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Figure 9.18 |  Increases in atmospheric CO2 and changes in aridity are projected to shift the geographic distribution of major biomes across Africa (high confidence). Arrows in the diagram indicate possible pathways of biome change from current conditions resulting from changes in CO2 and aridity. Changes need not be gradual or linear and may occur rapidly if tipping points are crossed. Currently, widespread greening observed in Africa has been at least partially attributed to increasing atmospheric CO2 concentrations. Future projected increases in aridity are expected to cause desertification in many regions, but it is highly uncertain how this will interact with the greening effect of CO2. Inset maps show the projected geographical extent of changes in CO2 concentrations and aridity. CO2 is projected to increase globally under all future emission scenarios. Aridity index maps show projected change in aridity (calculated as annual precipitation/annual potential evapotranspiration) at around 4°C global warming relative to 1850–1900 (RCP8.5 in 2070–2099) from 34 CMIP5 models (Scheff et al., 2017). Shaded areas indicate regions where >75% of models agree on the direction of change.