Why are glaciers in southeast Tibet melting so fast?

Millions of people depend on water from the glaciers of the high peaks of Asia. But southeast Tibet has some of the fastest melting glaciers in the entire continent. This phenomenon is explained by the decrease in snowfall in summer, as shown by a study carried out by the Federal Institute for Forest, Snow and Landscape Research WSL.

Unlike alpine glaciers, glaciers on the Tibetan Plateau are mainly snow-fed during the summer months, which are the wettest but also the hottest. Glaciers in the southeast Tibetan Plateau feed the Brahmaputra River, which millions of people downstream depend on for domestic, agricultural and industrial uses.

Earth observation satellites have recently revealed that the rate of glacier mass loss in this region is among the highest in Asia and that this loss has accelerated in recent decades. Rising temperatures due to climate change are known to cause glaciers to melt, but is this really the only factor behind the rapid retreat of glaciers in this region?

The answer is no, as indicated by research conducted by Achille Jouberton and Francesca Pellicciotti of the WSL, in collaboration with the Institute for Tibetan Plateau Research of the Chinese Academy of Sciences. Scientists have dissociated the different mechanisms at the origin of the great sensitivity of the glaciers of southeastern Tibet to warming.

45 years of modeled variations

To identify these mechanisms, they reconstructed the climate and mass variations of the Parlung No. 4 glacier in southeastern Tibet over the past 45 years. This is the longest reconstruction of the mass variations of a glacier in this region. The latest-generation glacio-hydrological model used by scientists was fed by a large set of data collected in the field and by remote sensing. Glacio-hydrological models incorporate both glaciological and hydrological processes, for example, glacier flow and river flow.

The long-term variations of the phases of precipitation – rain or snow – are however very difficult to observe, especially at high altitudes where the zones of accumulation of glaciers are located. These variations require continuous precipitation measurements, which are often costly and subject to high uncertainties. Therefore, using a well-informed model, calibrated with locally collected data, was the most practical way to quantify them.

Global warming cataclysm

Global warming has accelerated the rate at which glaciers are melting. Between 2000 and 2020, 85.3% of the planet's 1,704 glaciers have retreated. The Tibetan Plateau contains low-latitude glaciers that are vulnerable to global warming, and Indian glaciers are melting at a similar rate. This retreat will increase the volume of nutrients and microorganisms released into downstream ecosystems, posing a risk to life.

"Global warming is accelerating glacier retreat, and increased meltwater flow may increase the chance that these virulence factors will interact with local plants, animals, and humans," the Lanzhou University study says.

Menon points out that since humans have no defenses against these microbes because they come from unexpected sources, this will become a growing problem associated with global warming.

The Lanzhou University study reveals that some of these microorganisms have demonstrated "the ability to adapt to these extreme conditions and contribute to vital ecological processes, such as the carbon and nitrogen cycle. ", creating a vicious feedback loop. Naz explains that methanogenic microbial strains under melting ice caps that are exposed are accelerating global warming. As they produce methane, an extremely potent greenhouse gas with a global warming potential up to 34 times greater than that of CO2, this could cause the ice sheets to melt even faster in the future.

Rain instead of snow in summer

Scientists have found that the acceleration of mass loss results mainly from the fact that part of the summer snowfall is replaced by rain, which reduces the accumulation of snow - which later turns into ice - on glaciers. The results also show that glacier melt has increased, but that decreasing snow accumulation has been a more important cause of recent mass loss in this region.

The fact that snow accumulates less on glaciers has the side effect of exposing glacier ice to sun and heat for more of the melting season, which can be significantly accentuated. This phenomenon was also evident in the Alps this summer, where low winter accumulation led to early and excessive summer melt.

Overall, the results show how important it is for projections of glacial variations to take into account variations in glacier accumulation mechanisms. In addition, this study helps to explain why summer accumulation glaciers are particularly sensitive to warming.

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