Study by NUS researchers reveals climate
High Mountain Asia (HMA), which refers to the Tibetan Plateau and the surrounding high Asian mountains, is home to the third largest reservoir of ice in the world and the origin of many major rivers in Asia. In fact, these rivers are crucial lifelines for a third of the world’s population. HMA rivers are experiencing increased runoff and sediment fluxes due to amplified climate change, melting glaciers and thawing permafrost.
To examine the impact of these phenomena on HMA, Professor Lu Xixi and Dr Dongfeng Li of the Department of Geography, Faculty of Arts and Social Sciences, National University of Singapore (NUS) led an international team of researchers. to re-analyze the observations. of the region’s headwaters. The study found that river sediment loads increased dramatically, even much faster than river water flow. This is due to the recent warmer and wetter climate, and has important implications for water quality, hydropower development and maintenance, and for the river carbon cycle.
Dr Li, a researcher in the geography department of the NUS and lead author of the study, said: “Climate change is accelerating the retreat of glaciers and the thawing of permafrost, making previously frozen landscapes increasingly erodible. Our study shows that emerging processes, such as retreating glaciers and thawing permafrost, will improve sediment transport from slopes to river systems, especially when regional extreme rainfall storms also increase. This has important implications for the region’s hydropower, food and environmental security, potentially affecting millions of people in the HMA region and downstream regions.
The results of the study were published today in Scientific journal.
Analysis of flow and sediment load data in the headwaters of the AHM
The project was carried out in collaboration with some of the world renowned scientists in the field of sediment transport including Associate Professor Irina Overeem, Professor Jaia Syvitski and Professor Albert Kettner of the University of Colorado, Boulder (CU-Boulder ); Professor Des Walling of the University of Exeter; Professor Bodo Bookhagen of the University of Potsdam; and Professor Yinjun Zhou of the Changjiang River Scientific Research Institute in Wuhan, China.
The team gathered and analyzed available data on the flow and sediment load of HMA rivers over a period of six decades to study changes in runoff and sediment flow in response to a warmer climate and more humid. To exclude the potential impact of human activities, 28 almost virgin upstream basins were selected. The team then studied the sensitivity of the sediment flow to changes in temperature and precipitation in the HMA using observational data and a climate elasticity model.
Based on the study, the team estimated that the current flow of river sediment from the HMA is nearly two billion metric tonnes per year and could more than double by 2050 under a changing scenario. extreme climate.
“The cascade of climate change impacts, first through global warming, then through amplified changes in alpine temperature and precipitation regimes, followed by the melting and release of sediments from glaciers and frozen landscapes, and subsequently by increasing sediment transport by rivers, demonstrates how the planet Earth is being altered by our continued use of fossil fuels, ”said Professor Syvitski, who is also the former president of the International Geosphere Program -biosphere.
The team further estimated that the increased sediment loadings had profound impacts on the maintenance of downstream hydroelectric reservoirs. “The increasing influx of sediment into the reservoirs will reduce the storage capacity of the reservoirs and thus shorten their expected lifespan. As a result, all reservoir services such as water supply, irrigation, hydropower generation and flood control will be adversely affected, ”said Professor Lu, principal investigator of the study.
Professor Overeem added: “Since the 1950s, amplified warming in the headwaters of the HMA has increased sediment loads at an average rate of 32% for each degree of warming. The thaw of this landscape has already triggered profound changes in soil erosion and sediment delivery to rivers. Glaciers are known to be efficient sediment producers, but this study suggests that the role of thawing permafrost in longer-term climate and sediment cycles may be more important than previously thought.
The team noted that increasing sediment concentrations will likely have a negative impact on water quality and aquatic ecosystems. Fine suspended sediment particles are an important vector for the transport of phosphorus and most heavy metals, such as mercury, chromium, arsenic and lead. Thus, climate change is likely to increase the fluxes of nutrients and contaminants associated with sediments. In addition, suspended sediments are a key vector for the transport of organic carbon; and the precise role of erosion and sediment input in mobilizing organic carbon from permafrost landscapes and its contribution to the river system remains uncertain. The team said more observations are needed to assess the positive feedback between global warming, permafrost degradation and the carbon cycle.
The researchers also added that a substantial proportion of the increased sediment could be temporarily deposited in the river system, broad alluvial valleys, and river floodplains, aggravating river beds, potentially triggering river avulsions and increasing flood plains. risk of flooding, especially during the monsoon. However, increases in sediment are not always bad. Riparian sediments can be used as soils for local agricultural practices in high mountain areas and are important materials for construction and coastal protection structures in small island countries like Singapore. For example, a billion tons of sediment can be used for the construction of two Great Walls of China.
“These unique long-term river datasets from many headwaters of the high mountains of Asia have helped to determine how the landscape is rapidly changing under the effect of amplified climate change. This amplified climate change is also observed in the polar regions, where unfortunately long-term river datasets are scarce. However, it would be very interesting to see how the landscape reacts in these polar regions, if there are any similarities and what might be the downstream implications for communities and the environment ”, said Prof. Albert Kettner, research professor. at the Institute of Arctic and Alpine Research at CU-Boulder.
Dr Li commented, “This study highlights the importance and potential implications of marked increases in recent and future sediment fluxes that have not been fully recognized by scientific communities and have not been fully addressed. account in assessing potential changes in the global carbon cycle. We hope this will encourage more observations of river sediments in cold environments around the world.
The next step for the research team is to develop a dynamic sediment transport model suitable for cold environments. This would help better understand the seasonality of future river sediments in a rapidly warming world.
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