Human activities drive the ongoing intensification of the rainfall regime in the Sahel, and we better understand why

Human activities influence the climate in different ways : greenhouse gas (GHG) emissions, especially from the burning of fossil fuels, warm it, while sulphate aerosols (fine suspended particles) emitted by industry cool it. The direct and indirect effects of these external climate forcing factors interact with its internal variability (human activities or not, temperatures are, on average, always different from one year to the next, it never rains the same amount, etc.).

Understanding the effects of human activities on climate, and in particular on precipitation at regional scales (e.g., the Mediterranean rim, the Sahel) is both a scientific challenge and a social issue. The first is because at the scale of a region, precipitation signals are largely influenced by internal climate variability, and because climate models have difficulty in representing them. The second is because a good understanding of these effects over the past period is essential to anticipate the future climatic evolution (bearing in mind that GHG emissions are likely to continue). It is also necessary to focus on specific climate indicators in order to provide useful information for decision making, and thus support the implementation of climate change adaptation strategies. For example, a flood risk manager in a city or small watershed will need indicators of rainfall extremes at time steps typically ranging from hourly to daily.

In this paper, we analyze in an original way idealized climate simulations, allowing to separate the effects of the different external climate forcings and to take into account its internal variability. We are interested in a wide range of spatial and temporal scales of rainfall, over a highly hydrologically vulnerable region, the Sahel. This has led to a better understanding of the evolution of the rainfall regime over the last 70 years, marked by alternating wet periods (50s-60s), dry periods (70s-80s) and intensified rainfall (since 1990). The latter is characterized by :

• a slight recovery in annual accumulations compared to the dry period
• a larger variability at the inter-annual timescale (wet year followed by dry year) and the intra-seasonal timescale (dry sequence followed by a strong rainfall episode) than in the past
• an intensification of extreme rainfalls.

While the oscillation between wet and dry decades is largely the result of a mode of internal ocean variability (especially in the Atlantic), which has led to a sharp reduction in the average number of rainfall events per year, the recent period seems to be strongly influenced by human activities, at various levels : the decrease in aerosol emissions (or their precursors) following the introduction of regulations since the end of the 1970s (particularly in the USA and Western Europe) has led to an increase in the intensity of rainy days and their average number per year, which is relatively homogeneous over the Sahel (see Figures a & b). On the other hand, GHGs are at the origin of a well-marked dipole between the western Sahel (Senegal, Mauritania) and the eastern Sahel (Burkina Faso, Niger, northern Nigeria). More precisely, the increase in GHG concentration leads to a strong reduction in the occurrence of rainy days over the western Sahel and an increase in their intensity over the eastern Sahel (see Figures c & d).

In addition to providing an in-depth understanding of the "Sahelian dipole", which is present in the future projections provided by successive generations of climate models, these results have very concrete implications for a wide range of actors : agricultural producers, water resource managers, designers of hydraulic structures, etc. Mitigation efforts are also highly important, because in a region where water is a naturally uncertain resource, serious disruptions (social, economic, political) are to be feared if the future hydro-climatic situation becomes too severe compared to today.

Reference : G. Chagnaud, G. Panthou, T. Vischel, and T. Lebel. Capturing and Attributing the Rainfall Regime Intensification in the West African Sahel with CMIP6 Models Journal of Climate (2023).
https://doi.org/10.1175/JCLI-D-22-0412.1

Scientific contact : Guillaume Chagnaud, IGE

Author : G. Chagnaud - 08/03/2023