African climate response to orbital and glacial forcing in 140,000-y simulation with implications for early modern human environments

Significance

A computer model calculates the changing climate/vegetation from 140,000 y ago to the present for Africa, Arabia, and the Mediterranean Basin. The results illustrate how and when changes in Earth’s orbit, greenhouse gases, and ice sheets change the climate. The model makes this long calculation using the full set of dynamic/thermodynamic equations with sufficient spatial resolution to calculate monsoon and storm track rainfall over this region. The results explain when and where the climate was wetter or drier and how the vegetation changed. The simulated environmental changes agree with observed paleoenvironmental data in most areas. The results will help assess whether and how climate, hydrology, and vegetation changes may have influenced human dispersal out of Africa.

Abstract

A climate/vegetation model simulates episodic wetter and drier periods at the 21,000-y precession period in eastern North Africa, the Arabian Peninsula, and the Levant over the past 140,000 y. Large orbitally forced wet/dry extremes occur during interglacial time, ∼130 to 80 ka, and conditions between these two extremes prevail during glacial time, ∼70 to 15 ka. Orbital precession causes high seasonality in Northern Hemisphere (NH) insolation at ∼125, 105, and 83 ka, with stronger and northward extended summer monsoon rains in North Africa and the Arabian Peninsula and increased winter rains in the Mediterranean Basin. The combined effects of these two seasonally distinct rainfall regimes increase vegetation and narrow the width of the Saharan–Arabian desert and semidesert zones. During the opposite phase of the precession cycle (∼115, 95, and 73 ka), NH seasonality is low, and decreased summer insolation and increased winter insolation cause monsoon and storm track rains to decrease and the width of the desert zone to increase. During glacial time (∼70 to 15 ka), forcing from large ice sheets and lowered greenhouse gas concentrations combine to increase winter Mediterranean storm track precipitation; the southward retreat of the northern limit of summer monsoon rains is relatively small, thereby limiting the expansion of deserts. The lowered greenhouse gas concentrations cause the near-equatorial zone to cool and reduce convection, causing drier climate with reduced forest cover. At most locations and times, the simulations agree with environmental observations. These changing regional patterns of climate/vegetation could have influenced the dispersal of early humans through expansions and contractions of well-watered corridors.