The Tibetan Plateau and surrounding mountains are nicknamed the “Third Pole” or the “Water Tower of Asia” because of the immense volumes of ice stored in the area’s snowpack, glaciers and permafrost. Annual spring thaws send meltwater into the tributaries of many of Asia’s largest rivers, such as the Yangtze, Indus and Mekong. Together, streams and rivers snaking out of what is called High Mountain Asia provide freshwater for nearly one third of the world’s population, and are crucial for hydropower and farming. But springtime meltwater is increasingly bringing potentially destructive stones, sand and other sediment along for the ride.
Local temperatures have risen more than 2 degrees Celsius since the 1950s, more than twice the global average, amplifying melt and intensifying rains. This has caused a dramatic increase in the amount of earthen material coursing through the region’s rivers over the past 60 years, according to a new analysis published Thursday in Science. As global warming continues, the amount of sediment flushed down High Mountain Asia’s rivers could more than double by 2050. The researchers warn this will diminish water quality, shorten the lifespans of dams and reservoirs used for hydropower and irrigation, and increase flooding and mudslides, ultimately affecting the lives of millions of people.
“The type of change that they're talking about here—that’s happening within decades to maybe a century—is the type of change that we would normally expect to see when we're coming out of an ice age,” says Tara Jonell, a geologist at the University of Glasgow who was not involved in the study. But today’s melting and large-scale sediment flushing is happening much faster than what would occur at the end of a glacial period, she adds. “Rapid climate change is not just affecting communities at sea level; it’s also affecting people high up in the mountains.”
Study co-author Dongfeng Li, a geomorphologist at the National University of Singapore, and his colleagues sought to better understand how much more sediment is washing down rivers in High Mountain Asia today, compared to the past. They analyzed roughly 60 years of temperature, rainfall, river flow and sediment data from 28 rivers across the area, looking only at data from sites with no upstream human activity (such as dams), in order to isolate the impact of climate change on the rivers involved.
The team found river flows had risen by about 5 percent, and sediment flows by 13 percent, in each decade since the 1950s. These increases accelerated after the mid-1990s, paralleling an uptick in regional warming and rainfall rates. As temperatures rise, glaciers—which are often covered and embedded with sediment—melt and leave behind large piles of this loose material, says study co-author Irina Overeem, a geomorphologist at the University of Colorado Boulder. Soils that typically remain frozen year-round thaw instead, making them easier to flush away, Overeem adds. Rainfall and snowmelt then wash all this sediment into river systems. The researchers estimate that about 2 billion metric tons of such material is now flowing out of High Mountain Asia each year. That is about the weight of 20,000 U.S. aircraft carriers, or 350 Pyramids of Giza.
High Mountain Asia’s topography and atmospheric systems form such a complicated picture that it is hard to group all of the glacier systems in one category. Each seems to have its own “personality,” as Jonell puts it. The fact that all of these very different rivers are seeing big increases in sediment speaks volumes about how much climate change is altering the region, Jonell says.
Using these observed trends, along with projections of future temperatures and rainfall, Li and his team also estimated how the amount of sediment moving through the rivers might change in the future. They projected that if temperatures rise 3 degrees C by 2050, relative to the period between 1995 and 2015, and if rainfall simultaneously increases by 30 percent, then the amount of sediment flushing through the region’s rivers each year would increase to about 5 billion metric tons. When the researchers assumed a more conservative climate change scenario, in which temperatures rise just 1.5 degrees C and rainfall increases just 10 percent, they found that the amount of sediment moving in the rivers each year would increase to more than 3 billion metric tons.
Muddier rivers pose a major problem to communities that rely on their waters. Sediment can carry heavy metals such as arsenic, mercury and lead. More sediments in rivers likely means more of these contaminants in the water, and Li says this is a big problem when it comes to sourcing drinking water and growing crops. Filtering water from affected rivers will become more difficult and expensive, he adds.
Sediment in High Mountain Asia is also rich in hard minerals such as quartz. As the amount of such material rushing through the rivers increases, their flows become more abrasive and can damage costly parts of hydroelectric dams that provide essential power to local populations. “Hydropower is one of the ways in which smaller countries are getting a leg up economically, especially as we're trying to transition towards cleaner energies,” says Sarah Schanz, a geologist at Colorado College who was not involved in the study. “What they’ve found is that the increased sediment is taking away that hydropower potential from these countries.”
Increasing sediment in rivers could also accumulate behind dams and cause reservoirs to fill up faster than expected. This may become most apparent in the arid sections of Central Asia, including parts of Pakistan and Afghanistan where people depend on dams and reservoirs to provide water for crops. “In those regions people rely heavily on the damming of the rivers for irrigation,” says study co-author Xixi Lu, a physical geographer at the National University of Singapore. “This could become a huge issue.”
Hazards associated with large amounts of water and sediment may also worsen. “Flooding will increase in terms of magnitude and frequency,” Lu says. Mudslides could additionally become more of a threat, Jonell notes, as more rain falls across the region and more sediment becomes available to be swept away.
Jonell says she hopes engineers and policy makers can use this study’s observations and projections to help people in and around High Mountain Asia adapt. In the future, she adds, the sediment coursing through the area’s rivers might become a large enough problem that many people now living near these rivers will have to relocate. “These communities are not the ones that are most responsible for climate change,” she says, “and yet they themselves could be displaced by it.”