10 October 2016

Land Degradation Likely Worsened Sahel Droughts

Land degradation associated with population growth likely exacerbated the late 20th century weakening of African Monsoon precipitation.


The impact of land degradation on Sahel rainfall is estimated and compared with the impacts of decadal sea surface temperature (SST) variations. While SST warming over tropical oceans was the main driver of the late 20th century Sahel droughts, land use/land cover change likely exacerbated the droughts. According to the multi-model assessment, the observed land-use and land cover change could cause between a four and twenty-five percent reduction in climatological precipitation.


The significant impact of increased population and associated land-use change on Sahel precipitation has important implications for water and land management, such as combatting desertification over the region. As Sahel precipitation and the associated African easterly wave activity generate much of the tropical cyclones (TCs) over the Atlantic, assessing the impacts of land degradation over the Sahel is also important for understanding the variability of Atlantic TC activity.


The West African Monsoon Modeling and Evaluation Project Experiment (WAMME) is an international multi-model study aimed at improving our understanding of the roles of sea surface temperature (SST), land-use/land-cover change (LULCC), and aerosol forcings on the  climate of African Sahel at seasonal to decadal scales. In the second phase of the WAMMEII project, scientists, including US Department of Energy researchers at Pacific Northwest National Laboratory, designed the multi-model experiments to evaluate the relative contributions of SST variability and land surface degradation on the Sahel. In the sensitivity experiments, LULCC was prescribed based on the population growth and expansion of agriculture over the second half of the 20th century. While the models consistently demonstrated that SST forcing is a major contributor to the 20th century Sahel drought, the prescribed LULCC caused reductions in the surface net radiation and surface evaporation. This in turn led to rainfall reductions of 4 to 25 percent over the Sahel as the West African monsoon rainfall peak shifted southward. These changes produced a precipitation dipole pattern and equatorward shift of the African Easterly Jet, both consistent with the observed change patterns associated with the dry climate anomalies over this region.

Ruby Leung
Pacific Northwest National Laboratory (PNNL)
Xue, Y, FD Sales, WK Lau, A Boone, K Kim, CR Mechoso, G Wang, F Kucharski, K Schiro, M Hosaka, S Li, LM Druyan, IS Sanda, W Thiaw, N Zeng, RE Comer, Y Lim, S Mahanama, G Song, Y Gu, SM Hagos, M Chin, S Schubert, P Dirmeyer, L Leung, E Kalnay, A Kitoh, C Lu, NM Mahowald, and Z Zhang.  2016.  "West African monsoon decadal variability and surface-related forcings: second West African Monsoon Modeling and Evaluation Project Experiment (WAMME II)."  Climate Dynamics, doi:10.1007/s00382-016-3224-2.