Growing patches of coloured snow in the French Alps could be a sign of the impact climate change is having in the mountains.
It is a shocking, garish sight to come across on a peaceful mountainside. Hike high enough in the French alps during the late spring and early summer, and there is a good chance that you will come across some rather strange patches of snow among the grey limestone and stunted clumps of vegetation. This snow isn't white – it's blood red.
The peculiar phenomenon – sometimes known as blood snow – is the result of a defence mechanism produced by microscopic algae that grow in the Alpine snow. Normally these microalgae have a green colour as they contain chlorophyll, the family of pigments produced by most plants to help them absorb energy from sunlight. However, when the snow algae grow prolifically and are exposed to strong solar radiation, they produce red-coloured pigment molecules known as carotenoids, which act as a sunshield to protect their chlorophyll.
Just two years ago, botanists at Charles University, Prague, in the Czech Republic, identified an entirely new genus of microalgae that is responsible for causing red and orange snow in different parts of the world, which they named "Sanguina" in reference to the blood-red colour they produce. The researchers found forms of Sanguina algae that cause red snow samples from Europe, North America, South America along with both polar regions. A species of Sanguina that causes an unusual orange snow was also found in Svalbard.
Researchers discovered a new genus of algae called Sanguina that is responsible for the red snow in the French Alps above 2,400 metres (7,874ft) (Credit: ALPALGA)
But understanding more about red snow algae carries a significance far greater than simply explaining the existence of strange-coloured patches in the Alps and near the poles. Its appearance and disappearance are important markers of climate change and how it is affecting the delicate ecosystems where the algae are found.
According to Liane G Benning, professor of interface geochemistry at the German Research Centre for Geosciences in Potsdam, red snow is becoming more common due to global warming. "The rise in the atmospheric carbon dioxide levels increases the temperature, which leads to more snow melting," she says. "The moment there is liquid water on the snow, the algae start growing."
This increasing abundance of red snow algae may also be contributing to climate change too. The red pigment turns the snow surface dark, reducing the amount of light and heat it reflects back into space – something known as the albedo effect. By trapping more of the Sun's heat, the snow melts even faster, allowing the algae to proliferate further. "There is a runaway effect in which the algae melt their preferred habitat," says Benning. "It’s as if they are destroying their own house."
On a wider scale, the extra heat absorbed by the tinted snow can alter the temperature in the wider environment, speeding up the melting of snow packs and glaciers. One study estimated that over a single melt season, red pigmented algal blooms could reduce the snow albedo by 13%, suggesting it plays an important role in how the effects of climate change can be amplified within mountain environments.
Tests on samples collected in June have revealed the presence of unicellular animals, called zooplankton, with the algae cells.
Studies have shown red algal blooms occur on glaciers all over the world, from Antarctica to the Himalayas and in the Arctic. So one question that scientists like Benning and Eric Maréchal, director of the Cell and Plant Physiology Laboratory in Grenoble, France, are keen to answer is whether red snow algal blooms are becoming more widespread and occurring more often.
One way of doing this would be to use satellite imagery to study the albedo-reducing effect of the red snow. A study using satellite imagery of snow fields on Fildes Peninsula on King George Island, off the coast of Antarctica, revealed that in January 2017, 26% of the snow was darkened by algae.
Although there is little widespread data to show if red algae are becoming more common globally, both Benning and Maréchal believe they will occur more often as our planet warms, and this will need to be taken into account as scientists try to estimate what the impacts will be.
But even laying aside their role in climate change, scientists are unpicking other mysteries surrounding red snow.
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Maréchal and his colleagues recently found that red snow algae appear to only grow at elevations above 2,000m (6,562ft) in the French Alps, and particularly flourish at around 2,400m (7,874ft). According to Maréchal, the Sanguina algae is found at high elevations because of the quantity, quality and longevity of the snowpacks present at these heights.
Puzzlingly, scientists have so far failed to grow these algae on real snow in a laboratory.
"It is for this reason that researchers need to collect as many samples as possible for a more refined study," Maréchal says.
Algae that grow close to penguin colonies near Antartica produce vivid red pigments (Credit: Robert Harding/Alamy)
During a recent two-day expedition to the Lautaret pass in Hautes-Alpes, southeastern France, in June this year Maréchal and his colleagues in the ALPALGA consortium of five French institutes dedicated to the study of mountain algae, collected their first samples of 2021. Unlike previous years, however, the snow didn't have its typical red hue. Instead, it was dominated by ochre yellow.
The yellow tinge, they believe, was due to the presence of sand on the snow that interfered with the colour imparted by the algae. While not an unusual phenomenon, this year was exceptional as strong winds carried plenty of Saharan sand to the Alpine heights.