A third of the Himalayan glaciers will melt at the end of the century due to climate change, threatening the water reserves of 1.9 billion people, although current efforts to reduce climate change are successful, according to a study.

If these efforts fail, the impact can be far worse: two-thirds of the region’s glaciers will be lost by 2100, said the Himalayan and Hindu Kush Assessment, published on Monday by the International Center for Integrated Mountain Development.

“Global warming is on the way to transforming the cold peaks covered by glaciers of the Himalayas and the Hindu Kush in eight countries, into simple rocks in less than a century,” said Philippus Wester, who led the analysis.

The five-year study reviewed the effects of climate change in a region that crosses Asia through Afghanistan, Pakistan, India, Nepal, China, Bhutan, Bangladesh and Myanmar. The area, which has the highest mountains in the world, has glaciers that feed fluvial systems such as the Indus, the Ganges, the Yangtze, the Irrawaddy and the Mekong.

According to the analysis, the impact of the melting could leave from floods due to the increase of the flow to an increase of the pollution by the black carbon and the dust deposited in the glaciers.

Saleemul Huq, director of the International Center for Climate Change and Development, a center for environmental research in Dhaka, described the findings of the study as “very alarming” especially for nations that are downstream such as Bangladesh.

“All affected countries should prioritize addressing this next problem before it reaches crisis proportions,” he said via email, and Huq was one of the external reviewers of the study.

According to the analysis, even if the most ambitious goal of the Paris Agreement is met, to limit global warming to 2.7 Fahrenheit by the end of the century, more than a third of the region’s glaciers will be lost. If the increase in global temperature reaches F, two thirds of the Himalayan water reserves will disappear.