Aquatic Ecosystem Variability and Climate Change – A Palaeoecological Perspective - Climate Change Impacts on Freshwater Ecosystems - page 20

Figure 2.3 Exposed diatomites from a dry lake bed of mid-Holocene age in the Bodélé
Depression, Northern Chad. (Photograph by J. Giles, with permission from Nature
magazine.)
Convergence Zone (ITCZ). As the ITCZ moved to the south and the intensity of
monsoon rains decreased, the extensive freshwater lakes and rivers that had
formed across the Sahara, Arabia, north-west India and western China in the
early Holocene progressively shrank and dried out (e.g. Gasse 2002), leaving
behind residual, mainly salty, lakes and extensive dry lake beds (Fig. 2.3).
The North African region is probably the best documented, with evidence for
former freshwater lakes provided by the number and extent of exposed fresh-
water diatomites and other lake sediments across the Sahel and Saharan regions,
dating to the early and middle Holocene (Gasse 2000; Hoelzmann et al . 2004).
These early Holocene freshwater lakes and rivers not only supported aquatic
plant and animal communities but also provided a freshwater resource for
indigenous herdsmen, attested by the rock paintings of giraffe, sheep and cattle
in the Tibesti Mountains (e.g. Lhote 1959).
The disappearance of these freshwater systems during the middle Holocene,
between approximately 6000 and 5000 years ago, is one of the most
dramatic events of recent earth history. Considerable efforts have been made by
palaeoclimatologists and climate modellers to document its timing and understand
its causes and consequences (Hoelzmann et al . 2004). Of relevance to the present
day debate on climate change and its effects is the evidence provided by this mid-
Holocene desiccation of the rapidity with which a small change in insolation can
lead to such a major response. It is very important to understand the mechanisms
in the climate system responsible. In this case, climate simulations were found to
match the observations only when the models were modified to include feedbacks
between land cover and climate so that evapo-transperative losses from the land
surface were sufficient to sustain a positive moisture balance in the atmosphere
(Claussen et al . 1999).