What was done
The authors modeled the terrestrial carbon balance of Africa over the past century (1901-2002) using a spatially-resolved process-based vegetation model (ORCHIDEE), which is forced by changing climate, human-induced changes in land use, and a parameterization of natural fires.

What was learned
Overall, Ciais et al. report that the African net terrestrial carbon (C) balance increased from a net CO2 source to the atmosphere of 0.14 Pg C per year in the 1980s to a net sink of 0.15 Pg C per year in the 1990s." In addition, they say that the land use flux due to deforestation was "a source of 0.13 Pg C per year," and that "this implies that climatic trends (mainly increasing precipitation) and CO2 increase (the fertilization effect), are causing a sink of 0.28 Pg C per year which offsets the land-use source."

In further discussing their findings, the five researchers indicate that "the trend of gross primary production is closely matching the trend in satellite observed NDVI," and they write that their simulated trend in gross primary production "is also consistent with an increased vegetation activity over [the] Sahel reported by Eklundh and Olsson (2003) and Olsson et al. (2005)," while at the continental-scale the gross primary production trend can be largely (70%) explained by the CO2 fertilization effect.

What it means
Primarily in response to the ongoing rise in the air's CO2 content, it would appear that the African continent is significantly "greening up," and that it has recently been doing so at a significantly enhanced rate.

References
Eklundh, L. and Olsson, L. 2003. Vegetation index trends for the African Sahel 1982-1999. Geophysical Research Letters 30: 10.1029/2002GL016772.

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.