As the planet warms, glaciers are retreating and causing changes in the world’s mountain water systems. For the first time, scientists at the University of Oxford and the 91̽�� have directly linked human-induced climate change to the risk of flooding from a glacial lake known as one of the world’s greatest flood risks.

The study examined the case of in the Peruvian Andes, which could cause flooding with devastating consequences for 120,000 residents in the city of Huaraz. The , published Feb. 4 in Nature Geoscience, provides new evidence for an ongoing that hinges on the link between greenhouse gas emissions and particular climate change impacts.

“The scientific challenge was to provide the clearest and cleanest assessment of the physical linkages between climate change and the changing flood hazard,” said co-author , a 91̽��professor of Earth and space sciences.

In 2016, Roe and colleagues developed a method to determine whether an individual glacier’s retreat can be linked to human-induced climate change. The retreat of mountain glaciers has several consequences, including creating basins in the space left by the retreating glacier. Precipitation and meltwater collects in these basins to form glacial lakes. Recent work has shown a rapid of high-elevation glacial lakes.

“We believe our study is the first to assess the full set of linkages between anthropogenic climate change and the changing glacial lake outburst flood hazard,” Roe said. “The methods used in our study can certainly be applied to other glacial lakes around the world.”

The new study first calculated the role of human emissions in the observed temperature increase since the start of the industrial era around Palcaraju Glacier. It finds that human activity is responsible for 95% of the observed 1 degree Celsius (1.8 degrees Fahrenheit) warming in this region since 1880.

The authors then used the UW-developed technique to assess the relationship between these warming temperatures and the observed long-term retreat of the glacier that has caused Lake Palcacocha to expand. Results show it is virtually certain, with greater than 99% probability, that human-induced climate change has caused Palcaraju Glacier’s retreat.

Lead author Rupert Stuart-Smith, a doctoral student at Oxford, then used two methods to assess the hazard of glacial lake outburst flooding, in which an avalanche, landslide or rockfall induces a tsunami wave that overtops the lake’s banks, to pinpoint how Lake Palcacocha’s growth affects the flood risk faced by the city of Huaraz below.

“We found that human influence on climate — through greenhouse-gas emissions — is responsible for virtually all of the warming that has been observed in the region,” said Stuart-Smith, who spent the summer of 2019 at the UW. “The study shows that warming has caused the retreat of the Palcaraju Glacier, which in turn has greatly increased the flood risk.”

The study provides new evidence for an in which Saúl Luciano Lliuya, a farmer from Huaraz, has sued RWE, Germany’s largest electricity producer, for its role in creating global warming. The suit seeks reimbursement for current and future flood-risk reduction measures.

“Crucially, our findings establish a direct link between emissions and the need to implement protective measures now, as well as any damages caused by flooding in the future,” Stuart-Smith said.

This is not the first time Huaraz has been threatened by climate change. In 1941, an outburst flood from Lake Palcacocha, resulting from an ice and rock slide, killed at least 1,800 people. The study also found this flood to be influenced by human-induced climate change — making it one of the earliest identified fatal impacts of climate change.

The lake’s recent growth strains since the 1970s to contain the lake’s water.

“Around the world, the retreat of mountain glaciers is one of the clearest indicators of climate change,” Roe said. “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.”

Other co-authors are Myles Allen and Sihan Li at the University of Oxford. The study was funded by the U.K. Natural Environment Research Council, the U.S. National Science Foundation and a from the University of Oxford.

For more information, contact Roe at groe@uw.edu or Stuart-Smith at rupert.stuart-smith@ouce.ox.ac.uk.

Adapted from a University of Oxford .

But the scientific basis for their retreat has been less clear. Glaciers respond slowly to any climate changes, they are susceptible to year-to-year variations in mountain weather, and some of the largest are still catching up after the end of the so-called . Scientists can connect climate change to the overall retreat of glaciers worldwide, but linking an individual glacier’s retreat to climate change has remained a subject of debate.

The last report from the Intergovernmental Panel on Climate Change concluded only that it was “likely” that a “substantial” part of mountain glacier retreat is due to human-induced climate change — a much weaker conclusion than for other things, like temperature.

Now, using statistical techniques to analyze 37 mountain glaciers around the world, a 91̽�� finds that for most of them the observed retreat is more than 99 percent likely due to climate change. In the climate report’s wording, it is “virtually certain” that the retreat of these mountain glaciers is due to climate change over the past century.

“Because of their decades-long response times, we found that glaciers are actually among the purest signals of climate change,” said , a 91̽��professor of Earth and space sciences. He is corresponding author of the study published Dec. 12 in Nature Geoscience, and presented this week at the American Geophysical Union’s fall meeting in San Francisco.

The new study analyzes specific glaciers with a history of length observations, and nearby weather records of temperature and precipitation. The authors also sought different glacier locations, focusing on roughly seven glaciers in each of five geographic regions: North America, Europe, Asia, Scandinavia and the Southern Hemisphere.

“We evaluate glaciers which are hanging on at high altitudes in the deserts of Asia as well as glaciers that are being beaten up by midlatitude storms in maritime climate settings,” Roe said. “The thickness, slope and area of the glaciers are different, and all of those things affect the size of the glacier length fluctuations.”

Co-authors are , a 91̽��professor emeritus of atmospheric sciences and Earth and space sciences, and , an undergraduate student at the University of Innsbruck in Austria.

The authors used statistical tools to compare the natural, weather-induced variations in a glacier’s length with its observed changes over the last 130 years, and establish a signal-to-noise ratio. They then use that to calculate the probability that observed retreats would have happened without any background change in the climate.

The iconic in Austria has retreated 2.8 km (1.75 miles) since 1880. Results show that climate change is extremely likely to be responsible for its retreat, with the probability that the changes are natural variations being less than 0.001 percent, or one in 100,000.

Likewise, for the well-known in New Zealand, even though the glacier has experienced re-advances of up to 1 kilometer (0.6 miles) in a given decade, there is a less than 1 percent chance that natural variations could explain the overall 3.2 kilometers (2 miles) retreat in the last 130 years.

The least significant retreats among the glaciers studied were for in northern Sweden, and in Washington state, with probabilities of 11 and 6 percent, respectively, that their retreats might be natural variability.

“South Cascade is at the end of the Pacific storm track, and it experiences a high degree of wintertime variability. Average wintertime snowfall generates about 3 meters (10 feet) of ice per year, whereas for glaciers in desert Asia, ice accumulation might be as low as 10 centimeters (4 inches) per year,” Roe said. “So they’re experiencing very different climate settings. As a result, their variability, and also their sensitivity to climate change, varies from place to place.”

The method uses a signal-to-noise ratio that relies on observational records for glacier length, local weather, and the basic size and shape of the glacier, but does not require detailed computer modeling. The technique could be used on any glacier that had enough observations.

Overall, the results show that changes in the 37 glaciers’ lengths are between two and 15 standard deviations away from their statistical means. That represents some of the highest signal-to-noise ratios yet documented in natural systems’ response to climate change.

“Even though the scientific analysis arguably hasn’t always been there, it now turns out that it really is true — we can look at these glaciers all around us that we see retreating, and see definitive evidence that the climate is changing,” Roe said. “That’s why people have noticed it. These glaciers are stunningly far away from where they would have been in a preindustrial climate.”

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For more information, contact Roe at groe@uw.edu or 206-697-3298.