Ecological Modelling 220 (2009) 2819–2823 Contents lists available at ScienceDirect Ecological Modelling journal homepage: www.elsevier.com/locate/ecolmodel Short communication How deep is the footprint? A 3D representation V. Niccolucci a , S. Bastianoni b,∗ , E.B.P. Tiezzi c , M. Wackernagel d , N. Marchettini a a Department of Chemistry, University of Siena, via della Diana 2A, 53100 Siena, Italy b Department of Chemistry, University of Siena, via A. Moro 2, 53100 Siena, Italy c Department of Mathematical Sciences “Roberto Magari”, University of Siena, Pian dei Mantellini 44, 53100 Siena, Italy d Global Footprint Network, 312 Clay Street, Suite 300, Oakland, CA 94607, USA article info Article history: Received 14 May 2009 Received in revised form 16 July 2009 Accepted 22 July 2009 Available online 27 August 2009 Keywords: Ecological Footprint Footprint depth Footprint size Flows Stocks Natural capital Sustainability Biocapacity abstract Depletion of natural capital stocks and use of natural capital flows are the central issues in the sus- tainability 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. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The ability of natural systems to provide resources and absorb wastes is a major function of natural capital. As defined by Daly (1994), natural capital is the resource stock that yields flows of natural services and tangible natural resources. As pointed out by Pearce and Turner (1990, p. 44) “the resource stock should be held constant over time”, in particular, its ability to renew and regen- erate itself and all ecosystem services essential to human welfare should be maintained. Natural capital has become a limiting fac- tor for human welfare and sustainability (Farley and Daly, 2006). Ekins et al. (2003) report declining natural capital stock as evidence of environmental unsustainability (see also Niccolucci et al., 2007). Tiezzi (2002) identified a potential threat in the difference between fast technological use of resources by humans and slow biological regeneration times. Measures of human demand on, and nature’s supply of, natural capital are therefore necessary for tracking progress, setting targets and driving sustainability policies (Wackernagel et al., 2004). The Ecological Footprint (hereafter EF) was introduced by Wackernagel and Rees (1996) to evaluate and communicate human dependence ∗ Corresponding author. Tel.: +39 0577 234358; fax: +39 0577 234353. E-mail address: bastianoni@unisi.it (S. Bastianoni). on nature. The method is based on the idea that every human action of day-to-day living creates an impact or leaves a footprint on the environment (Rees, 1992). Such an account is possible because resources consumed and wastes produced can be tracked and asso- ciated with the amount of biosphere regenerative capacity they require (Monfreda et al., 2004). The EF of a single individual or a population quantifies the total area of different biologically productive ecosystems (terrestrial and aquatic) required, on a continuous basis, to provide the resources consumed and to assimilate wastes generated, using prevailing technologies and resource management (Wackernagel and Rees, 1996). While EF is the demand on natural capital, available Bioca- pacity BC is the supply of natural capital. From an anthropocentric point of view, BC quantifies the regenerative capacity of an area, that is, the ability to provide ecological resources and services used by humans (Monfreda et al., 2004). The two indices EF and BC are both expressed in terms of global hectares (gha), namely hectares of land normalized to world aver- age productivity of all biologically productive land and water in a given year (Galli et al., 2007). The use of space as a unit of measure reflects the fact that humanity is constrained by the Earth’s limits. The surface of the Earth is finite, and therefore the ecologically pro- ductive area available, as well as the annual quantity of resources produced and wastes absorbed, are also finite. EF and BC are therefore compared, as expenditure and income are compared in finance (Wackernagel and Kitzes, 2008), to define 0304-3800/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ecolmodel.2009.07.018