COPENHAGEN—The Altiplano, or high plain, of Bolivia and Peru is getting a new climate. In the past 60 years temperatures have risen, rainfall patterns have changed and soils have begun to dry out even further. As a result, farmers move their crops further up the mountainsides, like endangered species seeking refuge at cooler elevations. Floods and frosts remain the biggest threats but when the entire water system of your area changes, how do you adapt?

That is the question residents from the Andes to the Himalayas are asking as the climate changes. Water streams off the Pastoruri Glacier in Peru year-round now, even in July, which is the middle of their winter in the Southern Hemisphere. Some call it an "ice blanket" now, rather than a glacier, thanks to its steady retreat. And much like their Incan ancestors, the residents must build weirs to hold some of the water and save it for their daily lives.

Local farmers have also seized control of hydroelectric dams in the region, due to concerns that power producers, such as U.S.-based Duke Energy, might be holding back water needed for their crops. "The farmers felt shortages," says John Furlow, a climate change specialist at USAID who is attending the United Nations' climate summit here. "There's a realization of impacts getting ahead of where the science is."

At the same time, local residents rely on the governments of Peru or Bolivia for protection from avalanches and floods kicked off by newly formed lakes of glacial meltwater or thawing permafrost. "There's a fair amount of mistrust of the government and a reliance on it to protect people," Furlow adds.

In the Himalayas, temperatures have been warming since the Little Ice Age, with an acceleration in recent years, particularly at high elevations. While areas at 1,000 meters might see warming of 0.1 degree Celsius per year, those at 3,000 meters or more see as much as 0.3 degree C per year. As a result, many Himalayan glaciers "will not disappear completely but they will reduce in size," says Mats Eriksson of the Kathmandu-based International Centre for Integrated Mountain Development (ICIMOD).

So-called glacial lake outburst flooding—basically the collapse of a permafrost moraine bridge freeing up meltwater—is an increasing risk, and in some areas such as the Chitral district of Pakistan, it is now happening every year. "These are the people who are paying the price for a changing climate," Eriksson says. For example, the village of Brep in Pakistan was entirely wiped out by such a flood, though the inhabitants were able to escape harm by monitoring the mountain sounds as well as the smell and color of the meltwater to know when to move to higher ground.

And, ultimately, that means bad news for the several billion people who rely on meltwater from these glaciers via the major rivers of Asia, from the Indus to the Yangtze. Although water flows in these rivers is actually increasing right now thanks to increased meltwater, Eriksson notes, "eventually this will turn into a decline."