Module 114 · Earth Systems Intelligence
The Dust That Feeds
182 million tons of Saharan dust cross the Atlantic every year.
27.7 million tons fall on the Amazon.
22,000 tons of phosphorus replace exactly what the rainforest loses to rain.
The dead life of an ancient African lake feeds the largest living forest on earth.
Simulation
An invisible river of dust, 5,000 kilometres long
Each particle represents roughly 1.5 million tons of airborne dust. Particles fade and descend as they reach the Amazon canopy.
The Sahara Desert and the Amazon rainforest seem to inhabit different planets. One is the largest hot desert on earth. The other is the densest living biomass. They are separated by 5,000 kilometres of open ocean. And yet the forest depends on the desert to survive.
The mechanism is phosphorus. Amazonian soils are ancient and heavily weathered. Nutrients are locked in the living plants, not the ground. When leaves decompose, their phosphorus is rapidly reabsorbed. But some washes away in the torrential rain, draining from the basin like a slow leak. Without replacement, the forest would starve. The replacement comes from 5,000 kilometres away, in the form of dust.
The source is the Bodele Depression in Chad: the dried bed of Mega-Lake Chad, a body of water that 7,000 years ago was larger than all the Great Lakes combined. When the Sahara dried, the lake vanished. Its sediment stayed: billions of fossilised diatoms, single-celled algae whose silica-rich cell walls are loaded with phosphorus, iron, and calcium. One hundred dust storms a year rake across this lakebed. In winter, the depression produces 700,000 tonnes of dust per day. The Tibesti and Ennedi mountains channel the wind into a natural jet at 47 kilometres per hour. The dust rises to 5,000 metres. The trade winds carry it west.
The Numbers
Measured by NASA CALIPSO satellite, 2007–2013
Geography
The trans-Atlantic corridor
Seven waypoints from source to destination. The world's largest dust transport event, measured in three dimensions for the first time.
Composition
What the dust carries
Variability
86% variation in seven years
Dust transport is anti-correlated with prior-year Sahel rainfall. More rain in the Sahel means more vegetation, less exposed soil, less dust. The pattern is volatile.
Tg/year leaving Africa at 15°W. Source: CALIPSO 2007–2013 (Yu et al., 2015)
Seasonality
The Bodele jet peaks October to March
Peak dust arrives in the Amazon during the wet season (February–April), when phosphorus depletion from flooding is highest.
Balance
The phosphorus budget
The balance is exact. 22,000 tons in, 22,000 tons out. This equilibrium has held for millennia. If Sahel rainfall patterns shift under climate change, the dust supply changes. The forest has no backup.
A lake that died so a forest could live
Seven thousand years ago, the Sahara was green. Savannas, grasslands, lakes teeming with diatoms and microorganisms. A shift in Earth's orbital parameters changed the monsoon pattern. The rains stopped. The lakes evaporated. The largest of them, Mega-Lake Chad, was bigger than all the Great Lakes combined. It dried to a fraction of its former size. The organisms died. Their silica shells piled up in the sediment. The phosphorus stayed.
The Bodele Depression, 500 kilometres long and 150 kilometres wide, is what remains. It sits 155 metres above sea level, flanked by the Tibesti Mountains (3,400m) to the northeast and the Ennedi Massif (1,000m) to the southeast. Wind funnels through the gap between them at speeds exceeding 47 km/h, scouring the ancient lakebed and launching the dust into the upper atmosphere. The resulting dust plume is the largest single atmospheric transport event on earth.
The dust carries more than phosphorus. Potassium, calcium, magnesium, and iron ride the same wind. Some particles act as condensation nuclei, seeding the clouds that produce the very rainfall that washes the phosphorus out of Amazonian soils. A dead lake in Africa makes clouds in South America that cause the rain that creates the deficiency that the same dead lake fixes.
What happens when the dust stops
Climate models suggest the balance is fragile. If Sahel rainfall increases under global warming, vegetation grows, soil stabilises, and less dust is released. The Amazon loses its phosphorus supply. Simultaneously, deforestation weakens the Hadley circulation, which paradoxically increases dust transport by 27% under a total deforestation scenario. One feedback loop heals the wound. The other tears it open.
The Amazon's dry season is already extending by 13 days per decade. If 16–17% of the forest reaches a tipping point, the transition from rainforest to savanna becomes irreversible. The dust cannot save a forest that is no longer there to catch it.
The Sahara is not barren.
It is a pharmacy, a seed bank, a fertiliser depot.
A lake that died seven thousand years ago
is still feeding the largest forest on earth.
This is the kind of thing we mean when we say: everything is connected.
Sources
Yu, H. et al. "The fertilizing role of African dust in the Amazon rainforest." Geophysical Research Letters 42 (2015): 1602–1607.
Yu, H. et al. "Quantification of trans-Atlantic dust transport from seven-year record of CALIPSO lidar measurements." Remote Sensing of Environment 159 (2015): 232–249.
Koren, I. et al. "The Bodele depression: a single spot in the Sahara that provides most of the mineral dust to the Amazon forest." Environmental Research Letters 1 (2006).
Prospero, J.M. et al. "Characterizing and Quantifying African Dust Transport and Deposition to South America." Global Biogeochemical Cycles 34 (2020).
Barkley, A.E. et al. "African biomass burning is a substantial source of phosphorus deposition to the Amazon." PNAS 116 (2019): 16216–16221.
Washington, R. et al. "Dust as a tipping element: The Bodele Depression, Chad." PNAS 106 (2009): 20564–20571.
Bristow, C.S. et al. "Fertilizing the Amazon and equatorial Atlantic with West African dust." Geophys. Res. Lett. 37 (2010).
NASA CALIPSO mission data (2007–2013). Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation.