The recent decline in Horn of Africa rainfall during the March–May “long rains” season has fomented drought and famine, threatening food security in an already vulnerable region. Some attribute this decline to anthropogenic forcing, whereas others maintain that it is a feature of internal climate variability. We show that the rate of drying in the Horn of Africa during the 20th century is unusual in the context of the last 2000 years, is synchronous with recent global and regional warming, and therefore may have an anthropogenic component.

Proxy indicators of relative moisture balance, in combination with long control simulations from coupled climate models, show that the Indian Ocean drives multidecadal hydroclimate variability by altering the local Walker circulation, whereas the influence of the Pacific Ocean is minimal on these timescales.

Instrumental observations suggest that Lake Tanganyika, the largest rift lake in East Africa, has become warmer, increasingly stratified and less productive over the past 90 years. These trends have been attributed to anthropogenic climate change.

The processes that control climate in the tropics are poorly understood. We applied compound-specific hydrogen isotopes (D) and the TEX86 (tetraether index of 86 carbon atoms) temperature proxy to sediment cores from Lake Tanganyika to independently reconstruct precipitation and temperature variations during the past 60,000 years.