African monsoons, an immediate climate response to orbital insolation

Abstract

The Croll–Milankovitch astronomical theory of climate1–3 has received strong support from the evidence of a linear climatic forcing by obliquity and precession, although nonlinearity had to be assumed for eccentricity4,5. Moreover, interglacials have appeared to be controlled by the orbital insolation6 although a phase shift of 6,000–5,000 yr is seen between an astronomical climate index and terrestrial climate indicators, dominated by the northern ice-sheet dynamics. Climate proxy data of low latitudes are less directly dependent on the ice sheets. Here it is shown that during the past 464,000 yr, African monsoons signalled by the East Mediterranean sapropels were heaviest always and only when a northern summer monsoon index, computed from the orbital variation of insolation, reached maximum values. This occurred during all the interglacials, but also twice during glacial periods. Thus tropical aridity is not the single climatic pattern of glacial periods. An immediate terrestrial climate response to orbital variations of insolation is established.