US EPA

1606858 

Journal Article 

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

Timm, O; Koehler, P; Timmermann, A; Menviel, L 

2010 

Yes 

Journal of Climate
ISSN: 0894-8755
EISSN: 1520-0442 

23 

10 

2612-2633 

10.1175/2010JCLI3217.1 

WOS:000278782600011 

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.