Not all of the water from the planet’s melting glaciers is pouring into rivers and oceans. A surprising amount is building up behind unstable piles of rubble left behind by the retreating ice. As the Earth continues to warm, the swelling lakes threaten to burst through the glacial moraines holding them back and wash away the forests, towns and farms below.

The threat is particularly high in the Northern Andes, where some of the world’s last big tropical glaciers are dwindling even faster than those in the Alps or Himalaya. At low latitudes, the length of the day doesn’t change much during the year, so tropical glaciers don’t get a seasonal respite from the sun’s direct rays. That’s resulted in a rapid meltdown of glaciers near the equator, and the formation of hundreds of lakes in recent decades. 

New research presents evidence of a direct link between global warming and the growing risk of an outburst flood to the Andean city of Huaraz, Peru. About 120,000 people there live in fear of the sirens that would signal that Lake Palcacocha, which is constantly swelling with water from the melting Palcaraju Glacier, had burst through its banks and was surging toward them, down the steep valley northeast of the city. 

The flood would trigger a mass evacuation, threaten large parts of the city with destruction and wash away many residents’ livelihoods. In the longer term, Huaraz could lose part of its water supplies for drinking and farming. The study, published today in Nature Geoscience, found it more than 99 percent certain that human-caused warming is melting the Palcaraju Glacier and increasing the threat. 

Lake Palcacocha sits at 14,980 feet above sea level in the gleaming Cordillera Blanca mountain range. The Palcaraju Glacier towers another vertical mile above it, at about 20,000 feet. Huaraz is 5,000 vertical feet below the lake, near the mouth of a steep, 13-mile canyon. Since 1941, as the glacier has shrunk, the lake has grown 34 times in size. 

An outburst from the lake that year killed about 1,800 people, and the rapid meltdown of the glacier makes it more likely than ever that the waves from a large rock, ice or mudslide falling into the lake will easily breach the barriers holding back the water.

The new study showed the 1941 flood was also linked with human-caused warming, making it one of the earliest identified impacts of climate change to take lives. There was a series of glacial outburst floods around the middle of the 20th century, which previous research identified as early responses to human-caused warming.

“The ongoing and frightening, deadly flood risk is indeed a consequence of human influence on the climate,” said University of Oxford climate researcher Rupert Stuart-Smith, co-author of the research. The results show, with greater than 99 percent probability, that human-caused global warming has caused Palcaraju Glacier’s retreat, the swelling of the lake and the imminent risk to Huaraz, he said.

Climate Science Meets Climate Justice

The future of the town, the lake and the glacier are also at issue in a climate lawsuit in a regional court in Germany. In 2015, Huaraz resident Saúl Luciano Lliuya sued German energy giant RWE, asking the company to pay about $20,000 toward establishing better flood protection for the town. The amount is calculated based on an estimation of the share of global warming caused by RWE’s 0.5 percent contribution to the total greenhouse gas emissions from burning fossil fuels.

RWE hasn’t made many public statements about the lawsuit, but a few court documents show its lawyers have argued that the company’s emissions can’t be pinpointed to a level that would make it responsible for the risk, or even that its share of total greenhouse gas emissions can be estimated accurately. 

The trial has reached the evidence stage, farther than similar climate trials, and a verdict against RWE would open the floodgates even wider for other climate liability and damages lawsuits. The judges are planning to visit the town and the lake with experts to assess the risk as soon as possible. 

The new study could be something of a smoking gun in the trial, helping to show exactly how RWE’s emissions increased the melting of the glacier and increased the risk posed by the growing lake.

“Without climate change, this hazard would basically not exist,” said University of Oxford climate scientist Friederike Otto, who was not involved in the study, but is Stuart-Smith’s doctoral advisor. The new study helps answer the question: “What is the role of climate change for this particular hazard threatening the livelihoods of the citizens of Huaraz today?” she said.

The trial is currently on hold due to pandemic travel restrictions, but the court may continue the evidence phase because the threat to the people of Huaraz is imminent, said Noah Walker-Crawford, an anthropologist at the University of Manchester who has been following the lawsuit closely as he researches how people and communities are trying to hold fossil fuel companies accountable for climate harms.

It’s not scientifically in question that global warming generally raises the risk of glacier outburst floods, he said. All the scientific evidence submitted to the court shows that growing threat. But the study could be key in the second stage of the evidence phase, when Saúl Luciano Lliuya’s lawyers have to show that the risk to Huaraz can be directly attributed to RWE.

“A few years ago, we didn’t have attribution studies down to this scale,” he said. Advances in attributing individual extreme climate events to global warming will probably play an increasingly important role in other climate litigation, he added.

In the United States, some of the largest fossil fuel producers in the world, including ExxonMobil, Chevron, Phillips 66 and Shell, are facing more than two dozen climate lawsuits seeking billions in damages. At the heart of these lawsuits brought by state and local governments are claims the oil and gas companies knew their products drove sea level rise, stronger hurricanes and wildfires, and willfully misled the public about those and other dangers related to global warming.

‘The Forensics in Climate Science’

The new study found that human activity is responsible for 95 percent of the 1.8 degrees Fahrenheit warming in the region since 1880. Co-author Gerard Roe, an Earth and space scientist at the University of Washington, said the team used a three-part analysis: the human-caused component of the warming, the role of that warming in the retreat of the glacier and how the changing glacier reshaped the valley and increased the flood risk.

Since every glacier melts differently in response to similar temperature increases, Stuart-Smith combined that information with two data points to show how the warming and melting contributed to the increased flood risk: the shape and thickness of the glacier itself, and the topographic features of the valley, such as the underlying geology and slope angles.

Stuart-Smith said he started his research when he heard about the court case in 2017. 

“We realized we had the tools to evaluate the claims made in this case,” he said. “Attribution science is the forensics in climate science.”

Such investigations will be increasingly important as large populations around the world face the growing threat of outburst floods.

“The retreat of mountain glaciers is one of the clearest signs of climate change,” Roe added. “Outburst floods threaten communities in many mountainous regions but this risk is particularly severe in Huaraz, as well as elsewhere in the Andes and in countries like Nepal and Bhutan, where vulnerable populations live in the path of the potential floodwaters.”