Over 1.9 billion people, relying on mountains for meeting their water needs, could face water shortage as water stored in mountains are quickly disappearing due to rising demands and excessive melting of glaciers accelerated by climate change, a new international study has concluded.

The study, ‘Importance and vulnerability of the world’s water towers,’ assessed world’s 78 mountain glacier-based water systems for the first time and ranked their significance to the downstream communities and their vulnerabilities in terms of various factors, including future changes.

The study, published in the scientific journal Nature this week, concluded that the most critical water towers are also the most vulnerable ones because of climate change and other socio-economic changes.

The water tower is a term used to describe a mountain-based water system that stores and carries water in different forms to meet downstream communities and the ecosystem needs.

According to the study, the Indus water tower, made up of vast areas of the Himalayan mountain range and covering parts of Afghanistan, China, India and Pakistan, was found to be the most important yet the most threatened mountain system of the world.

The study has ranked five of Asia’s water tower—Indus, Tarim, Amu Darya, Syr Darya, Ganges-Brahmaputra—as the most important ones and also the most vulnerable.

“Indus basin is the most important water tower and also the most vulnerable one because the Indus has the largest irrigation system in the world,” Walter Immerzeel, a scientist and co-author of the study, told the BBC in an interview. “It has over 200,000 sq km of the irrigated area, which is exclusively fed by the mountain water. The current 250million people will be rising to 300 million by 2050.”

The finding is even crucial for the Hindu Kush Himalayan (HKH) Region—a region that stretches over 3,500 km covering eight countries from Afghanistan in the west to Myanmar in the east—which is already witnessing impacts of climate change.

A study conducted by the International Centre for Integrated Mountain Development (ICIMOD) in the region concluded that even the most ambitious Paris Agreement goal—of limiting global warming to 1.5 degrees Celsius—by the end of the century would lead to a spike of 2.1 degrees and the melting of one-third of the region’s glaciers.

“Four out of the five most relied-upon and vulnerable water towers in all of Asia are located in the HKH region,” said Santosh Nepal, water and climate specialist at ICIMOD, in a press release. “Even when we assess at a global scale, the Indus, in particular, is one of the most critical and vulnerable water towers in the world. This is something the region as a whole needs to take seriously and work on addressing collectively.”

The latest study has projected that rising pressure on water towers due to climate change and manmade activities would lead to affecting an estimated 1.9 billion people— 0.3 billion living in the mountain region and 1.6 billion downstream.

The study, authored by 32 scientists from around the world, highlights the threats posed by changing climate and socio-economic changes to water towers. It also warns of harsher impacts of climate change mostly for Asia in future.