French scientists have expressed concern about a rise in Alpine snow turning from white to pinkish red – warning that the colour shift could be a marker of accelerating climate change.

As the snowy slopes of the Alps warm near summer, some turn various shades of orange, red and pink.

Locals call the phenomenon “sang de glacier”, or “glacier blood”, while others refer to it as “watermelon snow”.

It has been observed for centuries, with Aristotle believing it was the work of “red and hairy worms” under the snow.

In fact, the pinker shade of white is caused by blooms of normally invisible algae.

French researchers say they change colour to protect themselves from ultraviolet light and that they may be proliferating due to global warming.

“When you ski, you slide over these micro-algae,” said Eric Marechal, the head of a plant physiology lab at Grenoble Alpes University and a leader of the research project.

“But you don’t notice them because they are green and less numerous,” he told Le Parisien. “They live off carbon dioxide and light. Then come bacteria that eat them.

“It’s when the sun’s rays become very strong, starting in May, they create a shield of red molecules that play the same role as sun cream.”

He and experts from several French scientific institutions decided to try to work out how they survive and why “glacier blood” has thrived in recent years. To do so, they took samples from soil found in five peaks at various altitudes to create a snow bloom map.

After studying their DNA, they found a variety of algae including one key blood-red type named Sanguina, which only grows above 1,980m.

Algae produce a large amount of the world’s oxygen through photosynthesis, and are present at the root of most food chains. However, in certain conditions they multiply wildly, causing toxic sludge.

Here, the experts believe the snow algae may be a marker of climate change but also that their rise could hasten the shrinking of glaciers and snow caps.

“Unfortunately, the red colouration favours the melting of glaciers by getting rid of the snow’s ‘albedo’ effect; it reflects the sun’s rays less, and heats up and melts more quickly,” said Mr Marechal. 

Telegraph Media Group Limited [2021]