Matching scope, purpose and uses of planetary boundaries science

From a structural perspective, articles categorised as commentaries (120 articles of the 224 identified, or 54%) focus mainly on the concept of the safe operating space, and much less on the boundaries themselves (i.e. the processes, proposed control variables and their boundary values). Dominant keywords of commentary articles are 'Anthropocene', 'resilience', 'sustainability' and phrases including 'governance' (figure 2(a)). These papers highlight a global context in which the PBc is evolving, issues that might prevent it becoming operational, and the needs it might be expected to fulfil. In this literature, we find that the humanities have recently engaged with the PBc (Brown 2017), discussing how to navigate today's framing of sustainability and the Anthropocene (e.g. Bennett and Teske 2017, Stubblefield 2018, Wakefield 2018), as well as the values and risks of the PBc framing (McAllum 2018), narratives (Kunnas 2017) and visualisations (Morseletto 2017).

Several articles focus on new governance challenges that the PBc's Earth system perspective brings. They point out that for institutions to support global sustainable development, they need to better understand the dynamics of critical Earth system processes, how they connect, and the scales at which they operate (Bogardi et al 2012, Galaz et al 2012a, 2012b, 2016, Pereira et al 2015, Nash et al 2017), as well as a need to understand how different institutions are themselves structured and connected (Galaz et al 2012b, Reischl 2012, Ahlström and Cornell 2018). Further governance challenges lie in identifying viable, compatible goals (Biermann 2012, Pereira et al 2015), and in the ability to manage transformative change (Folke et al 2011, Galaz 2012, Pereira et al 2015). From this perspective, there is a call for change: in order to navigate pathways towards resilience and global sustainability, governance should encourage learning and innovation, be flexible to uncertainty and encompass indicators and review mechanisms for the complex global processes that Earth system science now illuminates (Galaz et al 2012a, Hepburn et al 2014). These lines of enquiry have been reframed and emphasized since 2015 and the adoption of the United Nations' 2030 Agenda. 'Sustainable Development Goals' is a frequent keyword (figure 2(a)), and nearly a quarter of the review literature discusses the Sustainable Development Goals, despite Agenda 2030's relative youth.

Another area of focus addresses the practical and political challenges to applying the PBc (Mouysset et al 2018), such as perceived trade-offs between society and the environment (Messerli et al 2015, Saunders 2015), between economic growth and sustainable environmental and/or social development (Hepburn et al 2014, Gómez-Baggethun and Naredo 2015, Saunders 2015, Cumming and von Cramon-Taubadel 2018, Velenturf and Jopson 2019), and between differing North-South perspectives on development priorities, values, needs and rights (Kim and Bosselmann 2015, Saunders 2015, Figueroa-Helland et al 2016). The need for interdisciplinary research better linking human drivers and social and biophysical impacts is also highlighted (van Vuuren et al 2016). A further line of attention addresses scientific realities and challenges to applying the PBc, such as the global scale at which boundaries are defined, connectedness between boundary processes across spatial and temporal scales, uncertainty associated with connectedness, and current limits to our scientific knowledge of Earth process dynamics (Rockström et al 2014, Liu et al 2015).

We found three main areas of boundary development: firstly (re-) defining the metrics and control variables for the fundamental Earth system processes in the framework. For example, Mace et al (2014) challenge the usability of biodiversity loss rates as a metric and suggest loss of genetic and functional diversity as well as biome condition as variables that better reflect changes to core Earth system functioning. A second area of development lies in (re-) evaluating boundary values Gerten et al (2013), for instance, propose a new value for the freshwater boundary by including environmental flow requirements to the assessment, concluding that rates of human use of freshwater should be lower than previously estimated. The third line of development of the concept focuses on understanding interactions between Earth system processes or between Earth system states (Anderies et al 2013, Larsen et al 2014, Heitzig et al 2016, Hellmann et al 2016), which serves as a preamble to reassessing boundary values by accounting for the interdependencies of processes.

Biodiversity, land use and water use boundaries have received most constructive critique (Table S1, supplementary materials). Of the two unquantified critical Earth system processes, only chemical pollution has spawned further research (Persson et al 2013, Sala and Goralczyk 2013, Handoh and Kawai 2014, MacLeod et al 2014, Villarrubia-Gómez et al 2018), whereas atmospheric aerosol loading has not been developed. Although our classification does not give a fine-grained content analysis, we find that nearly half the articles that develop the concept address more than one boundary process, many of which address the broader set of processes (see table S1). Some recent publications seek to develop the 'safe operating space' concept, exploring how humanity can navigate between different dynamic operating spaces—rather than remaining within static boundaries—and devising techniques to map the numerical and theoretical stability of potential safe operating spaces (Heitzig et al 2016, Hellmann et al 2016).

A growing branch of research seeks to advance the applicability of the PBc by explicitly addressing the human dimensions of the biogeophysically expressed boundary processes. Studies point out that implementing the PBc will always encounter the need to deal with society's decision-making and action scales (Häyhä et al 2016), for example at national and regional scales (e.g. Kahiluoto et al 2015), or at the level of production systems (e.g. Sandin et al 2015). Studies present different methods for 'translating' the concept to sub-global levels, including such novel approaches as in Cole et al (2014) that proposes a decision-based methodology for the national level, or Dearing et al (2014) that links social well-being and sustainable resource management on a regional scale.

In the science advancing the applicability of the PBc, four terms emerge from the content analysis, these are: needs, aspirations, behaviours and impacts. We use these four terms to categorise the applied literature following these human perspectives (figure 1, table S1), and describe each one in the following text.

Needs

Aspirations

Behaviours

Impacts

Combined perspectives

A third of the literature that includes society addresses a combination of these four perspectives of needs, aspirations, behaviours and impacts (e.g. Gerst et al 2013, bridging aspirations and behaviours or Rockström et al 2012, Bogardi et al 2013, combining needs and impacts). All possible two-way combinations are represented (supplementary materials S1). We suggest that this indicates how the PBc (and its global biophysical framing) has catalysed discussions that bridge these social perspectives, and has raised fresh questions about sustainability. Indeed, we would argue that these four perspectives can and should inform one another more than they currently do. Put together, they have the potential to form a framework that deals with the missing human dimensions of the PBc (figure 3). This framework allows a continuous (re-) assessment of pathways to and lifestyles within sustainability.

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In much of the literature analysed, humanity is seen as a globally uniform biological and/or economico-political entity—as in the 'Anthropocene' (Stubblefield 2018): be it as an unspecified consumer and producer of resources (e.g. Kahiluoto et al 2014) or a global holder of basic human rights (e.g. Raworth 2012). For instance, O'Neill et al (2018)'s national-level assessments of social needs met versus environmental boundaries overshot reveal the heterogeneity in the realisations of the PB: few countries are equal in the needs met and boundaries transgressed. However, the 'human' remains generalised, and prone to exist either in a space where social needs are met at the cost of the environment, or within environmental boundaries but out of reach of basic needs. There is a disconnect between commentaries that discuss issues of fairness of resource allocations (Saunders 2015) and the understanding of governance as a complex, multi-scale system of systems (Galaz et al 2012c)—which imply diversity and heterogeneity of social organisation—and applications where humanity is simplified to a globally generalised entity subject to global policy. Yet it is this biological and economico-political organism that is seen as the potential operator of sustainable development. The only approach we uncovered that applies the PBc to best empower social actors to achieve sustainability is McLaughlin (2018), who downscales the PBc to a relatively homogeneous region from a biogeophysical perspective, and highlights the presence of diverse human actors within this region. Overall, most commentaries bring forward human dimensions that are only hinted at in applications of the concept. These dimensions underlies topics of fairness, subjective value, and ethics (Neuvonen et al 2014, Sandin et al 2015, Saunders 2015, Häyhä et al 2016, Mavrommati et al 2016). These dimensions are dynamic and evolving and complement biological and political human facets. They cannot be described by biological growth models or rulebooks and laws—and imply fundamental diversity in human aspirations, psychologies, needs, behaviours and thus impacts. We argue that it is primarily the absence of such human dimensions that prevents the effective realisation of global sustainability concepts at sub-global scales, as scaling the current global sustainability vision translates to top-down—and oftentimes North-South, wealthy-poor, industrialised-industrialising—control and decision-making (Saunders 2015). This dichotomy is made clear in the findings of O'Neill et al (2018). To make actionable the findings and inform action to reach safe operating spaces, it is essential to understand the diverse people underlying the PBc's generalised Humanity.

We suggest that an added human dimension could be represented into the four broad perspectives through which social applications of the PBc are currently addressed (figure 3). The four dimensions that emerge from our literature analysis have analogous dimensions that emerge from the social sciences, indicating that there are established tools and frameworks with which to address them. Attention to expanded framings of needs and impacts helps to articulate the rationale and motivation for taking a global viewpoint on sustainability, giving more depth and realism to the social component of sustainable development. Attention to aspirations and behaviours strengthen the bridge from knowledge of unsustainability to action-oriented research, and thus potentially inform new solutions and challenges to achieving sustainable lifestyles and societies. Using the reflective questions in figure 3 helps shed a light on people's agency in the context of global change (obviously while specifying who the 'we' and 'our' relates to), and can provide insight into possible ways of applying PBc science to reach safe operating spaces.

The PBc set off to frame a Safe Operating Space for humanity, but instead of describing this space, current PBc research clearly focuses on thresholds: either boundaries that must be avoided at all cost (e.g. Baum and Handoh 2014), or basic targets that must be reached (e.g. Gerst et al 2013, Raworth 2012, 2017). There is little description of a social-ecological Safe Operating Space, encompassing system dynamics that lie both above social foundations and below environmental boundaries. The lack of any clear vision(s) of such a space (or spaces) is recognised as a problem for governance (Biermann 2012).

The United Nations' 2030 Agenda is taken as a consensus global goal framework, with worldwide legitimacy and accountability. Even though its social targets reflect a reality that is still far from humanity's situation today—in terms of poverty, education, health etc—they represent only some of the most basic social foundations and needs. We can easily assume that people aspire to more than having these needs met. The literature suggests that satisfying humanity's needs provides only a pass-mark; it fails to include the diverse, dynamic, complex and cultural aspects of societal ambitions. In Fauré et al (2016) for instance, the analysis of the tensions between the need to reduce environmental impacts while maintaining relatively high social welfare and participation levels in Sweden, perhaps illustrates how aspirations—when achieved—become seen as necessities. This perspective showcases how development pathways are shaped by fluid aspirations, not just fixed social foundations. There is risk in this realisation, for instance when the unsustainable lifestyles of many in the global North and of an extremely wealthy minority are aspirational goals. There is also opportunity perhaps, that aspirations of sustainability, combined with sound and specified scientific groundings of what is sustainable—and for whom—can effectively characterise the safe and just operating spaces for humanity and show way-markers for pathways through their terrain.

A critique of the PBc is that Earth system boundaries are presented as a maximum allowance (e.g. Heijungs et al 2014), rather than as a signpost to a fundamentally different and sustainable development route, which gives the impression that they might even be negotiable targets. This critique is neither new nor unique to the PBc: it is shared by carrying capacity (Verhulst 1838), global warming limits set by the Intergovernmental Panel on Climate Change and UN climate agreements (IPCC 2014), the social foundations and environmental ceiling of Doughnut Economics (Raworth 2012, 2017) and the Sustainable Development Goals (UN 2015). It may be true that the focus specifically on boundaries constitutes a negative framing of the sustainability discourse, with strong potential for self-sabotage towards goals of sustainability. Indeed, using an analogy from climbing: a successful climber will remember to look up to her/his destination, not down the cliff-side.

A better understanding and integration of human dimensions in the PBc will help not only define sustainable aspirations that different people can strive for, but also help frame the sustainability discourse in a constructive way. To this purpose, we recommend shifting away from referring to the 'planetary boundaries' and instead talking more about the Safe Operating Space(s) (SOS).

Implementing the SOS concept will always encounter the need to deal with society's decision-making and action scales (Häyhä et al 2016). We find that the scale at which humanity is being described is often unclear, which has repercussions on how well sustainability science can be translated into action (Reischl 2012, Galaz et al 2012b). Indeed, when seen as a generalised global human, 'humanity' is mostly seen as an object of global change, manipulated by global policy. However, when seen as an individual, community, organisation and society, humanity can be the subject and director of change. To understand and act upon global sustainability challenges we need both the bigger picture of the global human and its place on the planet as well as the detail on how social organisations (as biological, economic, political and social entities) shape and are shaped by the world around them.

The SOS concept is currently expressed at a global scale and in purely biophysical terms. This gives poor insights into how responsibility or rights over the Earth system processes are distributed. Furthermore, there is a disproportionate influence of the wealthy on processes such as climate change, paralleled with a disproportionate effect of climate change on the poor (Boonstra 2016). Aligned with this understanding of heterogeneously overlapping processes, research translating the SOS concept to the national level (Häyhä et al 2016) recommends scaling its biophysical, socio-economic and ethical aspects separately. This is mostly seen as a process of 'scaling-down' to the sub-global 'action' level. Another approach, by McLaughlin (2018), scales the SOS concept to a relatively homogeneous region from a biogeophysical perspective, but where the 'action'—implementing measures to redress human impacts—is then distributed across the diverse actors/stakeholders in the regions. In a way, this approach turns the SOS concept on its head, by homogenising the environmental context and diversifying 'Humanity'.

To integrate context-specific needs, behaviours, aspirations and impacts, with global sustainability challenges, there is a need to distinguish processes that are generalizable from those that are scalable, and to complement current top-down perspectives with bottom-up perspectives that span from local to global scales.

As a global perspective on sustainability, the PBc could ignore issues relating to the heterogeneous distribution of processes. The SOS concept pays heed to the fact that the processes and their interactions vary at sub-global scales, and as such its relation to resilience thinking (Folke 2016) becomes explicit.

Many authors use the footprints and life cycle assessment approaches to scale the concept to national levels. There are nonetheless fundamental differences that distinguish these approaches from resilience thinking. For example, the gradient of resilience (systems are more or less resilient, until they collapse) and the fundamental shift in functioning of systems underlying the planetary boundaries (box 1) are absent from footprint and life cycle assessment approaches to evaluate environmental (and social) impacts. Also, Earth system processes are interdependent, co-evolving and influencing each other within and across multiple scales (Watson 1999), thus leading to the global picture of a SOS that cannot be downscaled or disaggregated (Steffen et al 2015), and where Earth system thresholds are neither fixed nor predictable (Steffen et al 2018).

By ignoring resilience and interconnections between processes, the implicit assumption of many environmental impact assessment frameworks is that individual impacts can simply add-up to form a global assessment of impacts. However, while some processes might be independent overall, many processes in complex adaptive systems either enhance, (e.g. Kirby et al 2009), complement (e.g. Gable et al 2012) or cancel each other out (e.g. Yachi and Loreau 1999). There is clearly a need to refine the sciences of cross-scale dynamics and complex adaptive systems to make SOS science applicable across scales and systems (box 2).