Chad → the Amazon · 23 January – 4 February 2025
A lake in Chad dried up, but its floor keeps leaving. Follow one shell of it for twelve days of real winter wind, and see how much of the story is still argued.
A shell
It is the house of a diatom, an alga that builds its wall out of silica. Samples of the Bodélé's sediment show hollow tubes of Aulacoseira, with rows of holes in the wall; in life the tubes link into chains.
In the species usually taken as the reference, one tube is between 4 and 17 micrometres across. The shells of this lake were not measured.
Lake Mega-Chad
From about 11,500 years ago the rains were strong and Lake Mega-Chad grew to 361,000 square kilometres, up to 160 metres deep. Shells sank through it for thousands of years and piled up as a pale sediment of hollow tubes, very light and easily broken.
5,000 years ago
About 5,000 years ago the rains weakened and the lake fell, abruptly. But the deepest part, the Bodélé basin, where most of the shells lie, stayed under water until about a thousand years ago.
Then that dried too, and the crust of shells lay open to the wind.
The Bodélé
At least four metres of that crust have been blown away in the past thousand years. What is left, a depression about 150 kilometres across, is called the world's greatest dust source.
Tibesti and Ennedi
The Harmattan, the dry winter wind, is squeezed between the Tibesti and Ennedi mountains into a fast current about 200 metres above the ground. When the wind at the surface passes about 10 metres a second, the floor lifts.
In winter that happens on four days in ten, and on those days more than 700,000 tonnes leave.
From here on, a stand-in
No one can follow a single grain. From here the shell stands for a parcel of air released over the Bodélé at midnight on 23 January 2025, about 1.5 kilometres up, and pushed along by that winter's real winds.
In one study of this route, dust reached the Amazon between 1.7 and 5 kilometres up.
Days 0 to 5
The gold on the globe is real: the dust the Copernicus atmosphere model put near the ground, every three hours, in cells 1.25° wide. The white line is the parcel.
It is out over the Atlantic, past 17°W, after 5.3 days and 3,900 kilometres. The median path in these months reaches the Atlantic in just under five days.
28 January
Where it leaves Africa, near 15°W, all of North Africa's dust adds up to about 182 million tonnes a year. Not all of it is from Chad: the Bodélé is one source of several.
At Dakar the model's dust stayed above 100 micrograms per cubic metre, as a daily mean, for eleven days in a row, from 25 January to 4 February: a long train, not one cloud.
31 January
Over the water the plume thins. Counting the whole band from 10°S to 30°N, the yearly flow is 132 million tonnes at 35°W and 43 at 75°W: dust falls out in rain and settles. Crossing takes about a week.
At the parcel's own position the model's ground-level dust goes from about 3,500 micrograms per cubic metre at the start to 2 at the end.
4 February
After twelve days and 8,800 kilometres the parcel is over the Amazon basin, about 300 kilometres from ATTO, a research tower in the forest north-east of Manaus. African dust is recorded there from December to April, most in February and March.
The model agrees that something arrived: at Manaus, dust peaked on 2 February at 13 micrograms per cubic metre, about 200 times less than over the Bodélé on the day of release. The parcel comes two days later. A single parcel in a coarse wind field is a sketch.
First light, in the forest
Most of what lands is not whole tubes. The dust that reaches ATTO peaks at 2 to 3 micrometres by volume.
But pieces of diatoms do arrive: in dust collected at Cayenne in 2015–16 they were 3.5% of the coarse particles by number, and 38% of those between 10 and 18 micrometres. The authors could not say where they came from.
Four studies
How much African dust reaches the Amazon? Four studies, four answers: 50, 40, 28 (range 8 to 48) and 8 to 10 million tonnes a year. They do not count exactly the same thing, and each draws the Amazon its own way. The later studies report lower numbers.
The forest
The dust carries phosphorus, which Amazon soil lacks. One estimate puts it at 23 grams per hectare a year (range 7 to 39), about what rivers and rain wash out.
But dust is no more than 13% of the phosphorus that falls from the air; smoke from African fires may supply up to half of what is carried over; a 2020 study found the Bodélé's long-range reach limited; and one lake-history paper calls the fertiliser story overstated, or true only of the last thousand years.
The Bodélé throws up dust every winter, and studies have followed it out over the Atlantic. Whether any of it matters to the forest, and how much, is still a live question, and the honest range is wide.
The gold on the globe is one real fortnight. The grain is a sketch of where the air could have taken it.
Every number on this page comes from one of these. The gold on the globe is drawn from the model's dust, smoothed between its cells; the shell and the landscapes are drawn, not photographed. The four estimates do not measure exactly the same thing: they differ in how they draw the Amazon and in what counts as arriving.