Changes in Precipitation over West Africa During Recent Centuries

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Internal Authors/Editors


Publication Details

List of Authors: Stefan Norrgård
Editors: Hans von Storch
Publication year: 2017
Publisher: Oxford University Press
Book title: Oxford Research Encyclopedia of Climate Science
Title of series: Regional and Local Climates


Abstract

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.


Keywords

Historical Climatology, Precipitation, West Africa

Last updated on 2019-26-08 at 05:51