| Summary: | The ecological footprint (EF) is an important tool for assessing ecological resource occupancy. The three-dimensional (3D) EF changes the EF from a plane to a column, and the bottom (EFsize) represents the human appropriation of the annual natural resource flow provided by the earth, while the height (EFdepth) represents the number of years required to regenerate the resources consumed within 1 year. According to the difference in the human demands for the productive functions of land, this paper improves the 3D EF model based on three sub-items, namely, the basic land footprint (including the footprint of cropland, grazing land and fishing grounds), which captures the demands of the physical part of food and clothing; forest land footprint, which captures wood and carbon absorption demands; and the built-up land footprint, which captures production and living space demands. The improved model is a 3D structure with three different heights. The bottom shows human appropriation of annual natural flows of the sub-items, and there is competition between the sub-items. The sub-heights are related to the overshoot of the sub-items. The forest land footprint depth is earlier and deeper than the 3D EF, and the footprint depth of the built-up land and basic land kept the natural depth. Therefore, humans perceive climate change, but their daily survival is not significantly affected. The flow occupancy ratio (orflow) and the accumulated ecological debt depth (EFdepthaccum) are introduced to analyse the closeness to the overshooting state and the years of using existing resources to eliminate historical ecological debt, respectively. © 2021
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