French scientists have expressed concern about a rise in Alpine snow turning from white to pinkish red, warning the colour shift could be a marker of accelerating climate change. 
The Alps are covered with a thick mantle of white snow from winter to spring but as the slopes warm with the approach of summer, some mysteriously shift to turn various shades of orange, red and red.

Locals call the phenomenon “sang de glacier”, or “glacier blood”, while others dub it “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.

They have just published their preliminary findings in Frontiers in Plant Science.

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

Algae are the foundation of most food webs

Credit: WILLIAM WEST/AFP

“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, that they create a shield of red molecules that play the same role as sun cream.”

He and experts from CNRS, CEA, Meteo France and Inrae, decided to try and 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.

“To date, and to our knowledge, no such systematic investigation has been attempted,” they said.

After studying their DNA, they found a variety of algae including one key blood-red type appropriately named Sanguina, which only grows above 2,000m (6,500ft).

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, for example.

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

A swimmer heads towards a red algae bloom in Sydney

“Unfortunately, the red coloration 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 Maréchal.

“In areas without glaciers, it shortens periods of snow cover with cascading consequences on (water) supply to dams, farming irrigation in the plains.

“It’s a paradox,” he went on. “The more the micro-algae multiply, the more they contribute to the disappearance of their own environment.”

The researchers have taken some species back to the lab to deepen their investigation into what triggers the blood-red blooms.

These occur naturally and have been observed for centuries. Indeed, Aristotle penned the first written observation of blood glaciers, hazarding a guess that the snow had grown hairy red worms. But human-produced factors may well be increasing their quantity and frequency.

Mr Maréchal’s team is about to take fresh findings to see how they have been affected by large quantities of sand blown from the Sahara onto Alpine peaks due to particularly strong sandstorms this year.

There may be, however, one silver lining to the rise of such bloody snow algae, according to the researchers.

“Sanguina molecules are rich in antioxidants and could interest the health sector to fight against the ageing of cells or even energy to synthesise new fuels,” said Mr Maréchal.