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A new study finds that climate change is reducing the supply of snowmelt water from many of Asia’s largest rivers, threatening the water security of millions of people.

Asia has the world's "third pole"-the largest fresh water outside the polar ice cap. Melting snow and glaciers in the region provide water for some of Asia's largest rivers, and more than one billion people rely on these rivers for water. However, due to the accelerated melting of the region due to climate change, the region’s ice and snow reserves are depleting, causing people to worry about water insecurity.

The new study found that between 1979-99 and 1999-2019, the “snowmelt water supply” of Asian mountain rivers dropped by an average of 16%. It added that even if the temperature rise is limited to 1.5 degrees Celsius, there will be a further loss of about 6% by the end of the century. At the same time, extreme warming scenarios in the future will lead to a 40% drop in meltwater supply.

The authors of the paper published in the journal Nature Climate Change added that although glacier melting often receives “a lot of attention” because of its contribution to water flow, the impact of snowmelt is often greater than that of glaciers.

The "Asian Mountains" are home to the 14 highest peaks on the planet, spanning 10 countries. The figure below shows the region, including 14 major rivers and their basins, including the Ganges, Yellow River and Yangtze River.

Every winter, the mountains in the area accumulate snow, which then melts as the summer temperature rises. Meltwater from the snow flows into the river, providing water for drinking, washing, agriculture, and energy for the 1.5 billion people downstream. 

There are three key sources of water—snowmelt, glacier melt, and rainfall—that keep the rivers in the region flowing. All three are important because “the melting of snow and glaciers ensures continuous water supply during periods of drought when rainfall is scarce,” explains Dr. Hester Biemans, a scientist at Wageningen University and Research Center, who was not involved in the study.

However, after warming by 1.8 degrees Celsius in the past half century, the third pole of the earth is also one of the most sensitive areas on the earth to climate change. Dr. Taylor Smith, a geological remote sensing researcher at the Institute of Earth Sciences at the University of Potsdam (who was not involved in the study), told Carbon Brief that rising temperatures have caused changes in the snowmelt season, making communities more vulnerable to "sporadic" impacts. "Rain. 

Biemans added that farmers will be affected by changes in the snowmelt season:

"It is important to understand how climate change will affect the water supply pattern in Asia. Many people downstream depend on this water and have also adjusted their water demand patterns based on this time pattern of available water... Farmers may need to find other sources of water , Such as groundwater, to water the crops or change their demand patterns."

The study’s lead author and assistant professor at Utrecht University, Dr. Philip Kraaijenbrink, told Carbon Brief that due to the “expected strong socio-economic development” in the region, water demand is expected to increase in the next century, thereby exacerbating this vulnerability.

The research aims to understand how climate change affects snowmelt and how future snowmelt patterns will change in the context of climate warming. To this end, the author combines regional-scale snow models with climate data, satellite snow observations, and global climate models to predict future warming.

By comparing the average snowmelt volume in 1979-99 with the average level in 1999-2019, the author found that the annual snowmelt volume in the area dropped by 16%. According to the study, this is mainly due to the depletion of snow reserves due to rising temperatures. However, it added that the changing precipitation patterns in the region also play a role in determining the amount of snow cover.

The author also assessed the future changes in snowmelt under a series of different emission scenarios and the “shared socioeconomic path” (SSP). In the SSP1-1.9 scenario, which is roughly consistent with limiting global warming to 1.5C, the amount of snowmelt will be further reduced by 6% by the end of this century. However, in the most pessimistic scenario, SSP5-8.5, it is expected to drop by 40%, and the global temperature will exceed 4C by the end of this century. The study also includes five other situations between these two situations.

The meltwater seasons in the high mountains of Asia are not consistent. In this study, the author divided the river into four "melt states" based on the time when the snow began to melt-short season, long season, evening peak, and morning peak. These systems are usually grouped by geography because the meltwater patterns are based on factors such as mountain elevation and precipitation patterns.

The four systems are as follows. The solid line represents historical snowmelt patterns, and the dotted line represents different warming paths in the future, from the low warming SSP1-1.9 path (blue dotted line) to the extremely high warming SSP5-8.5 path (dark red dotted line). The text on each chart shows which rivers are supplied by the regime’s snowmelt water.

For example, the author explained that the "short-season" meltwater comes from snow in low-elevation areas—because the temperature is higher in low-elevation areas. When spring arrives and snow starts to melt, processes such as snow albedo feedback (similar to the better known "ice albedo feedback") will cause the melting to accelerate. This means that the river flowing from the "short season" snowmelt will have a lot of water in March, but very little for the rest of the year.

Conversely, the mountains that provide nutrients to "evening peak" rivers (such as the Ganges) store most of the snow in places with higher altitudes and lower temperatures. Melting in this case also started in March, but will continue until August, making the flow of water into the downstream river slower and more stable.

The study found that as the climate warms, the four melting states will be affected differently. The graph below shows the percentage change in snowmelt in each case—including historical changes (grey) and seven future warming scenarios in 2100—from the lowest temperature increase (blue) to the highest temperature increase (pink).

Due to low-altitude snow accumulation, the "early peak" season is most vulnerable to climate change. Research has found that rivers in this state have lost one-fifth of their annual snowmelt on average. The author added that the system has been shortened by nearly a month, causing the start of the season to be delayed and to end early.

Therefore, rivers supplied by the early peak period are most sensitive to future warming. For example, the author found that for every degree of increase in the future, China's Yellow River and Yangtze River will lose 17% of their snowmelt contribution.

In contrast, high-altitude snowmelt is least affected by climate change. The study found that in the "upper basins"-those located above 200 meters above sea level-the contribution of snowmelt to streams is currently three to five times greater than that of glacier melting.

Therefore, the river's dependence on snowmelt water depends on its location. The authors found that in this study, the contribution of snowmelt to the river ranges from 7% of the Yangtze River to 75% of the Amu Darya.

Although both glacier melting and snowmelt are important factors in river flow, the paper points out that glaciers-slow-moving glaciers formed by years of snow accumulation-have been better studied. Smith told Carbon Brief that this is partly because the melting glacier is more noticeable:

"Melting and shrinking glaciers often become a very obvious sign of climate change; indeed, it is a very short spiritual leap from increasing temperature to shrinking ice. On the other hand, snow comes and goes every year, Both time and depth have changed a lot."

Kraaijenbrink adds that snowmelt is also more difficult to measure:

"It is difficult to quantify the amount of water stored in the snow, especially in such a large, inaccessible and data-scarce area. This has led to a lack of understanding of the exact impact of seasonal snowfall on the supply of water resources in the lower reaches of the Asian mountains and the impact of climate change. ."

Therefore, this study did not use observational data, but relied on the "temperature index method", which considered the relationship between air temperature and snowmelt and between air temperature and snowfall.

The authors pointed out that the paper provided an "incomplete representation" of snowmelt, but stated that their method was the "best way." However, Dr. Karl Rittger, a research assistant at the Arctic and Alpine Institute at the University of Colorado who was not involved in the study, warned that the results were "highly uncertain."

For example, he told Carbon Brief that the snow cover data used in the study "comes from a model developed in the 1980s and does not perform well during the melting season."

Professor Jeff Dozier is a snow hydrologist, distinguished professor emeritus, and founding dean of the Bren School of Environmental Science and Management at the University of California, and he was not involved in this research. He told Carbon Brief that he "appreciated [s]" the paper and pointed out that the "approximate answer" is still useful:

"Applying analysis to such a large field requires many approximations and therefore introduces uncertainty, but we should think that approximate answers to correct questions will promote scientific development."

He added that the author has identified potential areas for future improvements-including "efforts to effectively use satellite imagery to estimate historical snow water equivalent over the past few decades."

Smith told Carbon Brief that the paper "represents an important unfinished step-understanding which communities will be at risk of reduced snow water supply, which is very important for regional water resources management."

Kraaijenbrink, P. D A. et al. (2021) Climate change plays a decisive role in the supply of snowmelt water in Asia, natural climate change, doi: 10.1038/s41558-021-01074-x

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