Elevation of glaciers key to future runoff
Projecting global warming impacts in mountainous regions has been difficult for researchers, as drastic contrasts in terrain over short distances challenges climate models. Now, a new study by scientists from ETH Zurich and Utrecht University shows there will be different responses in different parts of the world.
In a new study, they looked closely at the water balance of two regions; the upper Langtang valley in Nepal and Juncal region of the central Andes in Chile, which are both important watersheds for millions of people. Both areas include peaks rising over 6,000 metres and glaciers that help sustain rivers.
Climate models project both regions will see similar warming the next few decades, with average temperature increases of anywhere between 1 and 6 degrees Celsius depending on future greenhouse gas emissions. But the impacts on water supplies in the two regions are quite different, the researchers determined.
The Juncal region will become even drier, with water shortages projected for the the dry season. Water discharge will hold steady through about 20230, but after that, there will be a steady decrease in water. Under the most extreme scenarios, water discharge from the drainage could shrink to a third of the current volume by 2100.
But in the upper Langtang valley, climate scenarios suggest that, in the first half of this century, water discharge will increase; in the extreme scenario, by as much as 70 percent. Maximum discharge could be reached by around 2050 to 2060, after which discharge is expected to remain the same or drop steadily the remainder of the century.
Glaciers are key factors for the different projections. They will shrink in both regions, but much more dramatically, by up to 70 percent in the Juncal region. In the Langtang Valley, the most extreme projections suggest the glaciers will dwindle by 55 percent because they are at a much higher elevation. As well, the tongues of the Langtang glaciers are covered with rock debris, which helps insulate them from warming air temperatures.
In Juncal, however, the highest glaciers are already melting, as they are situated lower than those in the Himalayas. Additionally, the summer dry season is expected to become longer and even drier.
“If the upper basins of rivers deliver less water in the future, it will be essential to take action to encourage conservation of the country’s water reserves,” said corresponding author Silvan Ragettli, post-doctoral student at the Institute of Environmental Engineering at ETH Zurich.
In the Nepalese Himalayas, resource managers will have to focus on flood management.
“Increased precipitation in the form of rain means that water runs off immediately, which in turn can lead to massive flooding,” Ragettli said.
“What’s new and unique about our model is that it can realistically illustrate many different processes,” he said. The models include factors like rain and snow volume, evaporation, groundwater, and also the ways in which glaciers expand and experience changes in volume.
The research started in 2005 in the Chilean Andes, and in 2012 in Nepal, in collaboration with Utrecht University and ICIMOD, an international organization based in Kathmandu that is committed to sustainable development in the Himalayan region.
The Andes and the Himalayas were chosen to provide reliable projections for the mountainous regions that surround urban centres in Chile and Nepal.