Monday, July 28, 2008

A new view on desertification: the greening of the Sahel

Climate change and stories of droughts and the encroachment of deserts go hand in hand. One related issue that has always intrigued me is whether desertification is a one way street. Once a desert always a desert (due to the removal of top soil etc.).

This article from EarthPortal (inbox) sheds some new illuminating light on the topic that certainly informs a lunch time conversation I had recently.

Greening of the Sahel [Earth Portal]

The Sahel region in Africa, spanning the entire continent from the Atlantic Ocean to the Red Sea, receives, in the main, the World’s attention in cases of drought, famine or political crisis. The Sahel is a dynamic ecosystem that responds not only to climatic variability bu to human exploitation of biospheric resources. Over the long-term, changes in rainfall may have resulted in changes in land use patterns. While there has been a tendency to refer such changes as the desertification of the Sahel, results from analysis of different types of satellite- and ground-based data have not resulted in consensus on the direction of changes.

Since the early 1980s, global satellite mapping of the biosphere has generated long time-series measurements of vegetation that can be used as proxies for understating the dynamics of variability of the Sahel’s ecological system. A number of studies using these and other data have shown the close coupling among rainfall, land use and primary production in the Sahel.

Here is the traditional model (as I now understand it):

The first and the most well known mechanism was proposed in the mid 1970s by Otterman and Charney and often referred to the “Charney hypothesis”, by which albedo increases, due to overgrazing and cultivation, leads to a general cooling of the land surface and thereby reductions of evapotranspiration and of surface air convergence, leading to reductions of cloudiness and rainfall. This chain of effects was hypothesized to cause a positive feed back loop responsible for the persistent drought in the Sahel.


After several decades of declining rainfall and dwindling food production in the Sahel, reports telling a different story started to appear. Analyses made by several independent groups of temporal sequences of satellite data over two decades since early 1980s, showed a remarkable increasing trend in vegetation greenness.

The first results results appeared in 2003 and showed a strong increase in seasonal greenness that was observed over large areas of the Sahel during the period 1982-1999 (Figure 2). These results were then followed up by more recent studies where the time series have been extended to cover the period 1982-2003, verifying the previous results.

Examination of the time series of greenness principally reveals two major periods: (a) 1982-1993 marked by below average vegetation and persistence of drought with a notable large-scale drought during the 1983-1985 period; and (2) 1994-2003, marked by a trend towards “greener” conditions with region-wide above normal vegetation conditions starting in 1994. Spatial patterns enable us to conclude that there is not a single footprint of desertification, rather they indicated the variability of green vegetation biomass over the region in response to inter-annual variations in rainfall.

The conclusion - I still not clear and need to read the full article.



Chester said...

HapiBlogging to you my friend! Have a nice day!

Dano said...

One of my colleagues worked in the Peace Corps with his wife, running tree-planting programs. This program has helped the greening.