Planetary boundaries revisited: a view through the 'water lens'

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.