The Greening Of The Sahel
By Paul Homewood
A new study confirms previous findings that the Sahel is regreening.
On regreening and degradation in Sahelian watersheds
Over many decades our understanding of the impacts of intermittent drought in water-limited environments like the West African Sahel has been influenced by a narrative of overgrazing and human-induced desertification. The desertification narrative has persisted in both scientific and popular conception, such that recent regional-scale recovery (“regreening”) and local success stories (community-led conservation efforts) in the Sahel, following the severe droughts of the 1970s–1980s, are sometimes ignored. Here we report a study of watershed-scale vegetation dynamics in 260 watersheds, sampled in four regions of Senegal, Mali, and Niger from 1983–2012, using satellite-derived vegetation indices as a proxy for net primary production. In response to earlier controversy, we first examine the shape of the rainfall–net primary production relationship and how it impacts conclusions regarding greening or degradation. We conclude that the choice of functional relationship has little quantitative impact on our ability to infer greening or degradation trends. We then present an approach to analyze changes in long-term (decade-scale) average rain-use efficiency (an indicator of slowly responding vegetation structural changes) relative to changes in interannual-scale rainfall sensitivity (an indicator of landscape ability to respond rapidly to rainfall variability) to infer trends in greening/degradation of the watersheds in our sample regions. The predominance of increasing rain-use efficiency in our data supports earlier reports of a “greening” trend across the Sahel. However, there are strong regional differences in the extent and direction of change, and in the apparent role of changing woody and herbaceous components in driving those temporal trends.
Why it should come as any surprises baffles me. Back in the 1970’s HH Lamb and other climatologists recognised that global cooling, and in particular a colder Arctic, shifted anticyclone belts towards the equator, with the result that rainfall increased in the equatorial zone, but decreased in areas further north such as the Sahel, not to mention vast tracts of Asia including India.
Going further back, we find a similar pattern in the Little Ice Age, when, for instance, Lake Malawi was 120m lower than before or after.
Across the Equator in Ghana, studies have found that “the most recent mega-drought [in the Sahel] was just 500 years ago, spanning 1400 to 1750 and coinciding with Europe’s Little Ice Age. At the time, Lake Bosumtwi, Ghana, dropped so low for so long that a forest sprouted on the crater’s edges. Those trees now stand in 15 to 20 metres of water .
The submerged trees on Lake Bosumtwi, Ghana, stand in 15 to 20 metres of water