Past, Present, and Future Climate Change: Can Palaeoecology Help Manage a Warming World? - Biodiversity Conservation and Environmental Change - page 95

greater food security. This case study has illustrated how a multidisciplinary approach,
involving the palaeoecology of interglacial periods and ethnobotanical studies, as well as
modern agroforestry, can all contribute to building adaptive strategies that help build resil-
ience in marginal environments.
Mid-Holocene Altithermal; the greening of the Sahara
From about 9,000 to 5,000 years ago, during the MHA, global temperatures were markedly
raised, as a consequence of orbital effects, which increased incoming solar radiation in the
northern hemisphere summer. This period of time, is especially useful in understanding
biodiversity responses to warmer climates. In North Africa, the MHA was associated with a
strengthening of the North African monsoon, bringing rainfall to latitudes that are pres-
ently occupied by the Sahara Desert. The fossil pollen record shows a green Sahara during
this time. As a result of increased moisture availability, savanna and even forest elements
spread as much as 400-500 km further north than their current range, greening areas that
are currently hyperarid and inhospitable to life, and forming plant communities that have
no modern analogue (Demenocal et al. 2000, Hailemichael et al. 2002, Cole et al. 2009, Wil-
lis et al. 2013).
From about 9,000 to 6,000 years ago, lakes and oases dotted the Sahara region, appearing
first in the east and moving west over the course of several millennia as groundwater
recharged (Lézine et al. 2011). Lake MegaChad covered an area of 330,000 km 2 , and further
east, many lakes overflowed and coalesced; for example, the central Afar lake system rose
150 m higher and was 13 times the surface area of today's lake (Demenocal et al. 2000, Hailem-
ichael et al. 2002, Cole et al. 2009). Southern forest taxa reached the Sahara at c. 9,400 years
ago; fossil pollen data from Lake Yoa, northern Chad, indicate regional open savanna, stream
vegetation, and tropical trees, typical of dry forests, from c. 6,000 years ago, the latter pres-
ently occurring only some 300 km to the south (Figure 5.7) (Lezine and Cazet 2005). The more
benign climate and vegetation resources enabled neolithic communities to flourish from
about 10,000 to 4,000 years ago in areas that are now desert (Gasse 2000).
As orbital effects diminished, lakes and waterholes dried up, again starting in the east and
moving westwards. The most water-loving, tropical trees began disappearing from Saharan
wetland from about 6,500 years ago, and inputs of wind-borne dust increased from 4,800
years ago (Kröpelin et al. 2008). The end of the African Humid Period (AHP), about 5,000
years ago, was marked by lake level decline, and a retreat of mesic vegetation to lower lati-
tudes. Open water surfaces fragmented and the desert boundary expanded south by about
5 degrees. Gallery forests in the central Sahel retreated from 4,500-3,500 years ago. Savanna
and grassland elements persisted longer, and C 4 grasslands spread at the expense of C 3 tree
taxa forests (Castañeda et al. 2009). Utilizing different photosynthetic pathways, C 4 plants are
more water efficient, and hence have an advantage over C 3 trees and shrubs in hotter, drier
conditions. This shift in vegetation composition resulted in migrations, population redistri-
bution, and socioecological adaptation as nomadic pastoralist cultures were more suited to
the utilization of patchy resources, while sedentary populations became concentrated along