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30°N
(a)
(b)
Mediterranean
Forest
Mediterranean
Scrub
Medite rrane an
Forest
M editerranean
Scrub
25°N
Extreme Desert
Grasslands
20°N
Semi-Desert
Savanna
Grasslands
Savanna
Tropical Rainforest
15°N
Tropical Rainforest
10°N
5°N
Fig ure 5.7 The Green Sahara. Distribution of African biomes (a) in the mid-Holocene (b) and present
day (c) Representation of biome shifts in the pollen record (Braconnot et al. 2000, 2006, 2007).
major rivers, and thrived though technical and political innovations (Demenocal et al. 2000,
Castañeda et  al. 2009, Bard 2013). Savanna trees declined gradually from about 4,300 years
ago and grass cover became increasingly sparse. The greening of the Sahara ended about
3,200 years ago and desert plant types, typically found in the area today, appear from 2,700
years ago (Peck et al. 2004, Kröpelin et al. 2008).
The orbital effects of the mid-Holocene, which initiated the greening of the Sahara, are well
understood and can be simulated with high precision, yet models that include orbital effects
alone underestimate the extent of desert retreat, which is known from the fossil pollen record
(Figure 5.7c). The expansion of surface waters and vegetation growth had an important feed-
back effect on climate, further enhancing monsoon rainfall through local recycling of evapo-
rated water, and possibly as much as doubling precipitation rates compared with orbital
effects alone (Lézine et  al. 2011, Krinner et  al. 2012). Evapotranspiration from plant leaves
would have contributed moisture to the air, allowing rainfall to be recycled many times across
the Sahara-Sahel region, and at the same time, the vegetation of the green Sahara also
reflected less solar irradiation compared with bare ground, further enhancing precipitation
by increasing the temperature contrast between land and ocean (Claussen and Gayler 1997,
Lézine et  al. 2011). This regional feedback reinforced the green Sahara state, showing the
important role of vegetation and surface water in generating rainfall at the regional scale. It
was not until the effects of land cover change were added to the models that the greening of
the Sahara could be accurately simulated (Doherty et al. 2000, Patricola and Cook 2007, Krin-
ner et al. 2012).
Without the palaeoecological record, the magnitude of vegetation-climate feedbacks
would not be known and our ability to predict a future green Sahara would be limited. The
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