Africa

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Ocean Currents

Ocean currents affect the climate of the Atlantic Coast from South Africa to southern Angola, and from southern Morocco to Mauritania. The former is affected by the Benguela Current, while the latter is influenced by the Canaries Current. These cold ocean currents result in somewhat cooler temperatures, very low rainfall, and the frequent occurrence of fog near the coast. Elsewhere, an area of surprisingly low rainfall along the coast near Accra, Ghana, has been attributed to localized upwelling of cold water.

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Clouds and Atmospheric Dust

Clouds help to reduce daily temperature fluctuations by reflecting solar radiation away during the day, and by slowing the loss of heat at night. The lack of cloud cover in Africa’s desert regions—and during the dry season in other climate regions—results in substantial day-to-night fluctuations of temperature.

The Sahara is estimated to generate 300 million metric tons of airborne dust each year, 60 percent of the worldwide total. During the dry season wide areas south of the Sahara are affected by the harmattan, dust-laden winds originating in the desert. Typical episodes last for three to five days, with a dusty haze obliterating the Sun, lowering temperatures, and sometimes reducing visibility to a kilometer or less. The frequency and intensity of the harmattan varies; regions near the desert margins are often affected for 20 to 30 days per year. Dust originating over the Sahara also affects North Africa, southern Europe, and the Arabian Peninsula. The hot, dust-laden winds that occur in North Africa between February and June are known collectively as sirocco, and locally by a variety of names (for example, khamsin in Egypt).

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Climate Change

The Sahel—the semidesert transition zone between the Sahara and the wetter tropical areas to the south—suffered from a severe drought from the late 1960s to the early 1980s. Scientists initially interpreted the drought as a new phenomenon in which reduced rainfall and inappropriate human use of the delicate environment were causing the desert to expand relentlessly. After greater research, however, many scientists now believe that the Sahelian drought is part of a long-standing cycle of change.

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The study of environmental change in the Sahel and in adjacent regions shows that the margins of the Sahara have shifted during the last 20,000 years. Some 18,000 years ago, glaciers covered much of northern Europe, and global climate zones shifted to the south. The boundary of the Sahara was far to the south of its present location, passing through southern Senegal and central Nigeria. Large areas of sand dunes in the Sahel demonstrate that the climate was formerly much drier than today. With the melting of the glaciers in Europe, the Sahara’s southern margins shifted to the north as the climate became warmer and moister. About 9,000 years ago the desert had shifted far north of its current margin. Evidence of this shift includes sedimentary deposits indicating that Lake Chad was much larger than today, and that other large lakes existed in now-dry Saharan basins. Many vivid rock paintings in the central Sahara show savanna animals and people herding livestock more than 3,000 years ago. Since then, the climate has become progressively drier, the desert margin has moved south, and most of the lakes and wildlife have disappeared.

Other climate changes in the Sahel have occurred over the past 500 years or so. These changes, especially in the amount of rainfall, have tended to be cyclical: a decade or two of poor rains, followed by moister conditions, followed in turn by drought. Evidence of these fluctuations comes from written and oral accounts of droughts and famines, archaeological evidence, and the analysis of fossils.

In addition, meteorological records show cycles of increasing and decreasing rainfall just since the early 20th century. For example, the Sahelian drought was preceded by two decades of far-above average rainfall, and was followed by several years of average rainfall. Records also suggest an overall downward trend in average rainfall since 1900. Scientists have suggested that the clearing of vegetation and global warming may account for this apparent trend. They argue that the removal of vegetation causes higher surface temperatures, increased evaporation, and reduced rainfall.

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Vegetation

African vegetation zones are closely linked to climatic zones, with the same zones occurring both north and south of the equator in broadly similar patterns. As with climatic zones, differences in the amount and seasonal distribution of precipitation constitute the most important influence on the development of vegetation. Moving across the continent into drier and drier climates, the typical sequence of vegetation is from tropical moist forest to moist savanna, dry savanna, semidesert, and finally desert.

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Tropical Moist Forest

Tropical moist forest occurs in humid tropical areas, usually with 1,500 mm (60 in) or more of precipitation and a dry season (or seasons) of three to four months or less. West of the highlands of East Africa and centered in the basin of the Congo River is a great tropical moist forest extending some 600 km (400 mi) north of the equator and a similar distance south of the equator. On the west, the forest extends to the Atlantic coast in the Congo, Gabon, and Cameroon, and stretches in an interrupted belt along the West African coast to Sierra Leone. Tropical moist forest also occurs along the eastern side of Madagascar.

Scientists recognize two major subtypes of tropical forest: tropical rain forest and tropical wetland forest. Tropical rain forests are characterized by a dense mass of evergreens, oil palms, and numerous species of tropical hardwood trees divided vertically into strata, or layers. The upper canopy of treetops forms a dense cover over the middle layer of treetops and the surface layer of shrubs, ferns, and mosses below. Rising above the canopy are scattered tall trees, known as emergents. In dense forest environments, the shrub layer tends to be quite sparse, except along streams, because the canopy limits the amount of light that penetrates to the forest floor. Tropical rain forests are extremely diverse in species; pure stands of a single tree species are rare.

Tropical wetland forests include both freshwater and saltwater subtypes. Freshwater swamp forests cover large parts of the Middle Congo River Basin. Saltwater swamp forests occur in many areas between Senegal and Angola on the Atlantic coast and between South Africa and the Red Sea on the coast of East Africa. Mangrove species, with their characteristic tall, arched roots, are highly adapted to the fluctuating water levels and brackish water found in estuaries and other tidal environments. Mangrove forests are tangles of roots, tree trunks, and branches reaching 8 to 23 m (25 to 75 ft) high. Significant areas of mangrove forest have been lost in order to clear land for rice cultivation, particularly in West Africa.

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