15 million people live in possible flood path for melting glaciers

As many as 15 million people live in areas that could be flooded by a natural dam failing at a glacial lake, according to the first global analysis of the hazard.

Tom Robinson at the University of Canterbury in New Zealand and his colleagues identified glacial lakes in satellite imagery. They then determined the number of people living within 50 kilometres of each lake, and within 1 kilometre of the river where water would flow in the event of a glacial lake outburst flood (GLOF). These floods can happen when an ice or rock dam holding back a glacial lake fails, or a rock slide sends water sloshing over a natural dam.

The researchers found up to 15 million people could be affected, with 9 million in the Himalayas, 2.5 million in the Andes and 2.2 million in the Alps. More than half of the 15 million live in either India, Pakistan, Peru or China.

Outburst floods are unlikely at many glacial lakes, and the study presents a simplified view of the areas any flooding would affect, says Simon Allen at the University of Zurich in Switzerland, but he says the global view helps draw attention to vulnerable and understudied regions.

Pakistan, for instance, has 2.1 million people living in potentially at-risk areas, but lakes there are less well understood than lakes in nearby Nepal, where the government has built expensive spillways at the most dangerous lakes and installed early warning systems. In some cases, a flood wave from an outburst could arrive in under an hour, with no prior warning.

Lakes in Peru and elsewhere in the Andes mountains, where thousands of people have been killed by glacial lake floods since the 1950s, also haven’t received sufficient attention, says Robinson.

Glacial lakes are growing rapidly in number and size as climate change melts more ice, but the frequency of glacial floods has decreased since the 1970s, says Stephan Harrison at the University of Exeter in the UK. As with warming after the 19th-century Little Ice Age, there may be decades of lag between warming and lakes growing large enough for an outburst, he says.