West Africa is a diverse climatic and ecosystem region. Dramatic change over the Sahel from wet conditions in the 1950s to much drier conditions in the 1970s and the 1980s, and then to partial recovery starting from the 1990s, represents one of the strongest interdecadal climate signals on the planet in the twentieth century. We apply an off-line biophysical/dynamic vegetation model (SSiB4/TRIFFID) over West Africa for this study. The offline SSiB4/TRIFFID is tested using the observed precipitation and reanalysis-based meteorological forcing from 1948 through 2006 with 1 degree horizontal resolution. The simulation results indicate that the model with relatively realistic forcing was able to produce reasonable vegetation plant functional type (PFT) and leaf area index (LAI) spatial distributions, generally consistent with the satellite products derived from satellites after the 1980s when the satellite data are available. The simulated LAI over the Sahel region exhibits strong interannual and decadal variability very well, consistent with the Sahel drought in the 1970s and the 1980s and partial recovery in the 1990s and the 2000s.

To further understand the cause of such decadal variability, a set of experiments has been design to investigate the relationship between the LAI and global warming and carbon increase. In one experiment, atmospheric carbon dioxide was increased as observed and in another one, CO2 was fixed at the 1948 level, i.e., 310 ppmv. The LAI difference between the elevated CO2 and fixed CO2 increases linearly, a clear fertilization effect. This increase was more related to the increase in shrubs and the decrease in C4 Plants. Most increases in LAI over Sahel occurred during the winter.

To understand how the warming trend affect the decadal variability. We also compared two experiments: one with observed temperature (with warming trend) to drive the model; another with the warming trend was removed. The simulation shows that after 1980, the LAI reduction due to the warming was evident, although it was not as strong as the carbon fertilization effects. High temperature created stress on vegetation over Sahel, especially over the Sahel transition zone. However, the fertilization effect dominates the global warming effect.