Rising atmospheric CO2 concentrations affect climate directly through radiative effects and indirectly
by changing plant water-use efficiency. Under global warming scenarios these widely reported changes will have a
substantial impact on future bush encroachment, crop yields, river flow and climate feedbacks. Tree-ring intrinsic
water-use efficiency (iWUE) records for Africa show a 24.6% increase over the 20th century. As high iWUE can
partly counterbalance projected decreases in regional precipitation, this research has important implications for
those involved in water resource management and highlights the need for climate models to take physiological
forcing into account.