Nigeria has a tropical climate with sharp regional variances depending on rainfall. Nigerian seasons are governed by the movement of the intertropical discontinuity, a zone where warm, moist air from the Atlantic converges with hot, dry, and often dust-laden air from the Sahara known locally as the harmattan. During the summer, the zone of intertropical discontinuity follows the sun northward. As a result, more and more of the country comes under the influence of moisture-laden tropical maritime air. As summer wanes, the zone shifts southward, bringing an end to the rainy season. Temperatures are high throughout the year, averaging from 25° to 28°C (77° to 82°F). In the higher elevations of the Jos Plateau, temperatures average 22°C (72°F). Northern Nigeria typically experiences greater temperature extremes than the south.
Rainfall varies widely over short distances and from year to year. Parts of the coast along the Niger Delta, where the rainy season is year-round, receive more than 4,000 mm (160 in) of rain each year. Most of the country’s middle belt, where the rainy season starts in April or May and runs through September or October, receives from 1,000 to 1,500 mm (40 to 60 in). Within this region, the Jos Plateau receives somewhat more rain, due to its higher elevation. In the dry savanna regions, rainfall is especially variable over distance and time. The region along Nigeria’s northeastern border receives less than 500 mm (20 in) of rain per year, and the rainy season lasts barely three months.
Vegetation also varies dramatically at both the national and local level in relation to climate, soil, elevation, and human impact on the environment. In the low-lying coastal region, mangroves line the brackish lagoons and creeks, while swamp forest grows where the water is fresh. Farther inland, this vegetation gives way to tropical forest, with its many species of tropical hardwoods, including mahogany, iroko, and obeche. However, only in a few reserves—protected from the chainsaw and the farmer—is the forest’s full botanic diversity intact. Elsewhere, forest is largely secondary growth, primarily of species like the oil palm that are preserved for their economic value. Forests now cover only about 15 percent (2000) of the country’s total land area.
Immediately north of the forest is the first wave of savanna: the Guinea, or moist, savanna, a region of tall grasses and trees. The southern margins of the Guinea savanna—which has been so altered by humans that it is also called the derived savanna—were created by repeated burning of forest until only open forest and grassland were left. The burnings decimated important fire-sensitive plant species and contributed to erosion by removing ground cover. Tropical forest is giving way to the Guinea savanna at such a rate that the only forests expected to survive the next generation are in reserves. Beyond the Guinea savanna lies the drier Sudan savanna, a region of shorter grasses and more scattered, drought-resistant trees such as the baobab, tamarind, and acacia. In Nigeria’s very dry northeastern corner, the semidesert Sahel savanna persists. Throughout these drier savannas, drought and overgrazing have led to desertification—the degradation of vegetation and soil resources.
