Does Rainfall Increase in the Sahel Mask a Degradation Trend?

There have been heated debates about the dynamics of land degradation and climate change in the Sahel region in West Africa. The region has suffered a number of extreme droughts since the 1960’s causing famine, loss of livestock and reduced vegetation. However, a ‘greening trend’ trend has recently been detected. To a large extent this trend appears to be driven by increased rainfall (although some scientists argue that this alone can not explain the full extent of the greening trend).

Several studies, based on remote sensing, have now analyzed the reduced vegetation during the drought years and compared it to current land cover. Interestingly, they have not detected any land degradation that can be attributed to land management, which is in contrast with earlier studies suggesting that livestock management in the region is reducing productivity and increasing the systems vulnerability to drought.

A recent paper ‘Desertification in the Sahel: a reinterpretation’ by Hein and De Ridder published in Global Change Biology, suggests that the analyses based on remote sensing may be flawed and that land degradation may have been masked by rainfall.

Hein and De Ridder’s reasoning builds on the way that previous studies linked net primary production (NPP) (or actually a vegetation index – NDVI) to rainfall. These previous studies assumed that for a given site with no land degradation a linear relationship exists between NPP and rainfall (i.e. the Rain Use Efficiency (RUE) is constant). When they did not see any change in RUE over time they assumed that there has not been any land degradation.

Hein and De Ridder studied RUE in six field sites and found that in the absence of land degradation the relationship between NPP and rainfall was non-linear (followed a quadratic curve). When they looked at expected RUE values based on their quadratic estimates they found that the RUE from satellite estimates were lower than the expected ones, and thus land degradation may have occurred. They conclude:

If anthropogenic degradation of the Sahel is demonstrated, this would have repercussions for the debate on the causes of climate change in the Sahel. Currently, a weakness in the argumentations … that anthropogenic land cover changes have contributed to the occurrence of the extreme Sahelian droughts of the last decades of the 20th century is a lack of evidence of degradation from remote sensing data. Hence, if new remote sensing analyses confirm anthropogenic degradation, this would support the hypothesis that degradation of the vegetation layer, in particular through sustained high grazing pressures, has contributed to the occurrence of the 20th century droughts in the Sahel. Furthermore, if degradation of the Sahelian vegetation cover is confirmed, this would indicate that Sahelian pastoralists may be more vulnerable for future droughts than currently assumed. Because degradation of the Sahel in the 1980s and 1990s has been masked by an upward trend in annual rainfall, the consequences of a future drought for the local population could be unexpectedly severe.

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