Figure 8.13: Annual mean
precipitation changes (mm/yr) over Africa (20°W to 30°E) for the mid-Holocene
climate: (upper panel) Biome distributions (desert, steppe, xerophytic and dry
tropical forest/savannah (DTF/S)) as a function of latitude for present (red circles)
and 6,000 yr BP (green triangles), showing that steppe vegetation replaces desert
at 6,000 yr BP as far north as 23°N (vertical blue dashed line); (middle panel)
6000 yr BP minus present changes as simulated by the PMIP models. The black hatched
lines are estimated upper and lower bounds for the excess precipitation required
to support grassland, based on present climatic limits of desert and grassland
taxa in palaeo-ecological records, the intersection with the blue vertical line
indicates that an increase of 200 to 300 mm/yr is required to sustain steppe vegetation
at 23°N at 6,000 yr BP (redrawn from Joussaume et al., 1999); (lower panel)
same changes for the IPSL atmosphere-alone (A), i.e., PMIP simulation, the coupled
atmosphere-ocean (OA), the atmosphere-alone with vegetation changes from OA (AV)
and the coupled atmosphere-ocean-vegetation (OAV) simulations performed with the
IPSL coupled climate model. The comparison between AV and OAV emphasises the synergism
between ocean and land feedbacks (redrawn from Braconnot et al., 1999).
