The Desert That Feeds a Rainforest
Every year the trade winds lift Saharan dust off Africa, carry it across the Atlantic, and drop part of it on the Amazon. The soils there are short of phosphorus, and much of the phosphorus they get comes in on this dust.
That is 27.7 million tons of Saharan dust settling on the Amazon basin each year: the 2007–2013 average NASA's CALIPSO satellite measured from space with lidar. It is about 15% of the 182 million tons that blow off the African coast.
Schematic map, not to survey scale · Dashed gates = where CALIPSO measured the plume · Plume shading is illustrative · Ochre = dust, green = Amazon basin
182 million tons leave Africa. 43 million make it past the Caribbean line.
CALIPSO measured the dust crossing three lines of longitude. Between the first and the last, three quarters of the plume falls out of the sky.
About one ton in seven lands on the rainforest.
Of the dust that leaves the coast, about a quarter reaches the Caribbean and about a seventh settles on the Amazon. Most of the rest rains into the Atlantic along the way.
* Derived here by subtraction (182 − 43 − 27.7). It is not a published figure.
A dry lakebed in Chad, 0.5% of the Amazon's size, may supply half its dust.
The Bodélé Depression is the dried bed of an ancient lake. Its fine, diatom-rich sediment is one of the dustiest surfaces on Earth.
Koren et al. (2006) estimated that about half of the Amazon's annual dust comes from the Bodélé (the Weizmann Institute's release said 56%). Later work in the NOAA repository argues that El Djouf, in Mauritania and Mali, is the main trans-Atlantic source. Both are marked on the map, and the split is still debated.
The dust brings back about the phosphorus the rain washes out.
Old, heavily leached Amazon soils lose phosphorus to rain and floods. Each year the dust brings about the same amount back.
NASA's estimate of the phosphorus in the dust deposited on the Amazon each year, which it describes as "about the same amount as that lost from rain and flooding." The phosphorus content comes from dust samples from the Bodélé and from ground stations on Barbados and in Miami. The "lost" bar shows NASA's wording, not a separate measurement.
The flow swings by a factor of six from year to year.
Over 2007–2013, deposition on the Amazon ranged from 8 to 48 Tg a year. After a wet year in the Sahel, the next year carried less dust.
change in dust transport between the highest year (2007) and the lowest (2011).
miles (≈2,600 km) of open Atlantic the dust crosses on the trade winds.
Why it matters
- Two continents, one system. The world's largest desert and its largest rainforest sit about 1,600 miles apart, and the trade winds connect them.
- Look at the gates. The plume carries 182 Mt at 15°W and 132 Mt at 35°W, but only 43 Mt at 75°W. Most of the dust ends up in the sea.
- Look at the dot in Chad. A small patch of dry lakebed may feed half of a basin of roughly 7 million km². Scientists still disagree about how big its share is.
- Rain in Africa affects the Amazon. A wetter Sahel means less dust the next year, so drought in one region changes how much fertilizer reaches the other.
Sources & notes
- Yu, H. et al. (2015). “The fertilizing role of African dust in the Amazon rainforest: A first multiyear assessment based on data from CALIPSO.” Geophysical Research Letters 42. CALIPSO lidar, 2007–2013: 7-year mean deposition into the Amazon Basin 28 (range 8–48) Tg/yr; deposition anti-correlated with prior-year Sahel rainfall.
- NASA Goddard / CALIPSO release (Feb 22, 2015), “NASA Satellite Reveals How Much Saharan Dust Feeds Amazon’s Plants”: 182 Mt/yr past 15°W; 132 Mt still airborne at 35°W; 27.7 Mt settle on the Amazon basin; 43 Mt travel past 75°W to the Caribbean; ≈22,000 t/yr of phosphorus, “about the same amount as that lost from rain and flooding”; 86% change between the highest (2007) and lowest (2011) years; ≈1,600 miles across the Atlantic. Phosphorus content estimated from dust samples from the Bodélé Depression and ground stations on Barbados and in Miami.
- Koren, I. et al. (2006). “The Bodélé depression: a single spot in the Sahara that provides most of the mineral dust to the Amazon forest.” Environmental Research Letters 1, 014005: about half of the annual dust supply to the Amazon comes from the Bodélé, a source ~0.5% the Amazon’s size. (The Weizmann Institute’s press release put the share at 56%.)
- Later work hosted in the NOAA Institutional Repository (doc. 25439) argues El Djouf (Mauritania/Mali) is the preferred source for trans-Atlantic dust rather than the Bodélé — the source split remains debated.
- * “Everything else” (≈111 Mt) is derived by this edition: 182 − 43 − 27.7. It is not a published figure; it includes dust that settles into the Atlantic and onto land outside the Amazon basin.
- The 27.7 Mt (NASA) and 28 Tg (paper abstract) are the same estimate at different rounding; 1 Tg = 1 million metric tons.
- The map is schematic (equirectangular, hand-simplified coastlines). Plume shapes and arrows are illustrative of the transport direction; only the gate values at 15°W, 35°W and 75°W are measured.