Bryan Mestre was shocked to discover the large pool of water at an altitude of 11,100ft (3,400m) in the Mount Blanc mountain range – claiming the unusual sight was a worrying sign.
Scientists have warned that heatwaves in Europe are becoming increasingly frequent, with the intense temperatures linked to climate change.
“Time to sound the alarm,” said Mr Mestre. “Only 10 days of extreme heat were enough to collapse, melt and form a lake at the base of the Dent du Géant and the Aiguilles Marbrées.”
He added: “This is truly alarming … glaciers all over the world are melting at an exponential speed.”
Sharing the image on Instagram, the French rock climber said he took the photo on 28 June – only 10 days after fellow mountaineer Paul Todhunter captured the same area covered in snow.
“Needless to say, the lake was a real surprise,” Mr Mestre told the IFLScience website.
“It’s located in the 3,400 to 3,500-meter area. You’re supposed to find ice and snow at this altitude, not liquid water. Most of the time when we stay for a day at this altitude, the water in our water bottles starts freezing.”
“I have been up there a fair amount of times, in June, July and even August, and I have never seen liquid water up there,” he added.
Glaciologist Ludovic Ravanel previously noticed a lake appearing high in the French Alps in 2015 and linked its formation to global warming.
According to the EU’s Copernicus Climate Change Service (C3S), last month was the hottest June ever recorded on Earth.
Data released by the satellite agency showed Europe’s average temperatures were more than 2C above normal, and temperatures were between 6C and 10C above normal over most of France, Germany and northern Spain during the final days of the month.