Past and Future Rainfall in the Horn of Africa

Abstract:

While the interannual response of rainfall in the Greater Horn of Africa (GHA) region to internal climatic forcings is generally well understood, it is unclear how external climate forcing from rising greenhouse gases will affect hydroclimate in the region. 20th century observations in the GHA are limited in number, giving us only a hint at recent trends. Here, we present the first high-resolution record of temperature and hydroclimate from the GHA spanning the last 2000 years. These data, consisting of biomarker proxies measured in a high-accumulation rate marine sediment core from the Gulf of Aden, provide a detailed view of both 20th century and last millennium GHA climate. Measurements of TEX86 show that the recent warming in upper ocean temperatures is unprecedented in the context of the last 2000 years. Paired measurements of the hydrogen isotopic composition of leaf waxes show that the 20th century warming in the region is associated with a drying trend that is also unprecedented in the last 2000 years. These data suggest that rising greenhouse gases drive drying in the Horn of Africa region, but the majority of climate model simulations from the CMIP5 archive (RCP 8.5 scenario) imply that the region will get wetter. We address this discrepancy by investigating both the ability of climate models to simulate the seasonal cycle in rainfall in the GHA region and the mechanisms driving model response. If the models are incorrect, and the GHA will indeed get drier as a consequence of global warming, this has major implications for projections of food security in the region.