March 9, 2007

Saharan Dust: Savior of the Amazon Rainforests?

Filed under: Climate History

One of the most common images linked to global warming is that of an arid, dusty, cracked soil complete with the skeleton of a dead cow resting on it. One would get the impression that a dusty plain is consequently one of the worst environments imaginable. Certainly one would not expect such a visually hostile environment to be absolutely vital in accomplishing something good for one of the globe’s more visually appealing biomes—such as the Amazon rainforest, right?

Not according to a recent study in a research journal entitled Environmental Research Letters, in which researchers from Israel, the United Kingdom, the United States, and Brazil report they have discovered that a single, very dusty, location in the Sahara Desert literally supplies half of the amount of the critical mineral dust needed by the Amazon rainforest to survive. The native soil of the Amazon rainforest, because of the continual leaching of the soils by heavy daily rainfalls, is extremely limited in the vital mineral nutrients needed by the abundant tropical vegetation of the rainforest to survive and flourish. So a fundamental research question can be asked: how does the Amazon rainforest survive without continual nutrient input? Well, scientists say that the rainforest actually gets that continual mineral input and from a most unusual source. Researchers now demonstrate that the Amazon rainforest owes most of its health to massive airborne dust transport from the Sahara desert!

Through analyses of satellite data, scientists from Israel, the United Kingdom, the United States, and Brazil have shown that an incredible 240 million tons of dust are blown out of the Africa Sahara into the Atlantic Ocean. Of that massive amount, an astounding 50 million tons actually reach—and fertilize—the Amazon rainforest! While such a figure (and the four thousand mile transport) is amazing in and of itself, the researchers writing in Environmental Research Letters add a surprising extra to this story: over half of that dust reaching the Amazon actually originates from a single tiny location in the Sahara, called the Bodélé depression. That small sandy valley is located on the southern border of the desert northeast of Lake Chad.

The researchers suggest that the Bodélé depression’s shallow topography—its shape and geology—is unique and therefore critical to its role in dust transport. The Bodélé valley is flanked on both sides by enormous magmatic mountain ridges (the Tibesti and Ennedi mountains), which create a cone-shaped “bottle neck” with a narrow opening in the northeast. Because of the world’s global circulations, winds are forced through this narrow opening and greatly accelerated in a phenomenon called a “jet wind.” As a result, surface wind gusts are accelerated and compressed into the narrow Bodélé valley (with consistent speeds of 24 miles per hour). There they pick up massive amounts of dust and blow that dust toward the Atlantic Ocean.

The team of researchers used satellite data to demonstrate that the dust from this single small valley supplies 56% of the total annual dust transport across the Atlantic—and consequently a massive amount of the mineral dust reaching the Amazon River basin. Data from the two satellite instruments (MODIS, the moderate-resolution imaging spectroradiometer, and MISR, the multi-angle imaging spectroradiometer) allowed the researchers to precisely track the airborne dust from the Bodélé valley to the Amazon rainforest. Most of the dust transport, according to the researchers, takes place in the stronger wind circulations associated with the Northern Hemisphere’s winter time (specifically between November and March).

Their research doesn’t exist in a vacuum. Atmospheric modeling has confirmed the researchers’ findings. A computer simulation identified that the small Saharan depression actually contributed more than forty percent of the dust over the Amazon in the winter. And, perhaps surprisingly to many, this transport of dust across the Atlantic from the Sahara has been noted for centuries. Charles Darwin, aboard the H.M.S. Beagle was one of the first to note (and later publish on) the aerial transport of dust from North Africa into the Atlantic Ocean. He described the atmospheric situation onboard the Beagle while in the Verde Islands in 1832:

“Generally the atmosphere is hazy; and this is caused by the falling of impalpably fine dust, which was found to slightly injured the astronomical instruments. The morning before we anchored at Porto Praya, I collected a little packet of this brown-coloured fine dust, which appeared to have been filtered from the wind by the gauze of the vane at the mast-head. Mr. Lyell has also given me four packets of dust which fell on a vessel a few hundred miles northward of these islands. Professor Ehrenberg finds that this dust consists in great part of infusoria with siliceous shields, and of the siliceous tissue of plants. In five little packets which I sent him, he has ascertained no less than sixty-seven different organic forms! The infusoria, with the exception of two marine species, are all inhabitants of fresh water. I have found no less than fifteen different accounts of dust having fallen on vessels when far out in the Atlantic. From the direction of the wind whenever it has fallen, and from its having always fallen during those months when the harmattan is known to raise clouds of dust high into the atmosphere, we may feel sure that it all comes from Africa.” (Darwin 1897, p. 4-5)

The intriguing new research on the importance of the Bodélé depression to the dust transport across the Atlantic builds on the observations begun by scientists as far back as Darwin. Given that the entire area of this African small dusty depression is less than one percent of the size of the entire Amazon River basin, it is truly astounding that the lush plant life of media-acclaimed extraordinarily sensitive biome of the Amazon rainforest literally owes its life to an African dusty plain.

Perhaps the next time you see an image of a “greenhouse-baked” parched desert, you might want to consider the interactions between all biomes on this Earth and that many lush tropical regions apparently owe their very existence to those much maligned dusty deserts.

References

Koren, I., Y.J. Kaufman, R. Washington, M.C. Todd, Y. Rudich, J.V. Martins and D. Rosenfeld, 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, doi: 10.1088/1748-9326/1/1014005.

Darwin, C., 1897. Journal of Researches into the Natural History and Geology of the countries visited during the Voyage of the H.M.S. Beagle round the world, under the command of Capt. Fitz Roy, R.N. Second edition. D. Appleton and Company, 519 pp.




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