Planetary boundaries revisited: a view through the ‘water lens’ Janos J Bogardi 1 , Bala ´ zs M Fekete 2 and Charles J Vo ¨ ro ¨ smarty 2 The recognition of the limits of Earth resources is a key to understanding the capacity of our planet to support a large and expanding human population with aspirations for improvements in well-being. This paper contributes to the ongoing debate on Earth system limits through a water focused crucial review. We argue that the degree of human appropriation of abiotic planetary and biotic ecosystem-based resources offers a useful framework to define sustainability, once societal aspirations and technology are taken into account. In this context, a ‘triangle’ consisting of coupled planetary, ecosystem-based Earth resources, and the human appropriation and stewardship may offer a pragmatic conceptual model for planetary sustainability with respect to freshwater. Because of these linkages, we find this new approach better suited to support policymaking for sustainability than a series of single-valued planetary boundary (PB) thresholds. To demonstrate the utility of the proposed approach, we explore human appropriation of water in the food production, industrial and domestic water sectors considering and modifying previously reported assessments. We do this for freshwater resources in light of anticipated population and economic growth. We find that a significant intensification of human appropriation of water will be necessary to support anticipated basic services and wealth generation over the coming decades. Furthermore, we foresee a major expansion of degraded water systems unless conscious preventive investments or costly remediation of impaired water quality are implemented. In this context, we see the need not only for technical innovation but improved governance as well. Addresses 1 Global Water System Project, International Project Office, University of Bonn, Walter-Flex-Str. 3, 53113 Bonn, Germany 2 The City College of New York City University (CCNY/CUNY), 160 Convent Avenue, New York, NY 10031, USA Corresponding author: Bogardi, Janos J (bogardi@ehs.unu.edu, jbogardi@uni-bonn.de) Current Opinion in Environmental Sustainability 2013, 5:581–589 This review comes from a themed issue on Aquatic and marine systems Edited by Charles J Vo ¨ ro ¨ smarty, Claudia Pahl-Wostl and Anik Bhaduri For a complete overview see the Issue and the Editorial Received 17 May 2013; Accepted 14 October 2013 Available online 8th November 2013 1877-3435/$ – see front matter, # 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cosust.2013.10.006 Introduction Collapses as a result of exhausting resources are not unprecedented in the history of human civilization, but occurred as isolated cases [1  ,2]. The specter of a break- down potentially affecting the whole planet represents a new challenge for humanity. The quest to identify the Earth’s limits is almost as old as the realization that our planet is a ‘sphere’ and hence finite. From Malthus through the Club of Rome a number of scientists [3,4] warned about the dangers of surpassing the carrying capacity of our planet. By introducing the concept of ‘planetary boundaries’ (PBs) the seminal work of Rockstro ¨m et al. [5  ] triggered considerable scientific and public debate [6  ,7  ,8  ,9]. Although not claiming to be exclusive measures of sus- tainability, PBs are recognized as (auxiliary) metrics defining a ‘safe operating space’ for humanity. How long, if at all, ‘business as usual’ can be continued? How can boundaries be defined and their transgression prevented? How many and which boundaries need to be defined and observed to keep the world on a sustainable track? These and similar questions can be answered through the perspective of PBs. Rockstro ¨m et al. [5  ,10  ] identified ten dimensions and proposed (except for two dimensions) global indicators and threshold values. The ten dimensions represent a practical compromise between the complexity of PBs and the desire to specify a ‘safe operating space’ that can be communicated to the public and policymakers. Blomqvist et al. [6  ] argued that many of these dimensions have arbitrary limits and no obvious tipping points. Rockstro ¨m et al. [10  ] recognized that the selected dimensions are not independent. Through the hydrological cycle freshwater use is linked to arguably all other dimensions. Therefore, any single boundary estimate for water is particularly problematical. We see additional limits to the original concept applied to water [6  ], including uneven distributions in time and space and entrenched local-scale management perspect- ives [7  ]. The current absence of limits recognizing the impact of impaired water quality and/or technological interventions and governance concepts also limit the current PB concept as applied to water. Although identifying PBs is essentially a scientific task, their acceptance is fundamentally a societal process reflecting human perspectives. PBs are inherently value judgments as they are associated with the preservation of a presumably desirable state. Consequently it is unlikely in our view that useful PBs could be established without explicit consideration of human activities, aspirations and Available online at www.sciencedirect.com ScienceDirect www.sciencedirect.com Current Opinion in Environmental Sustainability 2013, 5:581–589