Changes in Precipitation Over West Africa During Recent Centuries
- Stefan NorrgårdStefan NorrgårdÅbo Akademi University
Water, not temperature, governs life in West Africa, and the region is both temporally and spatially greatly affected by rainfall variability. Recent rainfall anomalies, for example, have greatly reduced crop productivity in the Sahel area. Rainfall indices from recent centuries show that multidecadal droughts reoccur and, furthermore, that interannual rainfall variations are high in West Africa. Current knowledge of historical rainfall patterns is, however, fairly limited. A detailed rainfall chronology of West Africa is currently only available from the beginning of the 19th century. For the 18th century and earlier, the records are still sporadic, and an interannual rainfall chronology has so far only been obtained for parts of the Guinea Coast. Thus, there is a need to extend the rainfall record to fully understand past precipitation changes in West Africa.
The main challenge when investigating historical rainfall variability in West Africa is the scarcity of detailed and continuous data. Readily available meteorological data barely covers the last century, whereas in Europe and the United States for example, the data sometimes extend back two or more centuries. Data availability strongly correlates with the historical development of West Africa. The strong oral traditions that prevailed in the pre-literate societies meant that only some of the region’s history was recorded in writing before the arrival of the Europeans in the 16th century. From the 19th century onwards, there are, therefore, three types of documents available, and they are closely linked to the colonization of West Africa. These are: official records started by the colonial governments continuing to modern day; regular reporting stations started by the colonial powers; and finally, temporary nongovernmental observations of various kinds. For earlier periods, the researcher depends on noninstrumental observations found in letters, reports, or travel journals made by European slave traders, adventurers, and explorers. Spatially, these documents are confined to the coastal areas, as Europeans seldom ventured inland before the mid-1800s. Thus, the inland regions are generally poorly represented. Arabic chronicles from the Sahel provide the only source of information, but as historical documents, they include several spatiotemporal uncertainties. Climate researchers often complement historical data with proxy-data from nature’s own archives. However, the West African environment is restrictive. Reliable proxy-data, such as tree-rings, cannot be exploited effectively. Tropical trees have different growth patterns than trees in temperate regions and do not generate growth rings in the same manner. Sediment cores from Lake Bosumtwi in Ghana have provided, so far, the best centennial overview when it comes to understanding precipitation patterns during recent centuries. These reveal that there have been considerable changes in historical rainfall patterns—West Africa may have been even drier than it is today.