pnas.orghttp://www.pnas.org/cgi/reprint/142033699v1.pdfTracking the ecological overshoot of thehuman economyMathis Wackernagel*†, Niels B. Schulz‡, Diana Deumling*, Alejandro Callejas Linares§, Martin Jenkins¶, Valerie Kapos¶,Chad Monfreda*, Jonathan Loh储, Norman Myers**, Richard Norgaard††, and Jørgen Randers‡‡*Redefining Progress, 1904 Franklin Street, 6th Floor, Oakland, CA 94612;‡Institute for Interdisciplinary Studies of Austrian Universities, Department ofSocial Ecology, Schottenfeldgasse 29, 1070 Vienna, Austria;§Centro de Estudios para la Sustentabilidad, Obreros Textiles 57 Departamento 6, ColoniaMarco Antonio Mun˜ oz, 91060 Xalapa, Veracruz, Mexico;¶World Conservation Monitoring Centre, 219 Huntingdon Road, Cambridge CB3 0DL, UnitedKingdom;储World-Wide Fund for Nature International, Avenue Mont-Blanc, 1196 Gland, Switzerland; **Green College, Oxford University,Oxford OX2 6HG, United Kingdom;††Energy and Resources Group, 310 Barrows Hall, University of California, Berkeley, CA 94720-3050;and‡‡Norwegian School of Management BI, Elias Smiths vei 15, Box 580, N-1302 Sandvika, NorwayEdited by Edward O. Wilson, Harvard University, Cambridge, MA, and approved May 16, 2002 (received for review January 17, 2002)Sustainability requires living within the regenerative capacity ofthe biosphere. In an attempt to measure the extent to whichhumanity satisfies this requirement, we use existing data totranslate human demand on the environment into the area re-quired for the production of food and other goods, together withthe absorption of wastes. Our accounts indicate that humandemand may well have exceeded the biosphere’s regenerativecapacity since the 1980s. According to this preliminary and explor-atory assessment, humanity’s load corresponded to 70% of thecapacity of the global biosphere in 1961, and grew to 120% in 1999.Accounting for Humanity’s Use of the Global BiosphereThe human economy depends on the planet’s natural capital,which provides all ecological services and natural resources.Drawing on natural capital beyond its regenerative capacityresults in depletion of the capital stock. Through comprehensiveresource accounting that compares human demand to the bio-logical capacity of the globe, it should be possible to detect thisdepletion to help prepare a path toward sustainability.The purpose of this study is to develop such an accountingframework, and to measure the extent of humanity’s currentdemand on the planet’s bioproductive capacity. We build onmany earlier attempts to create comprehensive measures ofhuman impact on the biosphere. For example, Vitousek et al. (1)used consumption estimates to calculate humanity’s appropria-tion of the biosphere’s Net Primary Productivity (NPP). Theyconcluded that the human economy co-opted organic materialequivalent to 40% of the NPP of terrestrial ecosystems in 1980.Odum developed a conceptual basis for accounting for energyflows through ecosystems and human economies, but did notproduce overall accounts (2). Fischer-Kowalski and Hu¨ttler (3)advanced the concept of ‘‘societal metabolism,’’ using materialflow analysis as a macro indicator for the environmental per-formance of societies. The Global Environment Outlook 2000 (4)and World Resources 2000–2001 (5) describe human impacts onvarious ecosystem types in detail, but both reports lack anaggregated summary of the impacts. Others have analyzed theintegrity of subcomponents of the biosphere, such as carboncycles (6), freshwater use (7, 8), and the nitrogen cycle (9), haveassigned approximate monetary values to the ecological servicesthat humanity depends on (10), or established frameworks formonetary natural capital accounts for nations (11).This preliminary and exploratory study demonstrates an ag-gregated approach to natural capital accounting in biophysicalunits. A wide variety of human uses of nature are identified,measured, and expressed in units that enable direct comparisonof human demands with nature’s supply of ecological services.The calculation results and annotated spreadsheet for 1999 arepublished as supporting information on the PNAS web site,www.pnas.org.Our global accounts build on assessments of the ‘‘ecologicalfootprint’’ of humanity (12, 13). Such assessments are based onsix assumptions:1. It is possible to keep track of most of the resources humanityconsumes and the wastes humanity generates.2. Most of these resource and waste flows can be measured interms of the biologically productive area necessary to main-tain these flows (those resource and waste flows that cannotare excluded from the assessment).3. By weighting each area in proportion to its usable biomassproductivity (that is, its potential production of biomass thatis of economic interest to people), the different areas can beexpressed in standardized hectares. These standardized hect-ares, which we call ‘‘global hectares,’’ represent hectares withbiomass productivity equal to the world average productivitythat year.4. Because these areas stand for mutually exclusive uses, andeach global hectare represents the same amount of usablebiomass production for a given year, they can be added up toa total representing the aggregate human demand.5. Nature’s supply of ecological services can also be expressedin global hectares of biologically productive space.6. Area demand can exceed area supply. For example, a forestharvested at twice its regeneration rate appears in ouraccounts at twice its area. This phenomenon is called ‘‘eco-logical overshoot’’ (14, 15).Thus, the ecological impact of humanity is measured as the areaof biologically productive land and water required to produce theresources consumed and to assimilate the wastes generated byhumanity, under the predominant management and productionpractices in any given year. Not only human demand on nature,but also nature’s supply changes over time because of innova-tions in technology and resource management, changes in landuse, and cumulative damage of past impacts.We recognize that reducing the complexity of humanity’simpact on nature to appropriated biomass offers only a partialassessment of global sustainability. It is a necessary, but notsufficient, requirement that human demand does not exceed theglobe’s biological capacity as measured by our accounts.The Impact ComponentsOur accounts include six human activities that require biologi-cally productive space. They are