Mountaineering in some parts of the world is becoming riskier because of climate change, climbing experts and scientists warn.
They say warming in the Alps has thinned ice and snow cover, resulting into frequent rock-falls and landslides.
Their concerns were voiced at a meeting of the International Climbing and Mountaineering Federation this week.
Melting glaciers have also added challenges for mountaineers.
Climbing routes have either had to be abandoned or changed because of the increased risks.
In some places, climbing seasons have had to be brought forward.
A study of mountaineering plans for climbers in the Mont Blanc massif showed numerous changes. Almost all climbing "itineraries" for the region had been affected since the 1970s and a few routes no longer existed.
In the journal Arctic, Antarctic and Alpine Research, Jacques Mourey from the University of Grenoble Alpes and colleagues wrote: "Moreover, periods during which these itineraries can be climbed in good conditions in summer have tended to become less predictable and periods of optimal conditions have shifted toward spring and fall, because the itineraries have become more dangerous and technically more challenging."
A 2017 study of several summits in the Mont Blanc massif by a French team showed significant degradation of permafrost between 1850 and 2015. This led to the slopes becoming unstable, causing rockfalls.
"Many of these routes have become extremely dangerous," Florian Ritter, from the University of Natural Resources and Life Sciences in Vienna, and colleagues wrote recently in the journal BioOne Complete.
"Numerous classical ice climbs in the Eastern Alps have become heavily affected by rockfall and falling stones, as well due to rocks melting out at the ice margins... during late summer and autumn."
The Austrian research also pointed to other factors. "While the described processes related to global warming might increase the potential, events are generally triggered by other processes that are less linked to global warming, such as extreme rain events," it said.
Analysis of guidebooks written for those climbing in the Bernese Alps, Switzerland, confirms the changes.
"In the climbing guides, multiple generations of climbers noted the climbing dangers for whole mountain ranges," said co-author Arnaud Tremme, a geographer with Wageningen University in the Netherlands.
"The oldest guide out of the dozens of guides used in the research was written 146 years ago, this allowed us to record the changes for a longer period and link these with climate change."
For the past 10 years, a network of wireless sensors on Mount Matterhorn in the Alps has been streaming data on the condition of steep rock faces, permafrost and prevailing climate.
"Based on the data we have received, we see that the rate of change is quite fast," said Jan Beutel, a Zurich-based researcher, who leads the monitoring project.
"As a result, some of the places can't be visited anymore."
A study on Aoraki Mount Cook in New Zealand also highlighted difficulties for climbers.
Climbers here have to navigate the Tasman Glacier, but this is now becoming more difficult because it's melting.
"One of the interesting results of our study was the spatial variability associated with climate change - the impacts are greater at lower elevations, but much higher on the mountains. Because of the large seasonal snowfall, the effects are somewhat buffered," Heather Purdie, one of the authors of a study on the mountain told the BBC.