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Rural societies in the face of climatic and environmental changes in West Africa

Benjamin Sultan
Richard Lalou
Mouftaou Amadou Sanni
et al.

Part I. Recent and future climate change in West Africa. Obvious features, uncertainties and perceptions


Théo Vischel

Texte intégral

1The last report of the Intergovernmental Panel on Climate Change (IPCC, 2014) confirms with ever-increasing certitude the global warming caused by the increased amounts of greenhouse gases (GHG) and the probable consequences for the environment and societies. In particular, it warns the international community once again of the trend towards a more extreme climate, accompanied by more frequent heat waves and extrem precipitation events. However, strong uncertainty still remains about the form of these changes in the various parts of the world. This limits scope for the reliable addressing of the question of impacts, the vulnerability and adaptation of populations and of the strategies to be set up to face them.

2Study of the regional climate in West Africa thus forms part of the major international issues at the hinge point between understanding the evolution of the climate on our planet and the societal challenges for populations and decision makers in order to face up to the global changes. On the one hand, the climate in the region is governed by a monsoon system which, like the Indian and South American monsoons, plays a key role in the redistribution of water and energy in the tropics and thus contributes to global climate dynamics. On the other West African societies are well known for their vulnerability to climate variability as economic activity is based mainly on rainfed farming that is rendered fragile by management and climate risk mitigation policies that are still in their early stages and little coordinated. The major drought that hit the region at the end of the 1960s and lasted for nearly 30 years is a dramatic illustration of the impact that a substantial change in climate can have on populations. Apart from its link with global warming, that is a question that has not yet been elucidated, this major climate event indicated the need to study the climate-environment-society system as a whole.

3The research conducted in the Escape project—and especially the work described in the first part—is based on an integrated approach of this kind. Here, we review the state of the climate in West Africa and its past and future evolutions in the light of climate observations and simulations, and also what is felt by the populations confronted with climatic uncertainties.

4The project started with four major questions for the scientists involved, on which focuses the five chapters of this first part of the book. What mark does global warming make on the evolution of temperatures in West Africa? Since the great drought, what do we really know about the evolution of precipitations in the region and especially at the scales of impacts on hydrology and agriculture? Is it possible to use global GHG emission scenarios to plot the main lines of the evolution of the climate in the region at the horizon of the next century? Finally, how do the populations subjected to the variability of the climate perceive recent changes?

5To answer these questions, researchers at the Escape project, with backgrounds in physical or human sciences, integrated the complexity and contrasts of the climate-society system in West Africa. First, because the monsoon is the result of numerous mechanisms (oceanic, atmospheric and continental) that interact at several scales of space and time. Climatology is thus marked by a strong seasonal factor and great temporal variability at the decadal, inter-annual and intra-seasonal scales and by the spatial variability from regional to local scales. Secondly, because this variability is accompanied by the sociological, economic and cultural diversity of the populations that will now live, feel and relate differently with regard to the effects of the climate and its changes.

6Temperature movement since 1950 is examined in Chapter 1. This is a central variable for the study of water and energy balances, it has an impact on agricultural yields and can be a very direct threat for public health. However, temperature climatology is very little documented in West Africa. It is shown in this chapter that warming is already visible in the region but is not homogeneous. It is stronger in the Sahel than in the Sudanian and Guinean regions. The hottest periods of the year are those most affected and especially spring, when constantly rising temperatures are 2°C higher than they were 60 years ago. This amplification of the annual temperature cycle results mainly in an increase in nocturnal temperatures, with daytime temperatures remaining stable, without it being possible to explain the reasons yet.

7The evolution of precipitations in the Sahel since the 1950s is reviewed in Chapter 2. In particular information is provided about the last two decades against the background of discussion of the presumed recovery of rainfall in the Sahel. In fact, analyses conducted using in situ rainfall data reveal regional disparities in the evolution of annual cumulated rainfall. A return of precipitation is indeed visible in the central Sahel (although this has not returned to the conditions of pre-drought years), but seems to be slow in the western Sahel where the drought continued until the end of the 2000s. Rainfall events are examined at a finer scale, showing that the recovery of rain in the central Sahel features a continued deficit in the frequency rainfall events close to that of the drought years. This is compensated by the more common occurrence of intense rainfall, with levels not seen since the 1950s. The intensification of precipitations thus better characterises the recent evolution of rainfall in the Sahel rather than a return to wet conditions.

8With rising temperatures and a more intensive hydrological cycle, the Sahel has displayed for the last two decades all the signs expected of warming at a global level. To see whether this singular evolution may be the presage of change in the longer term, Chapter 3 consists of analysis for West Africa of simulations using general circulation models (GCMs) in the CMIP5 project, on which the conclusions of the 5th IPCC report (IPCC, 2014) are based. Some progresses of the GCMs made since the last similar experiment, CMIP3, are highlighted in the chapter, and in particular their capacity for finding the spatial structure of precipitation events and average temperatures. However, intra-seasonal variability is still poorly simulated by many models. In response to the increase in GHG concentration, the models all converge towards a rise in temperatures in the region over the coming century, although there is no consensus with regard to the scale of warming. Rainfall projections seem to agree on the difference in behaviour between the eastern Sahel, which should become wetter, and the western Sahel, which should become drier. However, the very strong disparity between simulations means that a reliable quantitative analysis of these trends is not possible.

9Beyond the uncertainties that still remain with regard to the climate in the future, the ability of populations to perceive recent changes is very instructive for seeing how adaptation approaches can be established. This question is addressed in Chapter 4 with the evaluation in three West African countries with contrasted climates (Niger, Benin and Senegal) of how the population perceives climate changes in comparison with meteorological observations. In spite of strong local climate variability and the many factors that can interfere between the reality of measurement and cognitive experience, the surveys conducted show that perception by the population is a good reflection of recent measured climate changes. Perception is all the more accurate when changes are short and strongly marked, as is the case of the changes in the rainfall regime observed in the Sahel. Perception is less close to meteorological observations when changes—such as an increase in temperature—take place slowly or do not have an impact on the way of life of the persons concerned. It is also noted that access to meteorological information improves the ability of the population to detect changes.

10Finally, it is shown in Chapter 5 that this perception of climatic and environmental changes is extraordinarily rich and varied in the populations surveyed, as are the representations that explain these changes and the many practices aimed at making a response. Beyond this diversity, the persons met at the main locations in Niger, Senegal and Benin have a very clear perception of both the changes in rainfall, temperature and winds and also of environmental changes: new features appear such as dust and certain plant species and known features disappear, such as ponds, part of the fauna and shrub and grass species.

11The results described in the first part of the book are certainly of great value for the scientific community—both in the replies that they bring and the new questions raised. They show above all that the challenges raised by the complexity of regional climates, especially in West Africa, can only be revealed on the basis of long-term observation of the climate-society system. Continuing the meteorological and sociological observations is crucial for detecting changes, understanding their mechanisms, improving modelling and finally providing populations and decision makers with reliable climate information that is beneficial for their adaptation and development strategies.

© IRD Éditions, 2017

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