Two-thirds of Himalayan glaciers, the world's "Third Pole," could melt by 2100 if global emissions are not reduced, scientists warned in a major new study issued on Monday.
And even if the "most ambitious" Paris Agreement goal of limiting global warming to 1.5 degrees Celsius is achieved, one-third of the glaciers would go, according to the Hindu Kush Himalaya Assessment.
Glaciers in the Hindu Kush-Himalaya (HKH) region are a critical water source for some 250 million people in the mountains as well as to 1.65 billion others in the river valleys below, the report said.
The glaciers feed 10 of the world's most important river systems, including the Ganges, Indus, Yellow, Mekong and Irrawaddy, and directly or indirectly supply billions of people with food, energy, clean air and income.
Impacts on people from their melting will range from worsened air pollution to more extreme weather, while lower pre-monsoon river flows will throw urban water systems and food and energy production off-kilter, the study warned.
It was published by the Kathmandu-based International Centre for Integrated Mountain Development (ICIMOD) in Nepal, one of eight countries on the front line.
Five years in the making, it involved more than 350 researchers and policy experts, 185 organizations, 210 authors, 20 review editors and 125 external reviewers.
"Global warming is on track to transform the frigid, glacier-covered mountain peaks... cutting across eight countries to bare rocks in a little less than a century," Philippus Wester of ICIMOD said in a statement.
"This is the climate crisis you haven't heard of."
The 2015 Paris Agreement's central aim was to keep a global temperature rise this century well below 2 degrees Celsius above pre-industrial levels and to pursue efforts to limit the temperature increase even further to 1.5 degrees Celsius.
In December, world leaders at the COP24 talks in Poland agreed on a common rule book to implement the accord, in which countries committed to limiting global temperature rises to well below 2 degrees Celsius.
But major polluters, including the United States and Saudi Arabia, disputed a landmark scientific report released in October that suggested nations must slash fossil fuel use by nearly half in a little over a decade.
The new report said that even if the 1.5-degrees target is achieved, it would mean a rise of 2.1 degrees in the Himalayas region. If emissions are not reduced, the rise would be 5 degrees.
The Himalayan glaciers formed some 70 million years ago, are highly sensitive to changing temperatures. Since the 1970s, they have thinned and retreated, and areas covered by snow and snowfall have decreased.
As the glaciers shrink, hundreds of risky glacial lakes can burst and unleash floods.
Satellite data show that numbers of such lakes in the region grew to 4,260 in a decade from 3,350 in 1990.
Air pollution from the Indo-Gangetic Plains -- one of the world's most polluted regions -- also deposits black carbon and dust on the glaciers, hastening to melt and changing monsoon circulation, the ICIMOD study said.
The region would require up to $4.6 billion per year by 2030 to adapt to climate change, rising to as much as $7.8 billion per year by 2050, according to an estimate in the report.