On some mountains, the thick white snowpack can sometimes be covered with red spots, giving the impression of a real high-altitude crime scene. This natural phenomenon, both meteorological and biological, is in fact a very negative sign of the health of the affected glaciers.
Mountain hiking can hold surprises on certain massifs. In recent years, images of pink, red, or orange snow taken by hikers on glaciers have multiplied, particularly in the Alps. The so-called “red snow”, “blood of the glaciers” or “melon snow” (in English) is actually an increasingly common observation. Mountaineers used to think it was just mineral deposits, much like the sand from the Sahara is regularly blown up the mountain peaks by the wind.
A “living” red snow that moves
But then scientists discovered that this blood snow was actually alive. These are microscopic algae, most commonly Chlamydomonas nivalis or Ancylonema nordenskioeldii, that appreciate freezing temperatures.
This tiny plant organism is packed with carotenoid, the same red pigment that colors tomatoes and maple leaves in fall. This type of algae is also known to turn some lakes and streams red or pink when the weather conditions are right.
This alga does not appear suddenly, it is present on certain glaciers all year round. During his hibernation he is simply invisible. But as soon as spring arrives, the algae can move with the water of the glaciers to reach an altitude that is suitable for them: between 3,000 and 3,700 meters in general, but certain species are also visible below 2,000 meters. In contact with the strong rays of the sun, the red pigment is activated to protect the algae from burns: the color finally acts as a sunscreen.
The algae feed on the minerals present in the soil, and when the snow begins to fall in large quantities again and the sun goes down at the end of summer, the algae again go into a dormant phase, lose their colors and become buried in the snow.
A large amount of red snow is a bad sign
Even if the appearance of these algae is natural, the increased snowmelt caused by global warming seems to make them more and more visible: the more the snow melts and the more the winter season recedes, the more the algae spread. This red snow then has the effect of reducing albedo, with up to 13% less reflection. Because the heat is absorbed rather than diverted to the sky, the melting of snow and glaciers accelerates. Warming therefore increases the amount of red algae present, and as the algae spread, the snow melts more.
“Glacial blood” is very common in the Sierra Nevada mountains of California, Greenland, Antarctica, but also in the Italian Alps, such as the Presena Glacier. Just as tomato juice can permanently stain clothes, “glacier blood” can leave indelible marks on hiking pants. If everyone knows not to eat snow in the mountains, even less risk swallowing red snow: seaweed is a powerful laxative. Red snow, on the other hand, is safely consumed by certain worms, protozoa, and arthropods.