A marine sediment record from off the western coast of Africa clearly shows an abrupt decrease in Saharan vegetation about 5,500 years ago, however (Figure 9). The scientists who generated this record measured the terrigenous flux, or dust that is transported off Africa into the Atlantic Ocean. This variable is inversely related to the amount of vegetation. Prior to 5,500 years ago, vegetation was more extensive in northern Africa and there was little loss of sediment from the land. The reverse is true after 5,500 years ago.
Why did vegetation decrease abruptly?
Pollen records show that vegetation in North Africa has two stable states (Figure 10). Since 5,500 years ago, this area has been in a 'desert' state, with little to no vegetation in the Sahara and mixed steppe and savanna to the south in the Sahel. However, pollen records from this area show that a 'green' state prevailed during most of the time between 14,500 to 5,500 years ago. Steppe vegetation expanded across the Sahara, and the Sahel was covered by savanna. One exception is during the Younger Dryas, when conditions in North Africa were drier and the vegetation was more desertlike.
Slow variations in the Earth's orbit caused a gradual decrease in summer solar radiation in the tropics from the early to mid Holocene (Figure 9). This decreased the amount of summer monsoon rainfall in Africa and other parts of the tropics. Scientists hypothesize that as monsoon precipitation gradually decreased, at some point conditions became too dry for plants and a rapid transition to dusty, desert conditions resulted. Understanding the feedbacks associated with this transition is an active area of research.
Some important datasets related to the African Humid Period:
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