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Winter Dust Activity off the Coast of West Africa
Reference
Evan, A.T., Heidinger, A.K. and Knippertz, P. 2006. Analysis of winter dust activity off the coast of West Africa using a new 24-year over-water advanced very high resolution radiometer satellite dust climatology. Journal of Geophysical Research 111: 10.1029/2005JD006336.

What was done
The authors applied a new daytime over-water dust detection algorithm for the Advanced Very High Resolution Radiometer (AVHRR) to 24 years (1982-2005) of wintertime satellite imagery over West Africa and the surrounding Atlantic Ocean, comparing it with a similarly-derived Normalized Difference Vegetation Index (NDVI) that is shown to be responsive to vegetation variability in the Sahel.

What was learned
A strong relationship was found to exist between tropical North Atlantic dustiness and the vegetation index, "suggesting the possibility that vegetation changes in the Sahel play an important role in variability of downwind dustiness." In addition, the data revealed a jagged decline in dustiness over the period of study, which for the tropical Atlantic region (0-15N, 10-30W) exhibiting "a statistically significant downward trend over the data set."

What it means
Evan et al. conclude that "dust mobilization may be mediated by vegetation through increases in soil stability and reductions of wind stress on the surface, when more vegetation is present," which "would be consistent with the modeling studies of Gillette (1999) and Engelstaedter et al. (2003)."

So what do increasing atmospheric CO2 concentrations have to do with these findings? First of all, the well-documented increase in plant water use efficiency that results from increases in atmospheric CO2 (see Water Use Efficiency (Grassland Species) in our Subject Index) should allow more plants to grow in the Sahel, which should help to stabilize the region's soil and decrease its susceptibility to wind erosion. Second, the propensity for elevated CO2 to increase soil moisture content as a consequence of CO2-induced reductions in plant transpiration (see Water Status of Soil (Field Studies) in our Subject Index) should do likewise. Third, the ability of extra CO2 to enhance the growth of cryptobiotic soil crusts (see Deserts (Algae and Lichens) in our Subject Index) should directly stabilize the surface of the soil, even in the absence of higher plants.

All of these CO2-induced phenomena benefit the source regions of windblown Sahelian dust; but they also benefit the far-flung regions over which the dust ultimately falls. For more on this topic, see Dust (Biological Implications) in our Subject Index, where a host of dust-induced maladies that affect humans, other terrestrial animals and plants, as well as coral reef organisms, are discussed.

References
Engelstaedter, S., Kohfield, K., Tegen, I. and Harrison, S. 2003. Controls of dust emissions by vegetation and topographic depressions: An evaluation using dust storm frequency data. Geophysical Research Letters 30: 10.1029/2002GL016471.

Gillette, D. 1999. A qualitative geophysical explanation for "hot spot" dust emitting source regions. Contributions to Atmospheric Physics 72: 67-77.

Reviewed 25 October 2006