Impetus - Universität zu Köln

38IMPETUS Development of scenariosmodel. Recent studies show that regional climate models using a combination of greenhouse gasand land use scenarios can give reasonable simulations of future climate in Africa. Results forthe target year 2025 show a reduction of the annual precipitation averaged over Benin. There is,however, a regional difference with clearly reduced precipitations in the coastal areas and in theUpper Ouémé Valley, whereas in Central Benin slightly more humid conditions are expected.The trend towrd a drier climate is especially manifest in a reduction of intense rainfall events. Ingeneral the seasonality of rainfalls will weaken in the south and north of Benin, but strengthen inthe Middle Ouémé Valley. The onset of the rainy season is likely to be delayed in all parts of thecountry. At the coast the monsoon will begin up to 12 days later. Furthermore Benin will becharacterised by a significvant warming trend throughout the next two decades reaching morethan 2°C just within 20 years especially in the summer. The weakening of the hydrological cycleappears notably in the local recycling of precipitation and thus implies also a clear reduction ofevaporation in most parts of the country. We expect, however, no significant changes in themonsoon circulation.Uncertainties of modelled climate changes arise from the influence of mineral dust and biomassburning aerosols, which is not taken into account, and from the poor representation of the mostimportant rain-producing weather systems in West Africa, the organized meso-scale convectivesystems (MCS).Storyline for scenario Z: Persistence of recently observed trends (business as usual)For several decades the entire sub-Saharan West Africa has encountered an increasing number ofclearly below-normal amounts of annual rainfall. For Benin we also observe a trend toward reducedprecipitation amounts since the late 1960s. Since the mid- 1990s the water temperatures inthe tropical Atlantic have changed in a way that would favour higher precipitations throughoutthe entire tropical West Africa. However, rainfall amounts remain below the level they reachedduring the wet period in the middle of the 20 th century, despite the fact that in recent years a tendencytoward higher rainfall sums has been observed. The wet period then was associated with avery similar distribution of sea surface temperatures found in recent years. Model studies suggestthat the return to wetter conditions fails due to the degradation of the vegetation and the continuouswarming of the Indian Ocean. In this present scenario we assume that the two rainfallinhibitingfactors continue to exist. Hence we predict the persistence of below-normal precipationin all parts of Benin until 2025.In addition a progressive warming of the land surface was measured in the past. Under the assumptionof a continuous rise of global greenhouse gas concentrations and an increasingly uncontrolledland use change by a steadly growing population the warming trend will persist at arate of 0.2°C per decade.