Volume 9, Number 2: 11 January 2006
"The Sahel," in the words of Anyamba and Tucker (2005), "is a semi-arid region stretching approximately 5000 km across northern Africa from the Atlantic Ocean in the west to near the Red Sea in the east and extending roughly from 12°N to 18°N," which "forms an ecological transition between the Sahara desert to the north and the humid tropical savanna to the south (Le Houerou, 1980)." It was recently featured in a special issue of the Journal of Arid Environments entitled "The 'Greening' of the Sahel," which describes its recovery from what Hutchinson et al. (2005) describe as a run of "several devastating droughts and famines between the late 1960s and early 1990s."
Working with National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) data obtained from polar orbiting satellites, Anyamba and Tucker developed a Normalized Difference Vegetation Index (NDVI) history that stretches from 1981 to 2003. Comparing this history with the precipitation history of the Sahel developed by Nicholson (2005), they find that "the persistence and spatial coherence of drought conditions during the 1980s is well represented by the NDVI anomaly patterns and corresponds with the documented rainfall anomalies across the region during this time period." Thereafter, they also find that "the prevalence of greener than normal conditions during the 1990s to 2003 follows a similar increase in rainfall over the region during the last decade."
In another analysis of NDVI and rainfall data in the same issue of the Journal of Arid Environments, Olsson et al. (2005) also find "a consistent trend of increasing vegetation greenness in much of the region," which they describe as "remarkable," and they state that increasing rainfall over the last few years "is certainly one reason" for the greening phenomenon. However, they find that the increase in rainfall "does not fully explain" it. Why?
For one thing, the three Swedish scientists note that "only eight out of 40 rainfall observations showed a statistically significant (95%) increase of rainfall between 1982-1990 and 1991-1999." In addition, they report that "further analysis of this relationship does not indicate an overall relationship between rainfall increase and vegetation trend." So what else could be driving the increase in greenness?
Olsson et al. suggest that "another potential explanation could be improved land management, which has been shown to cause similar changes in vegetation response elsewhere (Runnstrom, 2003)." However, in more detailed analyses of Burkina Faso and Mali, where production of millet rose by 55% and 35%, respectively, since 1980, they could find "no clear relationship" between agricultural productivity and NDVI, which argues against the land management explanation.
A third speculation of Olsson et al. is that the greening of the Sahel could be caused by increasing rural-to-urban migration. In this scenario, widespread increases in vegetation occur as a result of "reduced area under cultivation," due to a shortage of rural laborers, and/or "increasing inputs on cropland," such as seeds, machinery and fertilizers made possible by an increase in money sent home to rural households by family members working in cities. However, Olsson et al. note that "more empirical research is needed to verify this [hypothesis]."
We also have speculated on the cause of Sahelian greening, suggesting that the aerial fertilization effect of the ongoing rise in the air's CO2 content, which greatly enhances vegetative productivity, and its anti-transpiration effect, which enhances plant water-use efficiency and enables plants to grow in areas that were once too dry to sustain them, may be playing prominent roles [see our reviews of Prince et al. (1998) and Nicholson et al. (1998)]. Be that as it may, whatever the reason for the greening of the Sahel over the past quarter-century, it is clear that in spite of what the world's climate alarmists claim were concomitant unprecedented increases in the "twin evils" of anthropogenic CO2 emissions and global warming, the Sahel experienced an increase in vegetative prowess that was truly, as Olsson et al. write, "remarkable."
Sherwood, Keith and Craig Idso
References
Anyamba, A. and Tucker, C.J. 2005. Analysis of Sahelian vegetation dynamics using NOAA-AVHRR NDVI data from 1981-2003. Journal of Arid Environments 63: 596-614.
Hutchinson, C.F., Herrmann, S.M., Maukonen, T. and Weber, J. 2005. Introduction: The "Greening" of the Sahel. Journal of Arid Environments 63: 535-537.
Le Houerou, H.N. 1980. The rangelands of the Sahel. Journal of Range Management 33: 41-46.
Nicholson, S. 2005. On the question of the 'recovery" of the rains in the West African Sahel. Journal of Arid Environments 63: 615-641.
Nicholson, S.E., Tucker, C.J. and Ba, M.B. 1998. Desertification, drought, and surface vegetation: An example from the West African Sahel. Bulletin of the American Meteorological Society 79: 815-829.
Olsson, L., Eklundh, L. and Ardo, J. 2005. A recent greening of the Sahel - trends, patterns and potential causes. Journal of Arid Environments 63: 556-566.
Prince, S.D., Brown De Colstoun, E. and Kravitz, L.L. 1998. Evidence from rain-use efficiencies does not indicate extensive Sahelian desertification. Global Change Biology 4: 359-374.
Runnstrom, M. 2003. Rangeland development of the Mu Us Sandy Land in semiarid China: an analysis using Landsat and NOAA remote sensing data. Land Degradation & Development Studies 14: 189-202.