Rooting depth determines how much water can be stored in the soil which is accessible to the vegetation for transpiration. Here, we derive a global distribution of rooting depth with an optimisation principle: A simple formulation of Net Primary Production (NPP) is incorporated into a General Circulation Model and then NPP is maximised in respect to rooting depth. The obtained rooting depths are considerably larger to those used in present-day models but are consistent with observations. NPP increases substantially with the use of optimised rooting depths, mainly in tropical regions during the dry season accompanied with enhanced transpiration. The increased flux of latent heat leads to a considerable decrease in 2m air temperature, which leads to a better agreement with observations. We conclude that rooting depth is an important vegetation property, especially in the tropics, and tropical deforestation might have a much larger impact on climate than previously thought. ¿ 1998 American Geophysical Union