Vegetation/precipitation feedbacks and the end of the mid-Holocene African humid period

Background/Question/Methods African landscapes changed dramatically between 11000 years ago and the present, as relatively wet conditions during the early Holocene gave way to dry conditions observed across North and East Africa today. Many paleoclimate records suggest that the termination of this early Holocene ‘African Humid Period' occurred very abruptly about 6000 yr BP, and climate models of intermediate complexity suggest that the abruptness of this change could result from a threshold response of the African monsoon to summer insolation in the northern subtropics coupled with a strong, positive feedback between precipitation and vegetation. Although this event has arguably become the archetype for abrupt climate change during the Holocene, as well as an important example documenting the importance of vegetation feedbacks to climate change, there are very few paleoclimate datasets that demonstrate coupled, abrupt changes in precipitation and vegetation. Did the African monsoon collapse abruptly in the mid-Holocene, and what is the role of vegetation in mid-Holocene precipitation changes? This presentation summarizes paleorecords from North and East Africa to examine the rate of climate and vegetation changes as well as their coupling. Results/Conclusions Lake level reconstructions indicate that although individual sites within North Africa may have experienced abrupt climate ‘collapses', precipitation over North Africa declined more gradually from north to south. These data suggest abrupt precipitation changes likely occur as the Saharan/sub-Saharan boundary passed individual sites during a gradual southward migration over the mid-Holocene. New high-resolution paleohydrologic and vegetation reconstructions from Lake Ounianga, Chad, located in the central Sahara, document gradual reductions in both rainfall and vegetation from 6000 yr BP onward with no clear evidence of abrupt transitions. Records from equatorial west Africa and east Africa indicate both gradual and abrupt changes in precipitation during the mid-Holocene between 6000 and 3000 yr BP. This asynchroneity further argues against an abrupt, synchronous collapse of the African monsoon and humid period. These data support recent results from coupled ocean-atmosphere climate models that indicate weaker coupling between North African rainfall and vegetation than simulations done with models of intermediate complexity. Thus, although regions within Africa may well have experienced abrupt, mid-Holocene climate transitions, the collapse of the African humid period was regionally variable and biogeophysical climate feedbacks are weaker than previously assumed.