Revue de Métallurgie 110, 79–88 (2013) c  EDP Sciences, 2013 DOI: 10.1051/metal/2013051 www.revue-metallurgie.org R evue de Métallurgie The footprint family: comparison and interaction of the ecological, energy, carbon and water footprints Kai Fang, Reinout Heijungs and Geert de Snoo Institute of Environmental Sciences, Leiden University, the Netherlands Key words: Footprint family; Key footprint; comparison; interaction Received 31 January 2013 Accepted 8 February 2013 Abstract – The year of 2012 marks the 20th anniversary since the concept of ecological footprint was introduced to the global community for the first time. Nowadays footprint studies have gained extensive debates as well as popularity. In this paper, we define the footprint family as an indicator system of selected footprints that measure some aspects of human impacts on the environment. A relatively comprehensive introduction and compari- son of four key footprints is represented by listing their characteristics in different aspects. The interaction among these footprints within the footprint family is classified into three types: overlap, contradiction and complement. The description of each type is provided in detail. Limitations and uncertainties of the footprint family, and priorities for further im- provement are also performed. This research makes a preliminary attempt at developing the conceptual framework for the footprint family, which allows us to examine the perfor- mance of footprints combination. The footprint family will serve as a tool for integrating footprints on human impacts assessment, and a support for environmental decision-making. O ver the past 30 years our Earth has witnessed a significant shift from environmental pollution restricted to local or regional area towards ecological degradation associated with the irreversibly decline in natural resources stocks and ecosystem services at global scale. Nowa- days environmental issues are more related to the impacts of human activities in multi- dimensions. Given this serious situation, it is necessary to develop a solid and integra- tive system where human impacts are mea- sured through appropriate indicators [1]. The footprints are such kind of indicators that capture the complexity of environmen- tal impacts derived from specific resources consumption or waste discharge. At present we celebrate the 20th anniver- sary of ecological footprint which has been formally introduced to the global commu- nity of ecological economics since 1992 [2]. Over the past two decades the ecologi- cal footprint has reached great popular- ity among academics, publics, organiza- tions and governments. The basic ideas and perspectives behind it have been explored through different approaches, leading to a variety of footprint concepts. So far there is not yet a completely sat- isfactory indicator that should be consid- ered as the “golden standard” [3, 4]. With no exception of the footprints, none is able to represent the whole human impacts on the Earth [5]. This requires the development of a combination of the existing footprints in or- der to be more informative for representing human-imposed environmental pressure in multi-dimensions. Based on a definition of what counts as a footprint family, a relatively comprehensive description of four key footprint indicators were provided in this paper. Then we estab- lish the comparison among them by listing their characteristics in different aspects, and explore the interaction within the footprint family by showing how the footprints over- lap, contradict, and complement each other. Limitations of the research and suggestions for further improvement are also presented. 1 The footprint family (I): definition & members 1.1 Definition The terminology of the “footprint family” was first described as an aggregation of the Article published by EDP Sciences