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France's longest glacier, the Mer de Glace, is being hit hard by climate change and is melting fast.

The "Sea of Ice" is one of the biggest glaciers in the Alps…but it gets smaller every year. Tourists come to marvel at this landscape but for the scientists working here, the view is a terrible demonstration of the effects of climate change.

Formed over thousands of years by snow freezing on the mountain top and being compacted into enormous masses of ice, the 5,000 glaciers of the Alps are all under threat. The Mer de Glace is just one of the most visible examples.

"The only solution is to reduce gas emissions," says Ludovic Ravanel, a glaciologist. "Even if today we stop all gas emissions, the glacier will continue to retreat for many decades. So if we want to do something for the glaciers here in the Alps, we have to have a revolution on a world scale."

There's another example of climate change here: the color of the glacier. It's not the typical tumbling blocks of blue and white ice. Rising temperatures melt the high ice holding rocks in place and Mer de Glace is covered with mountain debris.

The view is spectacular up at the glacier, with the peak of the 3,754m-high Aiguille du Dru overlooking the valley. Access is by mountain train, then cable car down to a cliff-side platform and finally hundreds of steps down the current level of the glacier.

There are signs pinned to the rocks as you descend showing where the glacier was in 1980, 2000 and 2010. It is a simple but sobering illustration of the impact of climate change.

"Every time I go there it breaks my heart," says Christian Dechavanne, the CEO of the company that runs all the Chamonix ski lifts and mountain trains. He says the situation is very serious but better education is the answer.

"The good news is, we can see that there is a demand for that. There is an expectation that wasn't there 10 years ago. Ten years ago it was like, 'ok, I can see this but don't talk to me about this – I'm on holiday.' "Nowadays, people come here to talk to the glaciologists at the top and to get the beginning of an answer."

Tourism powers much of the economy of the Alps. But while this region needs holidaymakers, the flow of visitors impacts the very beauty they come to see through erosion, energy use and greenhouse gas emissions.

There's also a push to change the make-up of the tourist community to dissuade visitors who take polluting long-haul flights and encourage more visits from those closer to home.

The melting of the glacier can't be halted but it can be slowed if action is taken to stop this turning into another dried-up valley.

This story is part of CGTN Europe's series The Alps: Timeless and Changing which was originally published in September 2020.