Mechanisms for the Onset of the African Humid Period and Sahara Greening 14.5–11 ka BP

Oliver Timm International Pacific Research Center, SOEST, University of Hawaii at Manoa, Honolulu, Hawaii

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Peter Köhler Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany

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Axel Timmermann International Pacific Research Center, SOEST, University of Hawaii at Manoa, Honolulu, Hawaii

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Laurie Menviel International Pacific Research Center, SOEST, University of Hawaii at Manoa, Honolulu, Hawaii

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Abstract

The mechanisms leading to the onset of the African Humid Period (AHP) 14 500–11 000 yr ago are elucidated using two different climate–vegetation models in a suite of transient glacial–interglacial simulations covering the last 21 000 yr. A series of sensitivity experiments investigated three key mechanisms (local summer insolation and ice sheet evolution, vegetation–albedo–precipitation feedback, and CO2 increase via radiative forcing and fertilization) that control the climate–vegetation history over North Africa during the last glacial termination. The simulations showed that neither orbital forcing nor the remote forcing from the retreating ice sheets alone was able to trigger the rapid formation of the AHP. Only both forcing factors together can effectively lead to the formation of the AHP. The vegetation–albedo–precipitation feedback enhances the intensity of the monsoon and further accelerates the onset of the AHP. The experiments indicate that orbital forcing and vegetation–albedo–precipitation feedback alone are insufficient to trigger the rapid onset of the AHP. The sensitivity experiments further show that the increasing radiative forcing from rising CO2 concentrations had no significant impact on the temporal evolution of the African monsoon during the last deglaciation. However, the fertilization effect of CO2 is important for the terrestrial carbon storage. The modeling results are discussed and compared with paleoproxy records of the African monsoon system. It is concluded that the model results presented here do not lend support to the notion that simple insolation thresholds govern the abrupt transitions of North African vegetation during the early to middle Holocene.

Corresponding author address: Dr. Oliver Timm, International Pacific Research Center, SOEST, University of Hawaii at Manoa, POST Bldg. 401, 1680 East West Rd., Honolulu, HI 96822. Email: timm@hawaii.edu

Abstract

The mechanisms leading to the onset of the African Humid Period (AHP) 14 500–11 000 yr ago are elucidated using two different climate–vegetation models in a suite of transient glacial–interglacial simulations covering the last 21 000 yr. A series of sensitivity experiments investigated three key mechanisms (local summer insolation and ice sheet evolution, vegetation–albedo–precipitation feedback, and CO2 increase via radiative forcing and fertilization) that control the climate–vegetation history over North Africa during the last glacial termination. The simulations showed that neither orbital forcing nor the remote forcing from the retreating ice sheets alone was able to trigger the rapid formation of the AHP. Only both forcing factors together can effectively lead to the formation of the AHP. The vegetation–albedo–precipitation feedback enhances the intensity of the monsoon and further accelerates the onset of the AHP. The experiments indicate that orbital forcing and vegetation–albedo–precipitation feedback alone are insufficient to trigger the rapid onset of the AHP. The sensitivity experiments further show that the increasing radiative forcing from rising CO2 concentrations had no significant impact on the temporal evolution of the African monsoon during the last deglaciation. However, the fertilization effect of CO2 is important for the terrestrial carbon storage. The modeling results are discussed and compared with paleoproxy records of the African monsoon system. It is concluded that the model results presented here do not lend support to the notion that simple insolation thresholds govern the abrupt transitions of North African vegetation during the early to middle Holocene.

Corresponding author address: Dr. Oliver Timm, International Pacific Research Center, SOEST, University of Hawaii at Manoa, POST Bldg. 401, 1680 East West Rd., Honolulu, HI 96822. Email: timm@hawaii.edu

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