Climate model study of the role of global climate in the late Pleistocene migration of anatomically modern humans out of Africa




Carto, Shannon Leigh

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According to the "Out-of-Africa 2" theory of human evolution all living humans today descend from a group of Anatomically Modern Homo sapiens or Anatomically Modem Humans that evolved in Africa 200,000 years ago and subsequently migrated out of Africa and spread into the rest of the World around 100,000 years. As a result an interest has developed in establishing a concrete theory of the factors that compelled and/or motivated our ancestors to venture out of their African origins at this time. Interestingly, the Earth's Last Glacial Cycle also dates from this period--stretching from 115,000 to 10,000 years ago. Current paleoclimate evidence suggests that the climatic repercussions of this glacial cycle in Africa resulted in a shift towards a drier and somewhat cooler climate state and the fragmentation of the formerly extensive forested African landscape. As a result, theories of early human migration have cited African climate change during the late Pleistocene as a determinant; however, the mechanisms responsible for the development of hyper-arid conditions in Africa at this time have remained unresolved. Although, past global climate change has been ascribed to changes in radiative forcing and changes in atmospheric carbon dioxide, recent paleoclimate studies have revealed that African climate is sensitive to changes in SSTs in the Atlantic, as it appears that subtropical Africa was more arid when North Atlantic sea surface temperatures (SSTs) were cold during glacial periods. The forcing mechanism believed to be responsible for the development of these cold SSTs are the so-called Heinrich Events that are documented as massive surges of icebergs (from high-latitude ice sheets) into the North Atlantic Ocean during high-latitude glaciations. These Heinrich events resulted in the release of large quantities of freshwater into the North Atlantic, which in turn led to a weakening on the global ocean thermohaline circulation and widespread cooling throughout the region. In particular, marine sediment records from the Nordic Sea document a widespread cooling and ice-rafting event that occurred around 105 kya, known as Heinrich event 9. In order to investigate the climate processes responsible for promoting cooler and drier conditions in Africa during the migration event of AMH (around 100 kya) I used the University of Victoria Earth System Climate Model (UVic ESCM) to conduct two climate model experiments that compared the global-scale response of climate at 105 kya, in particular the Atlantic Ocean and the African climate system, to: 1) orbitally-controlled solar radiation and atmospheric carbon dioxide forcing appropriate for 105 kya and 2) the combined effect of orbitally-controlled solar radiation and atmospheric carbon dioxide at 105 kya, and North Atlantic freshwater forcing. The ultimate goal of this study is to understand how low-latitude and high-latitude climate processes affect the African climate. Overall the comparative analysis of these two climate model states revealed that the complex interaction between orbitally-controlled solar radiation and atmospheric CO2 forcing at 105 kya produced a significant part of the cooling and drying in Africa at this time interval. However, the model also indicated that the climate perturbations, caused by the freshwater forcing, amplified the cooling and drying that was already taking place in Africa due to orbital and CO2 forcing. Guided by paleoclimate data, archaeological data and the results of this study, I consider it likely that the development of hyper-arid conditions in Africa around 100 kya served as the impetus for the migration event of AMH out of Africa, as these climate changes would have rendered Africa unsuitable for hominid occupation at this time. These climate model results also provide compelling evidence that high-latitude cold events, induced by Heinrich Events, are strongly covariant with African aridity, and thus provide support to previous assertions that North Atlantic climate changes can be effectively propagated throughout the globe to produce seemingly simultaneous climate change.



climate changes, Africa, history, prehistoric peoples