At least one third of the glaciers in the Hindu Kush and the Himalayas are expected to melt because of climate change, according to a landmark report.

At least 36% of the ice fields will disappear by 2100 even if carbon emissions are reduced dramatically and governments across the world succeed in limiting global warming to 1.5 degrees Celsius as agreed in the Paris Climate Agreement, the report said. If the emissions are not reduced, two thirds of the glaciers in the region will be at risk.

“This is the climate crisis you haven’t heard of,” Philippus Wester of the International Centre for Integrated Mountain Development, who led the study, told The Guardian. “In the best of possible worlds, if we get really ambitious [in tackling climate change], even then we will lose one-third of the glaciers and be in trouble. That for us was the shocking finding.”

The mountain ranges and the Himalayans are a critical source of water for the 250 million people who live in the Hindu Kush-Himalaya, or HKH, region. It runs from Afghanistan to Myanmar and is the planet’s “third pole”, harbouring more ice than anywhere outside the Arctic and Antarctica. Close to 1.65 billion people rely on the great rivers such as the Ganga, Indus, Yellow, Mekong and Irrawaddy that flow from these ranges into India, Pakistan, China and other countries. Any change in the source of these rivers is likely to cause massive devastation.

Since the 1970s, about 15% of the ice in the Hindu Kush-Himalaya region has disappeared with rise in temperatures. But the impact is variable since the region is about 3,500-km long. A few glaciers in Afghanistan and Pakistan are stable and a few are even gaining ice, most probably due to cloud cover that shields the sun and changed winds that bring more snow. But Wester warned that even these ice fields would start melting with as the planet warms even more.

Researchers took five years to prepare the report. It involved more than 350 researchers and policy experts, 185 organisations, 210 authors, 20 review editors and 125 external reviewers.