How deep is the footprint? A 3D representation

Depletion of natural capital stocks and use of natural capital flows are the central issues in the sustainability debate. Differentiation of these two components, considering natural capital and its limits, is important for planning and management of land use. This paper offers insights into this issue and proposes a new perspective of the Ecological Footprint (EF) in three dimensions, considering not only its size but also its depth: according to this viewpoint the footprint is not an area but a volume, although maintaining the same value as the one in two dimensions. Use of annual flows provided by the Earth is represented by the footprint size, expressed in global hectares ( gha) and plotted in the ( x, y) plane. Footprint depth represents the demand for extra land to meet human needs through depletion of stocks of natural capital. It is plotted on the z-axis. It can be seen as the number of years necessary to regenerate resources liquidated in 1 year (and to absorb the respective wastes) or as the number of planets necessary to support the inhabitants of the planet Earth. The evolution of these two components in the last five decades is studied and discussed.