Water and Environmental Change in the Sahel, Africa

In my previous blogs, I talked about how environmental changes affect the Sahel's rainfall variability and future projections and uncertainties. Over the past decades, the Sahelian people have also made many efforts to combat the challenges due to environmental changes. My last blog will be introducing some of the mitigations and adaptations that the Sahel has been doing.  In order to fight drought and restore land fertility, in 2007, 11 countries in the Sahel - Senegal, Mauritania, Mali, Burkina Faso, Niger, Nigeria, Chad, Sudan, Djibouti, Eritrea, and Ethiopia - launched the Great Green Wall (GGW) initiative together. This project aims to build a 15 km wide and 8000 km long belt of plants stretching across the Sahel ( Schleeter, 2013 ). Significant progress has been made since its launch. By 2020, the total intervention area of the GGW initiative expanded to 156 million hectares ( UNCCD ). In the meantime, millions of jobs have also been created and contributed to economic d...

Given the extreme vulnerability of the Sahel to rainfall variability (discussed in the overview ), accurate future precipitation projection is crucial for the  Sahelian people to develop adaptation and mitigation strategies. As can be seen from the previous blogs, anthropogenic activities have large impacts on Sahel's rainfall variability. However, the uncertainties in their projection simulations are substantial. Results under the framework of phase 3 and 5 of the Coupled Model Intercomparison Project (CMIP 3-5) vary from model to model, with s ome models projecting an increase in precipitation by the end of the 21st century, while others projecting a decrease in 2100 over the Sahel ( Caminade & Terray, 2010 ; Monerie et al., 2017 ;  Yan et al. 2019 ).  Monerie et al. (2020a)  examine the reasons behind the inter-model difference of Sahel's rainfall projections using CMIP 5 and the newly developed CMIP 6. They classify the causes of change of precipitation into ...

Greenhouse gases (GHGs) emissions-induced warming not only influences Sahel's rainfall variability indirectly through  sea surface temperature, as introduced in the previous blog   but also affects Sahel's rainfall variability directly.  An increasing number of studies links the partial recovery of Sahel's rainfall and the intensification of extreme precipitation in the Sahel with GHGs-induced warming (such as  Dong & Sutton, 2015 ;  Donat et al., 2016 ;  Giannini & Kaplan, 2019 ).  As mentioned in the last blog , aerosol reduction in the Northern Hemisphere removes the cooling effect and shift the tropical rain belt northward. This impact is also strengthened by the GHGs-induced warming over the Northern Hemisphere ( Giannini & Kaplan, 2018 ) . The warming over the African continent also increases Sahel's precipitation. B ecause  GHGs can absorb the longwave radiation reflected by the Earth's surface to space, trap the heat in the atm...

The anthropogenic and volcanic aerosols emissions also play significant roles in Sahel's rainfall variability.  As  mentioned in the overview , the Intertropical Convergence Zone (ITCZ) location greatly controls Sahel's seasonal rainfall.  Hwang et al. (2013)  found that sulphate aerosol-induced cooling in the Northern Hemisphere during the late 20th century shifted the tropical rain belt southward  and contributed to the profound 1970s-1980s Sahel drought. As shown in Figure 1, results from different models all show that there was drying on the northern side of the tropical rain belt and wetting on the southern side in the late 20th century.  Figure 1. Time series of zonal mean precipitation anomaly relative to the 20th-century mean based on a) the Global Historical Climatology Network (GHCN) gridded products, b) the 20th-century reanalysis project (20CR), and c) the ensemble mean of the 20th-century climate simulation from 14 GCMs in CMIP3 and 12 GCMs in ...