Fig. 4.

Pathways by which the Sahara and Hunshandake responded to precipitation change induced by Holocene monsoonal penetration/strength variations. (A) As precipitation is reduced, the system moves toward a drier state, resulting in a state switch from green to desert. Subsequent wetter conditions result in a second state switch (pathway 1), with green conditions reestablished (the green switch occurs at precipitation levels indicated by the dotted vertical line in A and C). (B) Groundwater elevation change coincident with changing climate under a model where there are no constraints on groundwater levels. HL indicates a linear response, whereas HN is one of many possible nonlinear responses. (C) Alternative pathway 2, seen in the Hunshandake in response to a regional inability to maintain high groundwater levels due to drainage redirection and groundwater sapping because pathway 1 is no longer possible. Increased precipitation beyond currently documented Pleistocene/Holocene values (left of the y axis in the graph) results in removal of the sediments necessary to support a higher groundwater table. (D) Hydrologic responses, HL and HN, under all conditions no longer intersect the green state switch (dotted horizontal line in B and D) and are maintained at levels that cannot support a green Hunshandake.