Bodélé Depression

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Dust storm in the Bodele Depression. This particular storm was blowing on the afternoon of 18 November 2004, when the Moderate Resolution Imaging Spectroradiometer (MODIS) flew over on NASA's Aqua satellite. The full-sized image has a resolution of 250 meters per pixel.

The Bodélé Depression (also Bodele), located at the southern edge of the Sahara Desert in north central Africa, is the lowest point in Chad,^[1] and one of the largest sources of airborne dust in the world.^[2] Once serving as part of the floor of a much larger Lake Chad,^[3] the Bodélé Depression now provides dust for many of Central Africa's dust storms, and was the source of the massive dust storms that swept over West Africa and the Cape Verde Islands in February 2004.^[3]^[4] As the wind sweeps between the Tibesti and the Ennedi Mountains in Northern Chad, it is channeled across the depression. The dry bowl that forms the depression is marked by a series of ephemeral lakes, which may be infrequently flooded with mud-laden water from the mountains. When the lakes dry, a layer of loose clay is left behind. The dirt is easily carried on the wind that blows down from the mountains.^[5]

In the mid-1960s, Lake Chad was about the size of Lake Erie. But persistent drought conditions coupled with increased demand for fresh water for irrigation have reduced Lake Chad to about 5 percent of its former size. As the waters receded, the silts and sediments resting on the lakebed were left to dry in the scorching African sun. The small grains of the silty sand are easily swept up by the strong wind gusts that occasionally blow over the region. Once heaved aloft, the Bodélé dust can be carried for hundreds or even thousands of kilometers.^[3] In winter, the depression produces an average of 700,000 tonnes of dust each day.^[6]

Research published in the 25 March 2004 edition of Geophysical Research Letters, which used images taken by the Moderate Resolution Imaging Spectroradiometer (MODIS), aboard NASA's Terra and Aqua satellites, indicated that storms move across the Bodélé Depression at about 47 km/hr (29 mi/hr)—two times faster than previously believed. The research also found that winds have to whip across the region at a minimum of 36 km/hr (22 mi/hr), to kick up a dust storm.^[5]^[7] The pattern of air flow is so common that the winds have scoured a straight path in the ground, marking its southwesterly flow.^[3]

The same researchers who in 2004 more accurately determined the speed of wind through the depression also published in 2006 work showing that more than half of the dust needed for fertilizing the Amazon Rainforest is provided by the Bodélé depression.^[8]^[9] The research also shows that, contrary to what was previously thought, most of the Saharan dust that reaches the east coast of the United States originates from a single source—the Bodele depression.^[10]

The largest town associated with the Bodele dust source is Faya-Largeau ( 17°55′00″N, 19°7′00″E), located just to the north of the depression.^[11]