What do four decades of earth observation tell us about land degradation in the Sahel?
While many studies suggest that the Sahel is greening, others indicate that there is increasing land degradation, largely attributed to two severe droughts experienced in the 1970s and 1980s. However, recent findings show improved greenness as a result of increased rainfall.
The West African Sahelian landscapes are dominated by large plains and with small temporal water ponds surrounded by evergreen and dense semi-deciduous vegetation. Conditions are generally arid and semi-arid. Livestock production and subsistence farming are the main economic activity. The landscapes are characterized by woodland and savannah vegetation and with (semi) deciduous trees with evergreen forests bordering perennial water bodies. A major part of this zone is under cultivation with the remainder used for grazing and extraction of commodities such as wood fuel and non-timber products for food, fiber and medicinal plants.
Land degradation is mainly attributed to changing climate, as well as human activity as a result of land use change. Scientists from the World Agroforestry Centre assessed various studies carried on land degradation in the West Africa Sahelian zone, specifically Senegal, Mali, Burkina Faso and Niger, over a period of four decades. The team used a method known as Earth Observation, to collect data using remote sensing techniques, to assess the land degradation dynamics in these countries.
The Greening Sahel phenomenon
Most studies indicated a positive trend on greening the Sahel after 1982. Some analyses however showed the contrary particularly in northern Burkina Faso and the Dogon region in Mali. The greening trend is not only caused by rainfall, but also attributed to flourishing growth of herbaceous vegetation, as opposed to tree cover.
Main research gaps
According to Cheikh Mbow, a senior scientist at the ICRAF’s climate change unit, there is need for more information on the nature of interactions among drivers of land degradation in the Sahel. The study identifies the following seven emerging research gaps:
- There is lack of well-documented, comparable, time series of key indicators for many ecosystem features that increase the knowledge of condition and trends on land degradation in the Sahel.
- After many decades of remote sensing application in the Sahel, capacities are still limited for a rigorous and consistent monitoring of land use and land cover change.
- Information on land degradation in drylands is still poor due to scarcity of data, and this limits the ability to assess consistent baseline of the state of land degradation and desertification.
- There are no satellite images available for periods between 1976-1983 and 1989-1998 when major droughts occurred.
- The different scales used by both remote sensing specialists and botanists on the ground deliver inconsistent messages on land degradation and recovery
- Not enough attempts for long-term field based survey of land dynamics have been made. Existing surveys are limited to Mali, Niger and Senegal.
- Local perceptions of land degradation and improvements often conflict with earth observation analysis, calling for the need for more interdisciplinary studies.
Suggestions for improvement
The ICRAF scientists suggest improvements to adequately assess quality and develop a consistent message on the magnitude of land degradation. These include the harmonization of time-series data, promotion of knowledge networks, enhancement of access to data, filling data gaps, agreement on scales and assumptions, set-up of a denser network of long-term field surveys, and the consideration of local perceptions and social dynamics.
The study reveals that multi-scale earth observation analyses do not show any clear trend in neither the process of desertification nor the greening patterns, as both are simplifications of very complex realities.
“We found that heterogeneity is an issue of scale, and very coarse-scaled vegetation trend analyses reveal a greening Sahel, while local-scale studies are not uniform, observing greening and degradation at the same time,” added Issa Ouedraogo, a scientist with ICRAF.
This study was carried out under the BIODEV project, a high-value carbon development project implemented in Burkina Faso, Mali and Sierra Leone.
BIODEV is funded by the Government of Finland and implemented under the leadership of the World Agroforestry Centre (ICRAF), in collaboration with the Center for International Forestry Research (CIFOR), the University of Eastern Finland and the University of Helsinki, and in partnership with national organizations and government ministries at national and local levels in the three countries in which BIODEV works.
Download the paper here
Mbow, C.; Brandt, M.; Ouedraogo, I.; de Leeuw, J.; Marshall, M. What Four Decades of Earth Observation Tell Us about Land Degradation in the Sahel? Remote Sens. 2015, 7, 4048-4067.