Green water — terrestrial precipitation, evaporation and soil moisture — is fundamental to Earth system dynamics and is now extensively perturbed by human pressures at continental to planetary scales. However, green water lacks explicit consideration in the existing planetary boundaries framework that demarcates a global safe operating space for humanity. In this Perspective, we propose a green water planetary boundary and estimate its current status. The green water planetary boundary can be represented by the percentage of ice-free land area on which root-zone soil moisture deviates from Holocene variability for any month of the year. Provisional estimates of departures from Holocene-like conditions, alongside evidence of widespread deterioration in Earth system functioning, indicate that the green water planetary boundary is already transgressed. Moving forward, research needs to address and account for the role of root-zone soil moisture for Earth system resilience in view of ecohydrological, hydroclimatic and sociohydrological interactions. The planetary boundaries framework outlines a safe operating space for humanity according to key Earth system dynamics. This Perspective proposes the addition of a green water planetary boundary based on root-zone soil moisture and demonstrates that widespread green water modifications now present increasing risks to Earth system resilience. 

/pdf/a-planetary-boundary-for-green-water-2mi6x6yj.pdf

A planetary boundary for green water

In many regions and at the planetary scale, human pressures on the environment exceed levels that natural systems can sustain. These pressures are caused by networks of human activities, which often extend across countries and continents due to global trade. This has led to an increasing requirement for methods that enable absolute environmental sustainability assessment (AESA) of anthropogenic systems and which have a basis in life cycle assessment (LCA). Such methods enable the comparison of environmental impacts of products, companies, nations, etc., with an assigned share of environmental carrying capacity for various impact categories. This study is the first systematic review of LCA-based AESA methods and their applications. After developing a framework for LCA-based AESA methods, we identified 45 relevant studies in the existing literature through an initial survey, database searches and citation analysis. We characterized these studies according to their intended application, impact categories, basis of carrying capacity estimates, spatial differentiation of environmental model and principles for assigning carrying capacity. We then characterized all method applications and synthesized their results. Based on this assessment, we present recommendations to practitioners on the selection and use of existing LCA-based AESA methods, as well as ways to perform assessments and communicate results to decision-makers. Furthermore, we identify future research priorities intended to extend coverage of all components of the proposed method framework, improve modeling and increase the applicability of methods.

/pdf/review-of-life-cycle-based-methods-for-absolute-59rzatc5zi.pdf

Review of life-cycle based methods for absolute environmental sustainability assessment and their applications

Abstract The rapid growth of plastics production exacerbated the triple planetary crisis of habitat loss, plastic pollution and greenhouse gas (GHG) emissions. Circular strategies have been proposed for plastics to achieve net-zero GHG emissions. However, the implications of such circular strategies on absolute sustainability have not been examined on a planetary scale. This study links a bottom-up model covering both the production and end-of-life treatment of 90% of global plastics to the planetary boundaries framework. Here we show that even a circular, climate-optimal plastics industry combining current recycling technologies with biomass utilization transgresses sustainability thresholds by up to four times. However, improving recycling technologies and recycling rates up to at least 75% in combination with biomass and CO 2 utilization in plastics production can lead to a scenario in which plastics comply with their assigned safe operating space in 2030. Although being the key to sustainability and in improving the unquantified effect of novel entities on the biosphere, even enhanced recycling cannot cope with the growth in plastics demand predicted until 2050. Therefore, achieving absolute sustainability of plastics requires a fundamental change in our methods of both producing and using plastics. 

/pdf/towards-circular-plastics-within-planetary-boundaries-mzjler52.pdf

Towards circular plastics within planetary boundaries

The planetary boundaries framework contains regional boundaries in addition to global boundaries. Geographically resolved methods to assess regional environmental impacts are therefore needed. Existing planetary boundaries-based life cycle assessment (PB-LCA) methods have limited geographical resolution or are not applicable to full product systems, due to high spatial requirements on inventory data. Here, we enable PB-LCA of full product systems across a comprehensive set of regional and global PB impact categories. We propose comparing environmental impacts of individual processes within a product system to assigned shares of regional or global safe operating space (SOS). This is followed by aggregation of process-level results so that accumulated exceedance of assigned SOS is derived across the entire life cycle. We then present a procedure for aggregating geographically resolved characterization factors (CFs) and SOS to country, continent and global levels, ensuring compatibility with typical life cycle inventory results. We then apply the new techniques to a model laundry case study. It involves 61 selected processes, two geographically resolved PB-LCA methods, related to impacts from freshwater use and nitrogen emissions, and a largely spatially generic PB-LCA method that covers a comprehensive set of impact categories. The calculation of accumulated exceedance of assigned SOS may help inform decisions about where in a life cycle to focus impact reduction efforts most urgently. The number of case study processes that exceed their assigned SOS differs when applying the geographically resolved methods, as opposed to the largely spatially generic method. Case study results differ greatly across PB-LCA impact categories, reaffirming the importance of covering a comprehensive set. Geographically resolved methods are needed for all regional impact categories and software support would be advantageous. Existing methods will require periodic updates to reflect ongoing advancements in PB science. Best practice approaches or a consensus for assigning regional SOS to processes are needed. Our study provides a step towards greater operability of geographically resolved PB-LCA methods by enabling application to a full product system within an assessment that covers a comprehensive set of impact categories. The case study application shows potential advantages of the process-level approach and points to the need for quantifying uncertainties in such assessments. Future studies should seek to explore the potential role of PB-LCA in decision support compared with conventional LCA.

/pdf/life-cycle-assessment-applying-planetary-and-regional-35fvji9oho.pdf

Life cycle assessment applying planetary and regional boundaries to the process level: a model case study

A planetary boundary-based method for freshwater use in life cycle assessment: Development and application to a tomato production case study

A comprehensive planetary boundary-based method for the nitrogen cycle in life cycle assessment: Development and application to a tomato production case study.

Challenges and opportunities towards improved application of the planetary boundary for land-system change in life cycle assessment of products

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A comprehensive planetary boundary-based method for the nitrogen cycle in life cycle assessment: Development and application to a tomato production case study.

Challenges and opportunities towards improved application of the planetary boundary for land-system change in life cycle assessment of products