On some mountains, the thick white snowpack can sometimes be covered with red spots, giving the impression of a real crime scene at high altitude. This natural phenomenon, both meteorological and biological, is in fact a very negative sign on the state of health of the glaciers concerned.
Mountain hiking can hold surprises on certain massifs. In recent years, photos of pink, red, or orange snow taken on glaciers by hikers, especially in the Alps, have multiplied. The so-called “red snow”, “blood of the glaciers” or even “melon snow” (in English), is indeed an increasingly common observation. In the past, mountaineers thought it was simply mineral deposits, much like the sand from the Sahara regularly carried by the winds up to the mountain peaks.
A “living” red snow that moves
But then scientists discovered that this blood snow was actually alive. These are microscopic algae, Chlamydomonas nivalis for the most common, or Ancylonema nordenskioeldiiwhich enjoy freezing temperatures.
This tiny plant organism is packed with carotenoid, the same red pigment that colors tomatoes, and maple leaves in the fall. This type of algae is also known to color some lakes and streams red or pink when the weather conditions are right.
This algae does not appear suddenly, it is present on certain glaciers throughout the year. During its winter rest period, it is simply invisible. But, as soon as spring arrives, the algae is able to move with the water from the glaciers to reach an altitude that suits it: between 3,000 and 3,700 meters in general, but certain species are also visible below 2 000 meters. In contact with the powerful rays of the sun, the red pigment is activated to protect the algae from burning: the color finally acts as a sunscreen.
The algae feeds on the minerals present in the ground, and when the snow begins to fall again in large quantities and the sun goes down at the end of the summer, the algae again goes into a period of rest, loses its colors and gets buried in the snow.
A large amount of red snow is a bad sign
Even if the appearance of this algae is natural, the melting of snow accentuated by global warming seems to make it more and more visible: the more the snow melts and the more the winter season decreases, the more the algae spreads. This red snow then has the effect of reducing the albedo, with up to 13% less reflection. As the heat is absorbed, and not redirected to the sky, the melting of snow and glaciers accelerates. Warming therefore increases the amount of red algae present, and the more the algae spread, the more the snow melts.
“Glacier blood” is very common in the Sierra Nevada mountains in California, Greenland, Antarctica, but also in the Italian Alps, such as on the Presena Glacier. Just as tomato juice is capable of permanently staining clothing, “glacier blood” can leave indelible marks on hikers’ pants. If everyone knows that you shouldn’t eat snow in the mountains, you should even less risk swallowing red snow: seaweed is a powerful laxative. Red snow, on the other hand, is consumed without concern by certain worms, protozoa, and arthropods.
