UAE researchers have revealed new details about how dust is travelling from the Sahara to the Alps to cause snowy pistes and glaciers to turn a dramatic red, pink or orange.
The striking colouration, which happens when the dust causes the growth of microalgae, makes the snow melt more easily and is likely to become more frequent because of climate change.
Scientists at Khalifa University in Abu Dhabi reported that flows of air called atmospheric rivers are closely linked to the transport of dust from the Sahara to as far as northern Europe.
“In our study, we found an increasing trend in atmospheric rivers and associated severe dust transport episodes towards Europe,” said an author of the study, Dr Diana Francis, head of Khalifa University’s Environmental and Geophysical Sciences (Engeos) Laboratory.
“This implies that snowmelt can be expected to occur more frequently under global warming and since the presence of liquid water on the snow marks the onset of the algae growth, we can expect an increase in the frequency of pink or bloody snow.”
Titled, Atmospheric Rivers Drive Exceptional Saharan Dust Transport Towards Europe, the study is being published in the journal Atmospheric Research.
The study investigated, in particular, an “extreme dust transport event” in February this year linked to an atmospheric river that stretched from Africa to northern Europe.
Described as “elongated and narrow bands of clouds and high water vapour content”, the atmospheric rivers transport air from low latitudes towards the poles.
The dust and the microalgae make the snow darker. Because darker surfaces absorb more light, the red snow reflected 40 per cent less light than white snow, making it warm up and melt more easily.
This process amplifies the effects of climate change, which threatens glaciers and other areas covered with snow because of temperature increases.
Reduced snow in the Alps
The Alps have experienced a severe reduction in the depth of snow over the past 40 years, with an eight per cent fall recorded per decade in areas below 2,000 metres. This has badly affected many Alpine ski resorts.
While the influx of warm and moist air is a key cause of the melting of ice and snow, dust deposition adds to the effect.
Between 6 February and 6 March this year, snow depth over the Alps fell by half, partly because of the darkening from the deposition of dust.
Instead of being pink, the snow could turn orange or green depending on the type of microalgae that grow, said Dr Francis.
In March 2018, Saharan dust turned snow orange in eastern Europe and Russia and reduced the length of snow cover in some areas by as much as 30 days, the scientists said.
“Dust aerosols contain a lot of minerals such as clays, calcite, quartz, feldspars and iron oxides which could provide nutriments and other element for the microorganisms present in the algae bloom,” said Dr Francis.
“More scientific work is needed to link the type of microalgae growing on the snow to the impurities found on it.”
A similar phenomenon is seen in Greenland, where Dr Francis said dust and other impurities cause the formation of small black holes in the ice called cryoconites, which speed up melting.
Other authors of the new study work at universities in Los Angeles, Santiago in Chile and Grenoble, a city that sits at the foot of the French Alps.