PARIACACA — When the alarm sounds, it takes barely two minutes for the pupils of the "Lord of the Afflicted" school to empty out of their classroom. The older childen help the younger ones scale the grassy slope and rapidly move along a narrow path that leads away from the valley floor where they live, in the heart of the Peruvian Andes.

The students, all clad in their light blue and grey uniforms, make their way towards the "emergency operations center," a somewhat pompous name to refer to a tiny white canvas sheet on four wooden posts. Still, the place is synonymous with security. And everyone is where they should be, each playing his or her part to escape the "alluvium," or killer wave, loaded with ice, earth and rocks, that could emerge any moment from Lake 513.

"Everything went well," says Apolinario, a seventh grader who takes attendance. "We finished the drill in time." Flor, a young girl with an angelic face and sensibly tied-back dark hair, remarks, "I was there to help the wounded." In her hands is a red and white first aid kit containing a few cotton balls, some antiseptic and fever medication.

On this morning in Pariacaca, a modest village of the Cordillera Blanca (white range), nearly 200 grade-schoolers have once again practiced what to do in the face of the potential disaster that has been threatening their young lives since April 11, 2010. That day, a 300,000-square-meter block of ice broke off from Mount Wallqan and fell into Lake 513 below, unleashing a 20-meter-high wave that some describe as a "mountain tsunami."

Mountain lake near Pariacaca — Photo: delashinin via Instagram

"Luckily it was on a Sunday," Sally, 14, recalls. "We weren't in class. I was at home. I heard a huge noise. Mom was washing clothes at the river." The wave caused significant material damage but didn't take any lives. Had it ocurred on a weekday, the consequences no doubt would have been far worse.

Rapid retreat

Here in the Andes, children are acutely aware of the dangers associated with climate change, an issue that is coincidentally being discussed right now across the ocean, in Sendai, Japan, where experts are gathered for the third global conference on prevention of natural disasters.

Lake 513, which is located at an altitude of 4,400 meters, began to take shape about 30 years ago. Until 2010, the glacial lake's emerald waters had never worried anyone. It was even said to be one of the safest in the region thanks to several canals that were dug into its flanks to regulate the water level and prevent overflowing. But that sense of security proved to be frighteningly misguided.

The Cordillera Blanca, whose summits are a paradise for mountain climbers, is also one of the most spectacular examples of the effect climate change is having on tropical glaciers. Satellite images, weather stations, and field laboratories run by renowned glaciologists all tell the same story: The area's once-abundant glaciers are vanishing. What's not clear is what the outcome of this rapid environmental shift will be.

The Cordillera Blanca is just a sad example of a reality that concerns the entire Andean mountain range. Research published in November 2014 by Peru's National Water Authority of Peru showed that the total surface area of these ice caps decreased by 40% in 40 years. An unprecedentedly rapid regression. As a result, more than 1,000 lakes have appeared, raising concerns about other possible disasters.

Living in fear

In the village of Pariacaca, head teacher Juan Dextre lays out a map of the risk areas. "Pariacaca is the populated zone closest to the lake, and the school built near the river is the first building under threat," he says. "Even if we practice, not everyone will be able to leave on time. The youngest are 6 years old. We live in fear. There's no telephone network or doctor here. The wave can hit at any moment."

Dextre remembers another disaster, a terrible earthquake that struck in 1970 and killed tens of thousands of people. "I was 11," he says. "I remember the dead, the wounded. I don't want to go through that again.”

This risk mapping was conducted by Swiss scientists at the University of Zurich to set up a first alert system. Valley residents need to be able to evacuate the area in eight to 12 minutes, the time the wave would take to travel the nine kilometers separating the lake from the village.

Since August 2014, Pariacaca and the entire Rio Chucchun valley, down to the town of Carhuaz, where the 2010 alluvium came to an end, have been testing the system, the first of its kind in the Andes. Every site has its roadmap. The project cost about $330,000 and was funded by Switzerland.

With his colleagues from the national glaciology unit in the town of Huaraz, about 30 kilometers away, scientist Christian Huggel has modeled several disaster scenarios. In the worst case, the block that breaks away from the unstable walls of the Wallqan could be 10 times bigger than the one in 2010.

Watching and waiting

In Carhuaz's small town hall, which opens on a vast square surrounded by buildings of colonial architecture, Luis Mesa bears the heavy responsibility of managing the lake's surveillance system. "Everything is calm today," the young architect observes.

On the computer screen, in a cramped room that is also used for archives, there are two videos of the lake running continuously. The images appear as two green spots that are troubled only by a mild drizzle, typical in this rainy season. Next to the screen, curves reproduce the data recorded by four sound sensors set up at more than 4,700 meters altitude to observe the sounds produced by the glacier in case it disintegrates.

"They would be the first to sound the alarm," says Mesa, who admits he doesn't "sleep easily" now that the lives of the 11,460 people living along the Rio Chucchun depend so much on his vigilance. "We were given the technology, but we lack everything else. For this system to save lives, the local authorities need to get involved to be able to carry out the evacuation of the population."

In the streets of Carhuaz, large arrows painted in green on the white walls of the houses are a constant reminder of the road to take to escape the muddy wave.

Even with all the new precautions being taken, the children are anxious. "The disappearance of their glaciers, an ever-present risk, worries them," says Christian Rodriguez, the regional head of education who introduced lessons on the environment in school programs so pupils understand the issues of climate change.

"We've organized field trips to the lake so they could see where the danger comes from," he adds. "Talking about this makes me sad. I grew up here, in the middle of some of the most beautiful scenery in the world. I'm 38. I have two children. Aand I wonder if they'll be able to live in this world that is changing completely."

Water worries

In this rural region, the melting of glaciers also means water shortages. "During the dry season, from June to September, downstream from a 6,000-meter-high snowcapped summit, we paradoxically lack water," says Cesar Gonzales Alfaro, an agronomist who leads the adaptation to global warming program of an NGO called CARE. To help farmers handle the increasing irregularity of rain, temperature fluctuations and heavy frosts, CARE, which worked on the first alert system with the University of Zurich, suggests diversifying seeds and learning how to save water.

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The Pastoruri glacier — Photo: Taco Witte

"Here, tensions are rising between the poor inhabitants living in the mountains and the downstream regions where thousands of hectares of agricultural projects designed for export are being developed," explains Thomas Condom, a hydrologist of the Institute of research for development (IRD) and specialist of the region.

The inhabitants of the Cordillera Blanca will have to learn to live with this new reality. "In its initial phase, deglaciation provides additional water to the populations. But that stage is behind us," says Alejo Cochachin, coordinator of the Peruvian glaciology and hydraulic resources unit. "The glaciers have melted too much. They produce less and less water during the dry season."

Cochachin heads a team of a dozen scientists who have followed the evolution of 13 pilot glaciers that were chosen among the 2,679 that were recently counted in Peru. "The small glaciers located under 5,000 meters will have a lot of trouble surviving, even if we can't tell when exactly they will disappear," he says.

In Pariacaca, head teacher Dextre is not making plans for the future. "In 10 or 20 years maybe we won't have any water anymore, and we'll have to leave. Some have already started buying land far away from here," he says.

Twenty years ago, as soon as they got off the bus, the local children who went on field trips on the nearby Pastoruri glacier had snowball fights. Now, the ice is several kilometers away. The Pastoruri has become a tourist site along "the climate change road."