Floods from melting alpine glaciers look like one of the more obvious threats from global warming. Instead, they have turned out to be among the most controversial, even among scientists studying the topic. Now, for the first time, the flood risk to a specific city has been linked to the climate change-induced melting of a glacier above it, in research that could determine a court case of global significance.
The city of Huaraz sits in the Peruvian Andes at an altitude of 3,000 meters (10,000 feet). More than a kilometer above it lies Lake Palcacocha. In 1941 an immense chunk of ice fell into the lake and triggered an enormous flood, which killed at least 1,800 of the city/s residents.
Engineering works since the 1970s offer the city some protection. However, the Palcaraju Glacier that feeds Lake Palcacocha has been retreating rapidly since 1995, causing the lake to expand into the basin left behind. There are fears this melting could more than offset the flood mitigation efforts, once again putting Huaraz under threat.
A court case is underway alleging greenhouse gas emissions are to blame for Huaraz's perilous circumstances, and therefore the world's biggest polluters should pay to fix the problem.
Professor Gerard Roe of the University of Washington and colleagues have created a method to test the influence of the global climate on an individual glacier. "The scientific challenge was to provide the clearest and cleanest assessment of the physical linkages between climate change and the changing flood hazard," Roe said in a statement.
Oxford University doctoral student Rupert Stuart-Smith applied Roe's method to Palcaraju and published the findings in Nature Geoscience. Roe and Stuart-Smith observed that precipitation in the area has barely changed, so the retreat is almost entirely caused by local temperature trends – at least 85 percent of which is a result of human activities.
“We conclude that it is virtually certain (>99% probability) that the retreat of Palcaraju glacier to the present day cannot be explained by natural variability alone,” the paper states. Every tonne of carbon dioxide or methane added to the atmosphere from burning coal or burping cows adds to the danger Huaraz faces.
"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."
"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 added.
Plaintiff Huaraz resident Saúl Luciano Lliuya lost in lower courts. Even if the German regional court hearing the case accepts the paper's evidence, it is not certain they will agree the specific defendant – electricity producer RWE – should have to pay the damages Lliuya seeks. Nevertheless, the science is an essential step.
Among the thousands of pages of assessments in the Intergovernmental Panel on Climate Change's 4th Assessment report, the only aspect to be successfully challenged by its critics related to the melting of Himalayan Glaciers. It was found the science at the time on the topic was too uncertain to make conclusions about what would occur. Roe's work may change that for the future, giving us a clearer idea of the dangers we face on this front.