Our atmosphere is dirty. I’m not talking about greenhouse gases — those are a story for another day. I’m talking about tiny particles, just a few microns in size, known as aerosols. They’re so small that you can’t see them, but the evidence is everywhere: from the black residue when you blow your nose after spending the day in a busy city, to the hazy sky on a warm summer’s day.
The most abundant aerosol is mineral dust, tiny particles of sand and other surface materials, picked up by winds as they blow across the world’s desert regions. Dust. It might seem insignificant, benign. In fact, it could be responsible for the survival of the Amazon rainforest as we know it today.
The Sahara is the source of about half the mineral dust in the atmosphere, emitting on average over a hundred million tonnes per year. Some 50% of this dust comes from one single spot, in the southwest corner of Chad, called the Bodélé Depression. This depression — a topographic low in the surrounding landscape — is part of a vast dry lake bed. As recently as 6000 years ago, the whole area was submerged beneath the surface of Lake Mega Chad. Less a lake and more an inland sea, Lake Mega Chad was once larger than all of North America’s Great Lakes combined. As the world’s climate changed, the Sahara dried out and the receding waters left behind an abundant supply of fine sediment particles, just waiting to be entrained by the wind.
Many parts of the Sahara, and indeed other desert regions around the world, share a similar history to this. There are two things, however, that make the Bodélé Depression unique and explain why it produces such a vast amount of dust.
Firstly, the surface is covered in skeletons of microscopic creatures called “diatoms”, which reside in water bodies. Known as “diatomite”, this material is extremely light for its size, its density roughly half that of typical desert sand. It is therefore much more readily picked up by the wind than, relatively, much heavier sand particles.
Secondly, the Bodélé lies directly downwind of two Saharan mountain ranges — the Tibesti and the Ennedi. These mountains not only channel the prevailing northeasterly wind directly onto the exposed diatomite surface, but this channelling effect also accelerates the surface wind, ensuring that it is strong enough to entrain the surface material.
Dust emission from the Bodélé Depression can therefore be thought of as an intricate coincidence, a conspiracy of geology and climatology. And the coincidences don’t end there.
Because mineral dust particles are so small and light, once they are lifted from the desert surface they can drift in the wind for weeks at a time. Dust from the Bodélé usually finds its way into a river of air that blows towards the west, across the Sahara and out over the Atlantic. On this journey, the dust plays games with the atmosphere. For example, butts of dust will reflect and absorb radiation from the sun. One effect this can have is to reduce surface temperatures beneath the dust butt during the day, with drops in temperature of over 10°C not uncommon during extremely big dust outbreaks. Although such extreme effects are short-lived and localised, mineral dust actually has a cooling effect on global climate — albeit a very weak one in comparison to the warming caused by humans.
On the other side of the Atlantic Ocean lies the Amazon rainforest. Although a highly productive and lush ecosystem, the soils of the Amazon are naturally quite infertile, with vast amounts of nutrients being washed away by floodwaters each year. Without input from some other source, Amazon fertility would decrease, damaging regional ecosystems as well as a valuable carbon sink. Where do these nutrients come from? Amazingly, it turns out that at certain times of the year, the Amazon finds itself directly in the path of mineral dust from the Bodélé Depression, 3000 miles away! This is the mystery source of fertiliser. Compared to regular desert sand, diatomite is turbo-charged with iron and phosphorous, two essential plant nutrients. In an average year, 20 million tonnes of Saharan dust is dumped on the Amazon basin, quite literally washed out of the sky by rain. Around 20,000 tonnes of this is phosphorous — the same amount that is washed out of the Amazon basin by floodwaters each year.
So, dust. Seemingly doing little more than muddying our air, fertilisation of the Amazon by dust from the Bodélé Depression represents just one of the many complex and beautiful relationships that exist within the Earth system. However, this relationship is also extremely fragile. It would take just a minor shift in climate to disrupt this conveyor belt of nutrients from the Bodélé Depression to the Amazon, threatening the health of the rainforest and the important place it holds in the Earth system.