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The Green Sahara

Green Sahara—a phenomenon that is known to have occurred periodically over the past several million years is perhaps one of the most enigmatic transitions in our climate system.

During such events, vegetation, streams, rivers and lakes and as a result, animal life in all forms encroach on the otherwise dry and desolate landscape of the Sahara Desert. Several lines of evidence, such as reconstruction of past vegetation and water bodies, cave paintings depicting lush landscapes and animals, and several archaeological findings supporting human habitation deep in the Sahara offer a vivid illustration of what this landscape would have looked like during the most recent of such events, 10,000–5,000 years ago (known as the mid-Holocene). Unraveling the mechanism(s) of such dramatic changes is directly helpful to our quest to understand our climate system's propensity for widespread and sudden transitions.

This most recent Green Sahara phase occurred during a transformative time for the human race during which Neolithic humans living in Africa and the Near-East were starting to transition from a hunter-gatherer way of life to a more sedentary lifestyle marked by settled habitations anchored by farmlands and green pastures. These settlements laid the foundation for the emergence of the first city-states in the part of the world now known as 'the cradle of civilization'. It comes as no surprise that such a transformation in human lifestyle took place during the Green Sahara because the more fertile and productive land directly aided our ancestors' ability to settle and grow in a key junction along the migratory path out of Africa.

For these reason, both paleo-climatologists and archaeologists are deeply interested in studying the climate dynamics of the Green Sahara and the human dynamics of the people who lived at this time.

Recently, Dr. Deepak Chandan and Prof. Dick Peltier, who are members of the department's Earth Atmosphere and Planetary Physics group, published results in the Geophysical Research Letters providing new insights into the mechanisms that could be in play in order to maintain a Green Sahara. They showed that internal feedbacks in the climate system originating from the 'greening of the land' is much stronger than what decades-old results would lead one to believe. This feedback is critical to further strengthening the West African Monsoon that in the mid-Holocene is already stronger than the present day because of the Earth's different orbital configuration which led to warmer summers over the African landmass.

Going forward, in collaboration with another postdoctoral fellow in the group, Dr. Lynn Welton, and graduate student, Fengyi Xie, the authors will use the modeled climate of this time to drive "agent-based" models that simulate the dynamics of human "agents" as they respond to changing societal and environmental stimuli in the hopes of understanding the motivations of our ancestors to settle where they did.

The authors are part of, and the research funded by, the Computational Research in the Ancient Near East (CRANE) project which is an international and interdisciplinary investigation into the archeology of the Near East during the middle-late Holocene and is led by Prof. Tim Harrison from the Department of Near and Middle Eastern Civilizations, University of Toronto.

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