends and effects has been challenging because of the high altitudes and the rugged terrain. Now, with the help of mountain-top observatories, Indian scientists from the Snow and Avalanche Study Establishment in Chandigarh in the North of India have found that in the past 25 years, winters in the northwestern Himalayas are getting warmer and wetter with less snowfall. This warming is leading to a greater risk of wet avalanches in the western Indian Himalayas, reveals another study by a team of Swiss researchers from the University of Geneva who are examining tree-ring growth abnormalities.

The Himalayas are a massive 2,500-kilometre arc-shaped stretch of lofty mountains straddling Pakistan, China, India, Nepal, and Bhutan. Also, known as the “Third Pole”, the Himalayas are home to the largest concentration of glaciers outside of the poles.

Warmer and wetter winter

To find out how exactly the climate has changed high up in the mountains, Harendra Singh Negi and his team analysed trends in winter (November-April) temperature and precipitation in the northwestern Himalayas and Karakoram from 1991 to 2015 – a period of 25 years.

They collected data from “a network of observatories” established by the Snow and Avalanche Study Establishment on mountain-tops ranging between 2,000 to 6,000 metres. Compared with other studies that use indirect satellite-driven data or models, these observatories, he says, contain “precious data” that “reflect the true picture” of climate conditions on mountain tops rather than at valley stations.

Even within the Himalayas, the climate varies according to the altitude of the mountains, so they divided the region into zones with similar elevations. The Lower Himalayas (observatories at mostly 2000-3000 m) are characterised by moderate temperatures and high rainfall, the Greater Himalayas (observatories at 3000-4000 m) are colder with dry snowfall, and the Karakoram region (observatories at 4000-6000 m) is further up North where it is frigid and largely blanketed with swathes of glaciers. The Karakoram mountain range – which harbours the world’s highest peak, K2, and the longest non-polar glacier, Siachen – spans the borders of Pakistan (Gilgit-Baltistan), India (Ladakh), and China (Xingjiang Autonomous region).

They found that overall, in the northwestern Himalayas, average temperatures rose by 0.65 degrees Celsius since 1991. This is “much higher than the global average” increase of 0.40°C during the same period, said Anil Kulkarni, a scientist from Divecha Center for Climate Change at the Indian Institute of Science, Bangalore, which published a policy brief on the state of Himalayan glaciers and future projections. While the Greater Himalayan region saw the highest rise in average temperature of 0.87 degrees Celsius, Karakoram came second with an average increase of 0.56°C.

The warming trend agrees with other studies including his own, said Walter Immerzeel, a glacier hydrologist from Utrecht University in the Netherlands, although he cautions that summer temperatures also need to be considered and “very limited information is given about the stations, data quality and sensors used” in this study.

From 2000 onwards, however, the Lower Himalayas experienced an unusual cooling in minimum and maximum temperatures, according to the Snow and Avalanche Study Establishment research. The reasons are unclear, but according to past studies, Negi suggests that the drop in minimum temperatures might be due to “large scale deforestation and soil degradation” whereas the cooling in maximum temperatures could be because of higher aerosol emissions reducing incoming solar radiation. This shift in the trend warrants further investigation, said Negi.

Although total precipitation has increased over the 25 years, a greater part of it has been falling as rain and lesser in the form of snow. Citing a study from the National Centre for Atmospheric Research in the US, on changes in precipitation with climate change, Negi explained that “with every degree rise in temperature, the moisture holding capacity of air increases by almost seven percent which results in more precipitation, predominantly in form of rains than snow, since higher temperature hinders the formations of snow crystals.”

But, “the effect of increasing precipitation or temperature is not straightforward” said Mohd Farooq Azam, a hydro-glaciologist at the Indian Institute of Technology in Indore. Winter temperatures at glacial altitudes, he explained, “are always much below zero degree and an increase by half degree or one degree doesn’t change the conditions of precipitation from snow to rain.”

Over the last few decades, he said that “temperatures are surely increasing in the Himalayas, while most studies suggest decrease in precipitation, with few exceptions of increasing trends.”

“However a short time period or specific season (winter or summer) may have different results,” he added.

The Snow and Avalanche Study Establishment researchers noted decreasing precipitation in the Greater Himalayas and Karakoram from 2000 onwards. Again, the reasons are unknown, says Negi, pointing out that there are many studies reporting a hiatus in global warming after 2000. He cautions that this needs to be validated with data on glacial and vegetation cover for example.

More rainfall leads to a greater frequency of landslides and avalanches during late winter, warn the authors.

Greater risk of avalanches

Coincidentally, a team of researchers from Switzerland examining tree-rings has found just that: high avalanche activity over the past few decades.

“Trees are eyewitnesses of past environmental changes,” said Juan Antonio Ballesteros-Cánovas, a researcher at the Institute for Environmental Sciences at the University of Geneva in Switzerland, and lead author of the study. When snow gushes downhill, it is often the trees that bear the brunt of the impact. The damages are manifested as scars, loss of branches and tilting of stems. They are registered in the tree-ring records, explained Ballesteros-Cánovas.

By studying abnormalities in the rings of 144 trees on a slope between the villages of Solang and Dhundi in the Kullu district of Himachal Pradesh, the researchers were able to reconstruct a record of avalanches that occurred as far back as 150 years ago. This slope affects the road that provides access to the newly completed and “strategically important” Rohtang tunnel that will enable year-round connectivity to Ladakh in the neighbouring northern state of Jammu and Kashmir.

Almost no avalanches occurred in the region from 1940 to 1960, but high activity was recorded between 1970 to 1977 and 1989 to 2003 with more than 0.87 avalanches per year, on an average.

Using statistical modeling, they linked increased air temperatures in late winter and early spring to a higher probability of avalanches. The authors stress that “the transformation of dry snow packs into wet snow packs is decisive for the release of snow avalanches in the region.” Specifically, the “rise in liquid water content of the snowpack”, said Ballesteros-Cánovas, makes it “unstable and thus prone to trigger wet snow avalanches.” Also, moving snow with more water has less friction so it slides downhill easily, which can increase the distance traveled by the snow, known as run-out distances.

According to Immerzeel, this is “an excellent study on a very important topic” and one of the first to quantify an “increase in avalanching.”

“On the one hand, warming leads to less snow. On the other hand, it increases the risk of wet avalanches, which have a greater run-out distance,” he noted.

The team highlights the need for immediate measures to prevent blocking of roads in an area with rising traffic. Structures such as dikes and galleries can be installed based on the impact pressures, which could be determined using the tree-ring records, added Ballesteros-Cánovas.

This article first appeared on Mongabay.