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In July, Lorenzo Pernigotti, a seasoned mountain climber who has��summited��over 100 peaks in the Alps, scaled��Aiguilles du Diable—“devil’s horns” in English—a set of rock towers on one of 11 major high points��in the Mont Blanc massif, which straddles the border of Italy and France. The climb grew more difficult as he and his partner, Davide Iacchini, neared the top. At 13,000 feet, they reached the final leg, where they would climb out of a narrow corridor and onto the jagged ridge.��

Pernigotti, 30, took the lead on��the wall of the corridor, scanning for knobs and crannies where he could place his hands and feet��while bound to his partner by a short length of rope. Above him, he spied a large rock jutting from the wall, like a handle, that he could use to hoist himself up. He pulled on it with one hand, then two, testing to see if it would hold his weight. After a couple of sharp tugs, he hung from it. But as he dangled there, the rock gave way.��It slid down a few inches, and Pernigotti yelled at Iacchini��to shorten the rope connecting��them. As the rock began��crashing down��into the corridor, Pernigotti��let go, falling 30 feet before the rope went taut,��with other��rocks, snow, and ice tumbling��into the chasm all around him.

He was lucky. Pernigotti��suffered only a ruptured Achilles tendon��and, with his partner’s help, made it to the crest, where a helicopter came to his rescue. But the experience rattled him. Even the most experienced climbers are now struggling with increasingly perilous conditions in the Alps. Experts say that the problem is climate change. As temperatures rise and the peaks thaw, rocks that were previously held firmly in place by ice are coming loose.

During the 20th century, temperatures in the Alps rose by 3.6 degrees Fahrenheit, double the amount of warming seen across the Northern Hemisphere on the whole. If humans make only modest cuts to pollution, the Alps are expected to��warm by close to another six��degrees Fahrenheit by the end of this century, according to a of climate research. Climbers��are already seeing the effects of global warming.

Jacques Mourey, a geographer at the Université Savoie Mont-Blanc, has�� how rising temperatures have altered climbing conditions on the Mont Blanc mountain range over the past 40 years. He and his colleagues identified 25 changes affecting mountaineering. Among them,�� is melting snow and spurring avalanches; melting ice is producing wider crevasses; and rising temperatures are yielding less snowfall. For a recent��, he focused on one of the most dangerous changes: falling rocks.��

“It’s a very tricky condition,” Mourey said. “You think the rock is safe, and then it moves.”

This��mountain range is partly covered by permafrost, areas where the landscape is constantly frozen. Ice within rock fissures acts like cement, keeping parts of the mountain together. But around 20 years ago, the��—the altitude at which air temperature remains at or below 32 degrees Fahrenheit throughout the day—began to creep upward. In the 1990s, the summer freezing level was typically around 11,000 feet, but in 2003, it was closer to 13,000 feet, according to��, an organization��supporting research on the Alps. That summer, the permafrost started to thaw. Scientists say it will�� as the climate warms, causing more rocks to come loose, like the one that nearly killed Pernigotti.��

Recent years have seen massive��. From 1990 to 2017,�� and 230 were injured in the��Grand Couloir, on the popular route to the 15,774-foot peak of Mont Blanc, the highest summit��in the range and the tallest mountain in Western Europe. Rockfall accounted for 30 percent of the accidents.

Mourey said that while the number of yearly injuries on the Grand Couloir has remained roughly flat over the past two decades, fewer people are staying overnight on the mountain, a figure that has decreased from 62,000��between��1999 and��2001��to less than 51,000��between 2014 and��2016. In other words, smaller groups of climbers are��racking up the same number of injuries, which means the probability of getting hurt has increased��over time.��

“On a trafficked route, the rock was previously glued with ice, and mountaineers were using it. That rock had never moved before��but suddenly fell,” Mourey��explained. He believes that at least two deadly accidents in 2018 can be blamed directly on climate change. In��, a climber grabbed a huge stone that slid unexpectedly. It was precisely what happened to Pernigotti, except the��other climber was crushed and killed by a falling rock.

Alpine guides spend most of the year in the mountains, where they see the growing danger firsthand.��, 44, started climbing in the Alps with his parents when he was five and became a guide in 2002, when he was 26. He compared the cold summers of his childhood with the unusual heat he saw last January, “when the sun felt closer and closer.”

“The day might be fantastic, and you are at 13,000 feet wearing shorts. But how many risks have you taken to get there?” he said. Lately, Rossi has avoided leading��his clients on old-favorite high-mountain routes due to the dangerous, unpredictable changes.��

Rossi has begun fighting to make the mountain more secure by advocating for a forecasting system that can predict the danger of rockfall given the current weather conditions, something similar to the�� ratings available for many mountain environments.��

In Courmayeur, on the Italian side of Mont Blanc, some alpine guides are being trained to take climate change into account while planning trips for their clients. Jean Pierre Fasson, secretary general of��, a foundation dedicated to making mountaineering in the Alps safer, has added a weeklong module on climate change to his organization’s alpine guide course.

“Disasters are part of nature, and it’s wrong trying to stop them,” said Fasson, who believes that mountaineering will survive on Mont Blanc, but that climbers will need help navigating the growing risks. Between 1999 and 2017,�� of accident victims were amateurs who attempted to climb Mont Blanc without a guide. Fasson said it’s up to alpine guides to understand the rapid changes��and avoid routes that might pose greater danger.

Pernigotti said that only those who regularly climb in the Alps can appreciate how warmer weather has made the mountains more treacherous.

“Not everybody sees what’s happening. Only people who live daily in strict contact with nature can understand the impact of climate change,” Pernigotti said. “If you live in the city, you don’t see the stars. You don’t feel the wind. You can’t understand what’s happening.”

This story was published in partnership with��.