Paleo-ENSO influence on African environments and early modern humans

Abstract

is manifested via the El Niño Southern Oscillation (ENSO) (15, 16; see SI Appendix for more details). ENSO originates from sea surface temperature (SST) anomalies in the equatorial Pacific Ocean, and these changes impact the atmospheric WC, which in turn alters the location and strength of tropical convection cells (Fig. 1) (17). Through this coupled ocean–atmosphere system, ENSO events are propagated around the globe by Kelvin and Rossby waves (16), eventually reaching the African continent (see SI Appendix for more details). Here, changes in ENSO state alter the east–west trending moisture gradient across Africa (18–20). This leads to opposing dry and humid conditions between eastern and western Africa so that, during La Niña, eastern Africa experiences drier conditions than western Africa and vice versa during El Niño events. For instance, during El Niño years, eastern Africa experiences positive precipitation anomalies of up to 60% (or 200 mm per year) relative to the yearly precipitation budget during non–El Niño years, while western Africa experiences a 20 to 40% precipitation reduction (21, 22). Besides these modern driving mechanisms, spatiotemporal precipitation changes in Africa on much longer time scales have also been attributed to changes in Atlantic meridional overturning circulation, global atmospheric CO2 concentrations (pCO2), and/or the waning and waxing of global ice sheets (23–26). Significance Our results identify the prime driver of climate variation in Africa’s low latitudes over the past 620 ky—the key time frame for the evolution of our species. Warming and cooling of the tropical Pacific Ocean paced by insolation changes modulated the tropical Walker circulation, driving opposing wet–dry states in eastern and western Africa. We show that the effects of glacial/interglacial cycles were not the predominant source of environmental change in most of the continent. Africa’s environmental patchwork driven by low-latitude climate processes should therefore be a critical component in conceptual models of human evolution and early demography over the past 620 ky. Author contributions: S.K.-B. designed research; S.K.-B. analyzed data; and S.K.-B., W.D.G., R.V., A.B., E.M.L.S., A.A., A.S.C., W.D., V.F., H.F.L., M.A.M., H.M.R., F.S., and M.H.T. wrote the paper. The authors declare no competing interest. This article is a PNAS Direct Submission. J.T.F. is a guest editor invited by the Editorial Board. Published under the PNAS license.