Ecological footprint depth and size: new indicators for a 3D model
生态足迹深度和广度:构建三维模型的新指标

The accelerating growth in natural resource exploration and consumption is creating serious environmental problems on a global scale, which forces people to re-examine the conflicting relationship between environmental quality and economic expansion. In this sense, it is of great importance to develop objective and forward-looking indicators and models that can be used to understand, measure, and predict the environmental impacts associated with human activities. Building on the achievements in evaluating net primary productivity of global ecosystem and its human appropriation, the ecological footprint was formally proposed by ecological economists Rees and Wackernagel. The ecological footprint is a biophysical quantitative assessment tool designed to measure the impacts of human consumption of energy and material on the global ecosystems. It measures the areas of land required to continuously supply for resource provision and waste assimilation to satisfy human consumption, compares the demand with the globe's biocapacity to regenerate the resources and to absorb the waste, by representing the amount of biologically productive land area still needed. Despite its success in the policy world and general public the method is riddled with problems. However, it is difficult for any single indicator to efficiently collect enough information related to sustainability. Progress has been made in further maturing the ecological footprint, but there is still much room for methodological improvement. From a viewpoint of ecological footprint, human society can be considered sustainable only if the demand remains within the regenerative capacity of the planet. All renewable flows and stocks of resources and ecological services may be consumed but non-renewable stocks should remain untouched. In particular, the ability to regenerate stocks necessary for human welfare must be maintained. Otherwise, the balance of ecosystems may strike. In this case, natural capital has become a limiting factor for human welfare and sustainability. Therefore, tracking depletion of natural capital stocks and appropriation of natural capital flows are central topics in the study of sustainable development, and has gained much attention from the scientific community.In this paper, the concepts and calculating methods of a 3D model of ecological footprint recently introduced by Niccolucci and Wackernagel, etc. are discussed systematically. In particular, a great deal of emphasis is put on two indicators of the 3D model-footprint depth and footprint size. The main strengths of this model are also studied by comparing it with the classical model. Some improvements are proposed using two new indicators-appropriation rate of capital flows and use ratio of stocks to flows. An empirical analysis on China based on the data of national footprint accounts during 1961-2006 illustrates and tests the methods discussed. It is concluded that the footprint depth has almost tripled since 1978 when