What was done
The authors used the ratio of net primary production to rainfall, or rain-use efficiency (RUE), to map the occurrence and severity of desertification in the Sahel from 1982 to 1990.  RUE was used for this purpose because it relates plant productivity to rainfall, which is a primary factor controlling plant growth in this arid region.  Plant productivity was assessed by mapping vegetative cover using satellite images, while rainfall was measured directly at various meteorological stations within the study area.

What was learned
There was a strong relationship between plant productivity and rainfall in the African Sahel during the period examined. Generally, plant productivity increased with rainfall and decreased with drought.  However, productivity recovered rapidly with rainfall, even after long periods of drought, indicating the resilience of the region's vegetation.

RUE did not decline during the 9-year investigation, indicating that widespread desertification had not occurred.  In fact, there was a small but steady increase in RUE for the Sahel as a whole, suggesting that plant productivity had risen during the time of the study.  The authors hypothesized that this increase in productivity may have resulted from fertilizer use, species compositional changes, and microclimatic effects.  One potential contributing factor not mentioned in the article was the aerial fertilization effect of the ongoing rise in the air's CO2 content.

What it means
Contrary to "the popular view of the Sahel, supported by many pronouncements of international agencies," the slight but steady increase in regional RUE from 1982 to 1990 indicates that no extensive degradation or desertification occurred within this area during this time period.  Furthermore, the upward trend in RUE values is exactly what would be expected as a result of the aerial fertilization effect and enhanced water-use efficiency that is typically brought about by the increasing CO2 content of the atmosphere.  Thus, as the CO2 content of the air continues to rise, we may see even further increases in RUE within the arid African Sahel.