Commonly viewed as a household nuisance, dust as we know it hardly seems worthy of serious study. Humble image notwithstanding, airborne dust has a dramatic global impact, affecting everything from the weather to marine food chains.
An additional, surprising role for dust was identified in the 1990s: As torrential rains in the Amazon region continuously wash minerals out of the soil, they are replaced by new minerals carried in dust blown over 5,000 km across the Atlantic Ocean from the largest desert in the world – the Sahara. Scientists believe that without a steady supply of vital minerals, the Amazon region would become a wet, but largely lifeless, desert.
In winter, seasonal winds lift dust into the air in the Sahel, the southern part of the Sahara, and carry it to the rainforest in South America. How much dust is expelled from the Sahara and how much of it reaches the Amazon rainforest? What turns particular desert regions into good sources of dust? These questions lie at the basis of research led by Dr. Ilan Koren of the Weizmann Institute’s Environmental Sciences and Energy Research Department. In a study conducted with the late Dr. Yoram Kaufman of NASA and other colleagues from Israel, the United Kingdom, the United States and Brazil, and published in Environmental Research Letters, the scientists focused on a particular desert region considered the largest source of dust in the world – the Bodele Valley, covering a 20,000-sq-km area in northern Chad.
Koren’s goal was to quantify, for the first time, the Bodele Valley’s contribution to the Amazon rainforest. An additional goal was to try to explain what turns this small valley into a leading “exporter” of dust. He and his colleagues combined the data collected by two different types of satellite sensors: One made it possible to cover a wide area and evaluate the extent of dust clouds and their movement; the other supplied precise optical information about the dust’s properties. In addition, satellite photographs taken at regular intervals allowed the scientists to evaluate the speed and direction of the winds and calculate the size of the dust “shipments.” Additional measurements at two spots above the Atlantic helped evaluate the amount of dust that is “lost” on the way to South America.
Analyses of the findings produced unexpected results: The Bodele Valley, which accounts for about 0.2% of the Sahara’s area, is responsible for 56% of the dust reaching the Amazon rainforest. Moreover, the total amount of dust arriving in South America from the Sahara each year is about 50 million tons – a much higher figure than the previously estimated 13 million tons and one that matches the amount thought to be needed to sustain the rainforest.
Why does the Bodele Valley supply such a significant amount of Amazon dust? “I looked at the satellite photos, and the answer was staring me in the eye,” says Koren, referring to Bodele’s unique geological shape. It is flanked on both sides by enormous basalt mountain ridges, with a narrow opening in the northeast. Winds that “drain” into the valley focus on this funnel-like opening, creating a large wind tunnel that directs the surface winds toward the dust source and accelerates them.
Though dust may not be a profitable export item, understanding its long-distance movement is a matter of global importance.
Dr. Ilan Koren’s research is supported by the Sussman Family Center for the Study of Environmental Sciences; and the Samuel M. Soref and Helene K. Soref Foundation.