Terrigenous dust collected from ocean sediment near the African coast reveals the Sahara became almost totally vegetated from around 14,800 years ago and returned abruptly to a more arid state around 5500 years ago.
1. Sahara unvegetated; pre and post-African Humid Period
2. Sahara vegetated (annual grasses, shrubs), perennial lakes, associated fauna; African Humid Period
Gradual changes to earth's orbit around 14,800 years ago increased the amount of summer seasonal insolation reaching northern Africa. This strengthened the African monsoons creating the more mesic environment of the African Humid period. The period ended equally abruptly (decades to centuries) around 5,500 years ago.
When the summer season insolation crossed a threshold in both directions of 470 W/m2 (current value is 415 W/m2), the system shifted. Climate modelling suggests that two positive feedback processes (coupled vegetation-albedo feedback and surface ocean temperature-moisture transport feedback) explains the abrupt and non-linear shift. During summer, North Africa is heated more efficiently than the adjacent oceans, establishing a strong low-pressure centre over North Africa. This land-sea pressure gradient largely drives the strong, moist onshore winds, bringing monsoonal precipitation in the summer months. Orbital changes affect the seasonal distribution of incident solar radiation, altering the land-sea pressure gradient and the subsequent strength of summer monsoons.
Contact
Jacqui Meyers
Email
jacqui.meyers@csiro.au
CSIRO Sustainable Ecosystems,
PO Box 284,
Canberra ACT 2601
Keywords
Pleistocene-Holocene, empirical data, model, North Africa, climate change, Sahara, Younger Dryas, monsoon, ocean temperature, feedbacks, sea-surface temperatures, ocean circulation, climate change, empirical, quantitative model
