The flash flood, which killed dozens of people on Sunday and hundreds were missing in the Himalayas of India, was far from the first such catastrophe among the high mountain glaciers in the world. In a world with a changing climate, it won’t be the last.
The shrinking and thinning of glaciers is one of the best-documented signs of the effects of global warming caused by greenhouse gas emissions, according to scientists. Glacier retreat in mountains around the world has been measured, sometimes at a rate of 100 feet or more per year. In the Himalayas, the most glaciated mountain range and home to about 600 billion tons of ice, the rate of retreat has accelerated over the past four decades.
In the long run, there are concerns about what the loss of glaciers will mean for billions of people around the world who rely, at least in part, on them for drinking water, industry and agriculture. The more acute fear, however, is the safety of the people who live near them.
The lost ice releases water, and in the Himalayas, as elsewhere, some of this water becomes trapped in lakes as it runs down the mountain slopes and is dammed up by rocky debris left by the glaciers. Globally, as more ice has melted, the number of lakes formed has increased, and the total volume of water in them has grown 50 percent since 1990.
“We believe that climate change must be 100 percent certain why these lakes are forming and increasing,” said Umesh K. Haritashya, who studies glacier hazards at the University of Dayton, Ohio.
Glacial lakes are a serious threat. The rubble dam can collapse from the weight of the water or from an earthquake. or an avalanche over the lake can shoot ice and stone into it. In either case, the result can be a sudden, catastrophic surge of water that can wipe out communities and infrastructure in valleys downstream.
Outbreak floods, as they are called, have occurred throughout history. In Uttarakhand, an outbreak in 2013 caused flooding, destroyed villages and killed several thousand people. Elsewhere on the subcontinent, floods in the mountains in 1929 affected the Indus, almost 500 miles away. In the Andes Mountains of Peru, an estimated 30,000 people have died from glacial flooding since the 1940s.
In Switzerland and a few other countries, engineers have built siphons to drain lakes that pose a particular threat to communities or infrastructure. However, there are only a few such projects around the world that are overwhelmed by the increasing number of glacial lakes.
However, rising temperatures affect more than glaciers. The thawing and refreezing of ice trapped in rock breaks on mountain slopes can cause the slopes to become destabilized and more prone to collapse.
“We’re seeing more and more high-mountain cases where rocks and mountains aren’t as stable as we thought,” said Dan Shugar, a geomorphologist at the University of Calgary in Canada.
While it is too early to link the Uttarakhand disaster directly to climate change, destabilization caused by the melting ice may have been responsible.
Although the Indian government claimed that an avalanche that streamed into a river and caused the flood was caused by the “calving” or breaking of a glacier at high altitude, scientists analyzing satellite images before and after the disaster said the collapse of one Likewise high rock slopes were more likely the cause.
Dave Petley, Vice President of Innovation at the University of Sheffield in the UK, who has long worked with landslides, said that such slopes often contain many rock breaks and ice acts like a glue holding them together. “As the climate warms up, this ice worsens in summer,” he said. “The rock becomes so weak because the glue that holds it together is no longer there.”
One mystery is the source of all the water for the flood, which, along with sediments, formed a wall of mud that flowed down the Rishiganga River, sweeping away houses and people, and leaving two hydropower projects in ruins.
Many scientists initially thought that such a large amount of water must be the result of a flood disaster. However, satellite imagery from before the disaster showed no evidence of large lakes.
Dr. Petley said it was likely that the rockfall – which was likely on the order of tens of millions of cubic feet of debris – hit a glacier and splintered it. “These rock avalanches are very energetic and chaotic,” he said.
What was now a rock-ice avalanche continued downhill and generated immense frictional heat in the process. That heat likely melted much of the ice. “That’s probably where all the water came from,” he said.
A thick blanket of snow in the area, some of which had begun to melt in the days leading up to the disaster, may also have contributed, some researchers said.
When this rock mass and the now melting ice reached the valley floor, in 2016 it most likely encountered large amounts of sediment deposited by a landslide, the scars of which are visible in satellite images. This sediment would have mixed with the incoming debris and worsened the effects of the flooding.
“From what I could see, the chain of events may have started in 2016,” said Mylène Jacquemart, a glacier researcher at the University of Colorado.
And while this particular chain of events seems like a one-off event, it isn’t, said Dr. Jacquemart. A deadly 2017 mudslide that struck the village of Santa Lucía, Chile, followed a similar pattern, she said.
“It’s not that we’ve never seen anything like it,” she said.
Outbreak floods and the collapse of thawing slopes are not the only glacier-related hazards associated with climate change. A glacier is a river of ice, and the ice acts as a buttress for the slopes on either side. When a glacier retreats and becomes thinner, these slopes lose their support. The result can be a sudden collapse and an ice rock avalanche when the sloping slope hits the glacier.
Scientists sounded the alarm last year about the potential for such a disaster in Prince William Sound, Alaska, not far from Anchorage. A mile-long slope along a fjord had lost most of its support because the ice had retreated, increasing the risk of a landslide into the fjord. The resulting sudden tsunami could be fatal to hunters or fishing boats in the area and destroy coastal villages.
A faster melting of the glaciers also causes some glaciers to flow faster as the meltwater acts as a lubricant between the ice and the rock below. In some cases, they have flowed so quickly that the front of the glacier simply breaks off unexpectedly, said Dr. Jacquemart.
Two such divisions occurred within two months in the Aru Range in Tibet in 2016, she said.