African Monsoon Variability over Past Millennia Peter B. deMenocal Jennifer Arbuszewski Jess Adkins Zhengyu Liu

Overview The African monsoon and orbital forcing The early Holocene African Humid Period Climate model simulations: … mechanisms for abrupt change

Sahel Drought (1960s onward) Giannini et al. (2003)

The African Monsoon and Tropical Atlantic Upwelling Cool SSTs Maximum upwelling Steep thermocline slope W. Ruddiman, “Earth's Climate Past and Future”

Orbital Precession and the Monsoon Kutzbach (1981) W. Ruddiman, “Earth's Climate Past and Future”

Linear (?) monsoon sensitivity

Precessional monsoon variability (McIntyre et al., 1989; Molfino and McIntyre, 1993; Pokras and Mix, 1985; Rossignol-Strick, 1983) Tropical Upwelling Med. Sapropels African Climate

…persisted for millions of years! Miocene Mediterranean Sapropels (Sicily; Frits Hilgen)

NW African dust export Site 658

African Humid Period AHP deMenocal et al., 2000

African Humid Period Africa was much wetter between ca kyr BP. Most of Saharan Desert vegetated, wetland fauna.

North Africa wet during early Holocene Paleolake sediments in Mali (NW Africa)

Climate-Controlled Occupation of the Eastern Sahara Kuper and Kroepelin, 2006

Faiyum Mirimde Badari Nagada Egyptian Neolithic ( ka BP)

AHP: A pan-North African event Liu et al., 2007

Claussen et al., 1999; deMenocal et al., 2000 Albedo-Vegetation feedback?

Fully coupled transient O-A-Veg GCM Liu et al. (QSR in press)

Climate Model insight African climate very sensitive to orbital radiation changes. Abrupt responses to gradual forcing due to some aspect of vegetation feedback. Humid-arid transitions occurred within centuries. SST influence uncertain but present in historical records.

Annual sea-surface temperature Annual Evap - Precip

Meridional ITZ displacements: LGM water-hosing experiments Change in surface temperature (°C) Change in P-E (m/yr) Vellinga and Wood (2002)

LGM Collapse of zonal SST Gradient

SST gradients along transect

Fresh Salty Large and abrupt Deglacial ITCZ Swings

Summary Past African climate changes were large. Moisture budget changes were abrupt. Likely importance of vegetation feedbacks, perhaps SSTs as well.