LIMA, Peru — Ever since marauding Spaniards first began exploring South America's rugged mountains and jungles in the 16th century, locating the source of the Amazon, the world's mightiest river, has been something of a geographic obsession.

Finally, in 1971, a National Geographic expedition identified the Amazon's starting point as being on the upper flanks of Mismi, a 18,465-foot mountain in the southern Peruvian Andes. A follow-up expedition in 2000, also by National Geographic scientists, confirmed that discovery, using the same methodology, tracing the furthest point upstream of the Amazon's longest continuously flowing tributary, the Apurimac River.

Today, the remote spot, at what was once the tongue of Mismi's high-altitude glacier, is marked by plaques from various scientific organizations.

But now, just as a consensus has emerged around the site, the ice there has all but vanished thanks to climate change.

"The glacier's almost gone completely. It will have vanished entirely by around 2021," says Luzmila Davila, a glaciologist with INAIGEM , Peru's National Research Institute for Glaciers and Mountain Ecosystems.

"It used to be that you could clearly see where the bottom of the glacier was melting and forming a stream. But now it is just dry rock, sometimes with water trickling down parts of it, but most of the time just dry."

Peru is — or at least was — home to some 70% of the world's tropical glaciers . But they are vanishing at an astonishing rate, with the high Andes experiencing higher-than-average temperature rises as global climate systems become increasingly chaotic. That is disrupting water sources for many of Peru's 31 million people, from subsistence farmers in remote mountain communities to the capital, Lima, where one third of the country's population live.

According to INAIGEM's latest national glaciers inventory, the Chila mountain range, where Mismi is located, has been the most severely affected by climate change. It has lost more than 99% of its original glacier cover, with 33.7 square kilometers (13 square miles) having disappeared, to leave just 0.19 square kilometer ( see pages 53 and 55 respectively ).

In total, according to the same study, Peru has now lost just over 53% of its original glacier cover, or 1,285 square kilometers of the original 2,399 square kilometers, since 1962, a period roughly equivalent to when industrial greenhouse gas emissions first began ramping up.

The process is something of a vicious circle, with scientists describing it as a "positive feedback loop."

That is because the sheer white of the mountains' snow and ice reflects much of the sun's energy back out into space, a mechanism known as the albedo effect. But the darker bare rock absorbs a much greater proportion of that energy in the form of heat, helping to speed up the process of deglaciation.

That problem is made worse by fine layers of soot, or "black carbon," that have been accumulating on the surface of the glaciers.

Even though usually invisible to the naked eye, these pollutants from the burning of fossil fuels, usually in smoke-stack industries and diesel and gas-powered vehicles, are thought to subtly but significantly reduce the albedo effect and therefore increase the rate of glaciers loss, according to research conducted by U.S. scientists in the Peruvian Andes .

The problem is also compounded by spreading Western lifestyles on the ecosystems immediately beneath the Andean glaciers.

High-altitude wetlands on the "paramo," or high plains, have traditionally acted as an enormous natural sponge, slowly releasing water accumulated during the wet season, which forms streams and eventually rivers lower down on the slopes.

But the spread of sheep and, particularly, cattle have had a series of negative impacts on the fragile, slow-growing paramo ecosystem. Cows' hooves crush the delicate grasses and compact the soil, unlike the much softer pads of the native llamas, alpacas, and vicuña, which they increasingly replace. Meanwhile, when the cows eat they tend to rip up the grass by the root, unlike those South American camelid species that only chew the tips.

But based just on the loss of glaciers alone, water supply is already down between 20% and 30% during the Peruvian Andes' dry season, roughly from May to October, Davila estimates.

Meanwhile the "equilibrium line," which divides the areas where glaciers accumulate ice and where they lose it through melting, has also risen dramatically in the country, she says.

Several decades ago, it was at 4,700 meters (15,419 feet). Now, it is at 5,040 meters (16,535 feet). By the end of the next decade, Davila predicts, it will be at 5,100 meters (16,732 feet). Eventually, in a couple of centuries or so, even Peru's highest mountain, Huascarán, at 22,205 feet, may only have seasonal snow and ice cover.