Every year, tens of thousands of pilgrims gather at one of Peru’s most sacred glaciers — the Colquepunco, located at the heart of the Cordillera Blanca, near Cusco. Draped in brightly colored ponchos, adorned with iridescent plumes, and carrying large religious symbols, pilgrims from all walks of life come to join the Quyllurit’i — a festival mixing Catholic, Incan, and local indigenous rituals and ceremonies.

The pilgrims chant, dance, pray, and make offerings to the glacier, which legend holds is inhabited by a divine spirit.

The most sacred part of the ceremony involves men dressed as mythical half-man, half-bear creatures known as the Ukuku, who cut large blocks of ice from the glacier and strap them to their backs. The Ukuku carry this ice — which, when melted, is thought to hold supernatural healing powers — down to the valley.

Bernard Francou, a French glaciologist who has spent more than 20 years studying Andean glaciers, has joined three of these celebrations in his lifetime. To his surprise, though, the ritual had changed by the last time he went, in 2012. “I expected them to take the ice from the glacier to bring it down on their backs,” he recalled, “so I asked them why they stopped, and they said: ‘We saw that the glacier had retreated, it is increasingly small, it is sick. So we don’t want to cause it any more harm by removing more of its ice.’ “

In recent decades, nearby residents of the Cordillera Blanca have watched in dismay as the Colquepunco and surrounding glaciers have steadily shrunk. Now, researchers in Germany and France have quantified just how rapidly ice in Peru and throughout the Andes is disappearing. Using high-resolution data generated by satellites and a 2000 Space Shuttle mission to create three-dimensional representations of Andean glacier change over time, the researchers calculated that the area covered by glaciers in Peru shrank by nearly a third from 2000 to 2016.

Across the Andes, glaciers have lost nearly 3 feet in thickness annually since 2000, according to Etienne Berthier, a glaciologist at the Laboratory of Geophysical Studies and Oceanography in Toulouse, France, who recently published his findings in Nature Geoscience. Warming temperatures also have caused glaciers to swiftly recede, particularly in the southern Andes, where some glaciers have retreated 5.5 miles in the past century. Ninety-eight percent of Andean glaciers have shrunk this century.

Glaciers are vital resources for communities in and around the Andes, where meltwater is used for drinking, irrigation, and hydroelectric power — especially in arid regions and during periods of drought. “The disappearance of glaciers will have an impact on the cities, but not just cities — locals, farmers, and people who do agriculture more broadly,” says Francou.

The loss of Andean glaciers also has global repercussions. Nearly all the world’s ice is locked up in the vast ice sheets of Antarctica and Greenland, with lower-latitude mountain glaciers and ice caps making up only 4 percent of the world’s land ice area. But because the world’s mountain glaciers — including in the Andes, the Himalayas, the Tibetan Plateau, the Alps, and various Alaskan and Russian ranges — are melting so rapidly, they have been responsible for a disproportionate share of global sea level rise in recent decades. No mountain region has lost more ice, relative to its size, than the Andes.

Until recently, information regarding the speed and quantity of Andean ice loss was generally restricted to more easily accessible sites, with scientists manually planting stakes in glaciers and recording changes in their mass over the years, says Berthier. But the recent satellite studies have greatly expanded scientists’ ability to track melting glaciers in the Andes and around the globe.

“Our study, and the one from Etienne Berthier, are the first studies that cover the whole South American continent based on measurements everywhere,” says Seehaus Thorsten, a glaciologist at Friedrich-Alexander-Universität Erlangen-Nürnberg, who recently published his findings in Nature Climate Change.

Berthier and his team were able to obtain data covering most sub-regions of Andean glaciers, giving the researchers a more accurate picture of the pace of glacial retreat and enabling them to better forecast how quickly glaciers will recede in the future.

What the new data shows is that while Andean glaciers overall are receding, they are doing so at varying rates in different regions. In the Desert Andes, for example, a small number of glaciers are actually expanding or holding steady, says Thorsten — though these account for only 1.3 percent of the glaciers studied.

The overall trend, though, is abundantly clear. Andean glaciers — from the small icy regions of Colombia and Venezuela in the north all the way to Patagonia’s glaciated expanses in the south — are rapidly shrinking.

The south Patagonian ice fields are the fastest-melting on the continent, thinning by an average of nearly 3.3 feet a year, according to Berthier’s study. Together with the northern Patagonian ice fields, these regions account for 83 percent of all ice loss in South America. The reason for this, explains Francou, is that the low-altitude glaciers of Patagonia make them particularly vulnerable to rising air temperatures.

While glaciers in Patagonia are steadily giving way, Berthier points out that relatively few people actually live in these remote regions. The few who do live there, however, are concerned.

Lukas Garcia Reyes, 27, has spent his whole life in the small northern Patagonian town of Futaleufu in Chile, near the border with Argentina. The Futaleufu River flows from Argentina through the town and empties into long, narrow Yelcho Lake. The river is surrounded by nature reserves and mountains and is fed by Andean glacial snow melt from lakes in Argentina and Chile.

Reyes says the signs of glacial retreat are hard to miss. “We go up to see the glaciers because we like to hike, and we’ve noticed the difference from when we were kids,” he recounts. “There are areas of ice that have retreated around 5 kilometers.” He adds that rivers, swollen with glacial runoff, have risen visibly over the years, preventing locals from fishing in some areas.

For now, the rapidly receding glaciers are not severely affecting the community, says Reyes. Patagonia’s gargantuan glaciers contain large quantities of ice and it usually rains year-round, so water is an abundant resource — for now. But the question on the minds of some of Futaleufu’s nearly 3,000 inhabitants is this: For how long?

“All the rain and snow that comes during winter melts before spring begins,” explains Reyes. “During the summer we subsist with the glacial reserve, but I don’t know how much longer this will last.”

Larger, highly populated cities that depend on water from neighboring glaciers during summer — such as La Paz, Bolivia; Santiago, Chile; Mendoza, Argentina; and Huaraz, Peru — face a serious and imminent risk, says Seehaus. The speed of ice loss may not be as dramatic as in Patagonia, but the sheer number of people who depend on glacial water to drink and grow crops suggests the impact will be severe, he adds.

At first, regions surrounded by vast glaciers will actually experience an increase in water availability as glaciers melt and the water descends into rivers and lakes, says Francou. But soon, this water will dry up, he adds.

One study suggests that nearly 4 million people in major cities of the tropical Andes use glacial melt as a basic water resource. In La Paz, population 2.3 million, about 27 percent of the water supply during dry seasons comes from glacial melt. Similarly, glacial melt provides nearly 20 percent of Huaraz’s annual water supply. Seehaus says melting glaciers could exacerbate already-existing water shortages caused by rising temperatures and a changing climate, such as the drought that hit Bolivia in 2016, the worst in a quarter century.

Andes residents also are concerned about the impact of glacial retreat on tourism, an important source of income in some regions, notes Berthier.

Huaraz, a tourist-driven city of about 110,000 people, is flanked by two of Peru’s tallest and most spectacular mountain ranges — the Cordillera Blanca and the Cordillera Negra. But, says Seehaus,”If the ice is gone, it’s not the Cordillera Blanca [White Range] anymore.”

Luciana Juarez, owner of an adventure tourism agency and hostel, has lived in Huaraz for 16 years and says receding glaciers are causing ice fields to develop large crevasses that make mountaineering expeditions too dangerous.

Tourists still flood into Huaraz every year, and water availability is not yet an issue, says Juarez, but the city’s future is uncertain. “I know the glaciers are melting and I know that will be a problem in the future,” says Juarez.

In Futaleufu, Reyes is skeptical that anything can be done to address the melting glaciers. “We’ve run into a wall,” he says. “There is no return, I think, because the climate has already changed.”

Some indigenous groups are so worried that they have turned to their most sacred cultural symbols for support, says Francou. They offer little statues, coca leaves, and other objects they deem sacred to the glaciers. In return, they ask for a healthy family, financial stability, and favorable weather for their crops.

“With the retreat of glaciers happening right now, the Andes people ask the mountains for help,” Francou adds. “They’re very worried.”