It’s so light that it easily rises above the Sahara and sets out on intercontinental journeys. The effects of its movements are astonishing. Alpine snow changes colour, Caribbean heatwaves become amplified, and beneficial elements circulate in oceans and plants.
The first people to study desert areas considered them empty – an example of rightly dusty Eurocentrism. Deserts not only have a complex biosphere, but are sometimes also inhabited by diverse communities. Speaking of dust, that of the largest hot desert in the world – the Sahara – recently found its way, not only metaphorically, onto the front pages of European newspapers. Not just common dust: ‘Saharan dust’.
Everything begins in the Chad Basin, or, really, in its lowest part: the Bodélé Depression. It was here that the largest of Africa’s four paleolakes, Lake Mega-Chad, existed around 5000 years BCE. Its area is estimated to have been over one million square kilometres, more than the area of today’s Caspian Sea.
What does a mega-paleolake have to do with dust in the atmosphere? Quite a lot: while drying out to the size of the modern Lake Chad, it left behind limnic sediment (or, to put it simply, sediment created at the bottom of a lake) – and here we have the source of our dust. Dust, not sand, as some media would have it. Why? Because grains of sand, even in the Sahara, are too large to be carried across large distances – that fraction makes the shortest journeys. Saharan dust is different: it measures between 100 nanometres to 50 micrometres, and it can travel even between continents, frequently astonishingly far. The dust particles are incredibly small, but their stream is spectacular, to say the least – 180 million tonnes a year leave the Sahel and, carried by the wind, make their way to various areas of the world.
From the Amazon to Japan
The complex atmosphere dynamics in the Sahel region determine the distance and direction of the dust delivery. Every year, Africa usually sees several instances of south winds so strong and so prolonged that they send dust northwards, causing mud rains in Spain, staining the snow in the Alps (which accelerates its melting) or giving Polish sunrises and sunsets beautiful colours. Have you noticed that recently, apart from the strange residue that needed to be washed off your car (its origin aeolian, which means carried in by the wind), an incredibly interesting colour appeared in the sky during the day? The usual dark-blue shades were nowhere to be seen. Soft blue hues, with a lot of white light, dominated – a phenomenon caused by the diffusion of sunlight on big, roughly one-micrometre dust particles high up in the atmosphere.
But the northward direction of the dust is not the only one; it’s not even the dominant one. Depending on the intensity of the so-called North African dipole, dust clouds of varying sizes find their way over the Atlantic, to the Canary Islands (where the phenomenon is called calima, and apart from lowering the air quality it also brings sweltering weather), and on to the Amazon, the Caribbean and the US. What is the dipole? It’s simply a difference in atmospheric pressure between the tropical and subtropical regions of North Africa. The higher it is, the larger the load of dust in the western direction. The season for such weather systems usually lasts from June until September. The furthest occurrence of Saharan dust particles was observed in Japan; of course, on the way there it mixed (in an estimated 50/50 proportion) with material from other tropical deserts.
How do we know all this? The observation of atmospheric aerosols is not new. We study not only the particles’ chemical and isotopic composition, but also, thanks to satellite images, the directions of the dust’s diffusion, its density in the atmosphere and its influence on radiation balance (one excellent observatory studying this is on the roof of the physics department of the University of Warsaw). A very good model of the way the dust is transported in the atmosphere has also been created; apart from physicists, it is also used by balloon crews when predicting their flight itinerary.
From iron to caesium
Apart from lowering air quality and adding colours to sunsets, Saharan dust plays an important part in the circulation of elements in nature, providing marine ecosystems with essential iron. Iron is vital for photosynthesis, so it determines the amount of phytoplankton, the foundation of oceanic ecosystems, and as such, it influences the whole ecological pyramid in that ecosystem.
Another crucial element carried by the Saharan dust is phosphorus – essential for plant growth. Obviously, I’m not speaking about the Sahel, but the opposite coast of the Atlantic Ocean, where most of the soil in the Amazon basin is poor in phosphorus. Geochemical estimates indicate that the delivery of phosphorus in the Saharan dust is of undeniable importance to the whole region. Although the dust only delivers 13% of all the necessary phosphorus, this amount balances out whatever is leached and eroded away by rivers, so it is significant for preserving the region’s geochemical balance.
But there is even more to the Saharan dust. Its presence in the atmosphere influences the planet’s radiation balance: it determines how much radiation (and of what length) penetrates to the Earth’s surface. This, in turn, lowers the surface temperature in oceans, allowing for vertical mixing of water. From there, it’s just a hop and a skip to changes in local and regional oceanic circulation. The dense dust in the atmosphere is also connected with intense rainfall in the northern part of the tropical Atlantic and the shift of the intertropical convergence zone (in simplest terms, the place where trade winds meet) a few degrees to the north.
A cooler ocean surface means a lower transfer of energy to the atmosphere, less evaporation, and hence a smaller likelihood of an intense hurricane season. Unfortunately, this process also has a flipside: it brings intense drought to the Sudan-Sahel belt.
In February 2021, the Saharan dust also made its way to France, which we might consider ironic. Among the many elements carried in the so-called Saharan Air Layer (the dust is transported at a specific altitude) that reached this country was caesium-137: a radioactive element created during nuclear tests conducted by France in the 1960s in Algeria (which was, at the time, its colony). Of course, the observed radioactivity levels aren’t dangerous to humans, but isn’t this a meaningful warning? Don’t litter, or it will find its way back to you…
Translated from the Polish by Marta Dziurosz
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