Matching scope, purpose and uses of planetary boundaries science

Background: The Planetary Boundaries concept (PBc) has emerged as a key global sustainability concept in international sustainable development arenas. Initially presented as an agenda for global sustainability research, it now shows potential for sustainability governance. We use the fact that it is widely cited in scientific literature (>3500 citations) and an extensively studied concept to analyse how it has been used and developed since its first publication. Design: From the literature that cites the PBc, we select those articles that have the terms ‘planetary boundaries’ or ‘safe operating space’ in either title, abstract or keywords. We assume that this literature substantively engages with and develops the PBc. Results: We find that 6% of the citing literature engages with the concept. Within this fraction of the literature we distinguish commentaries—that discuss the context and challenges to implementing the PBc, articles that develop the core biogeophysical concept and articles that apply the concept by translating to sub-global scales and by adding a human component to it. Applied literature adds to the concept by explicitly including society through perspectives of impacts, needs, aspirations and behaviours. Discussion: Literature applying the concept does not yet include the more complex, diverse, cultural and behavioural facet of humanity that is implied in commentary literature. We suggest there is need for a positive framing of sustainability goals—as a Safe Operating Space rather than boundaries. Key scientific challenges include distinguishing generalised from context-specific knowledge, clarifying which processes are generalizable and which are scalable, and explicitly applying complex systems’ knowledge in the application and development of the PBc. We envisage that opportunities to address these challenges will arise when more human social dimensions are integrated, as we learn to feed the global sustainability vision with a plurality of bottom-up realisations of sustainability.


Introduction
Achieving sustainability is a global concern because many environmental processes that shape and influence humanity, such as climate change, operate globally and connect across multiple temporal and spatial scales (Liu et al 2013(Liu et al , 2007. Science informing sustainable development must therefore be a concerted effort with a global vision.
The planetary boundaries concept (PBc), by Rockström et al (2009aRockström et al ( , 2009b represents such an effort. Indeed, PBc authors identify changes (climate change, disturbance to nutrient cycles, land use changes), uses (freshwater use, biodiversity loss) and absorption processes (ocean acidification, atmospheric aerosol loading, stratospheric ozone depletion and chemical pollution) for which there are limits to what the Earth can support while maintaining Holocene-like functioning. Global limits are quantified based on the precautionary principle, to avoid a rising risk of creating and/or reaching large-scale biophysical Earth system thresholds (Box 1; Rockström et al 2009a, Steffen et al 2015. According to the PBc, the relatively low variability in Earth system dynamics that was characteristic of the Holocene epoch represents a global safe operating space for humanity. Achieving sustainability is understood as an increasingly pressing concern, as four critical Earth system processes have already overshot their boundary values. The initial aim of Rockström et al (2009b) was to establish an agenda for global sustainability research, but the concept has become prominent in sustainability governance and science-policy initiatives (Galaz et al 2012b), even inspiring the mission statement of the United Nations 2030 Agenda (UN 2015). The PBc has also been debated extensively within academia, with more than 3500 academic citations (source: Web of Science, May 2019).
This concept is in line with today's dominant scientific and political discourse: for already more than thirty years, global sustainability policy (UN 1987) has recognized how the environment supports and shapes humanity, and how humanity in turn influences its environment. PBc serves as a tool with which to relate human impacts to biogeophysical dynamics that are ideal (or aspirational) for humanity (i.e. Holocene-like dynamics). In the concept, humanity implicitly underlies critical Earth system processes, for example land use change, which is seen entirely as anthropogenic. Also, humanity is an important driver of the control variables behind each Earth system process. However, though the PBc points to the clear need for constraints on the human perturbation of global environmental processes, it has a limited articulation of links between its biophysical processes and more specific human processes. Indeed, society-the social organisations that act and react to their environment-is absent from these Earth system processes.
Here, we analyse the PBc's development and uses, to determine how it is being applied, with a particular focus on missing human dimensions. We relate applications of the concept to articles that discuss and review its context and challenges (labelled commentaries-viewed as the concept's evolving mandate) and to developments of the concept's core framework (the concept's scope). This analysis informs us how PBc needs to develop to fulfil its mandate and thus points to new research directions and specifications to render the PBc operational.

Methodology-literature search and categorisation
We used Web of Science to identify references that cite the original PBc publications by Rockström et al (2009aRockström et al ( , 2009b)-777 and 3108 references respectivelyaccessed on 6 May 2019. We selected those that use terms 'planetary boundaries' or 'safe operating space' in title, abstract, and/or keywords. We manually excluded a foreign language reference, a reprint, as well as references that only summarily mention the concept. We added references we are aware of, that cite the concept and/or are themselves cited in this context-but that do not appear in Web of Science (e.g. Raworth 2012, Crépin and Folke 2014, Fanning and O'Neill 2016. We obtained a total of 224 references (figure 1-note that 2 response articles are excluded from the count). Our assumption is that we in this way selected only articles that explicitly apply and build on the PBc. Although our literature search potentially missed some relevant research, we believe that our selection gives a comprehensive overview of academic research carried out on the PBc. Figure 1 shows how we categorised the literature. 54% of the literature published to date discusses the PBc without advancing the scientific basis or applying the concept in practical or policy contexts. We labelled this category 'commentaries'. 14% of articles focus on further advancing the scientific underpinnings of the PBc as a biophysically-expressed framework, and 32% seek to use the concept to evaluate sustainability at sub-global scales. We group these last two categories -development and uses-as 'applications' of the PBc (see table S1 in supplementary materials, available online at stacks.iop.org/ERL/14/073005/mmedia).
We made simple word-clouds of keywords (when available) from commentary (figure 2(a)) and applied articles respectively (figure 2(b)). We omitted the terms Box 1. Planetary boundaries-semantics and science.
The concept is rooted both in Earth system science and in 'resilience thinking', the notion that systems can exist in functionally and structurally different dynamical states, and that a system can change state relatively suddenly in response to even gradual changes in conditions. As a system approaches a state transition, its resilience erodes (Berkes et al 2003). State transitions-or regime shifts-are not always directly or even at all reversible. Boundaries presented in the PBc are set at a cautionary distance from potential Earth system tipping points. Science has yet to uncover the conditions under which tipping points of each critical Earth system process might exist, or what lies beyond such tipping points (Hughes et al 2013). This knowledge is masked by another key characteristic of the concept: the recognition that all Earth system processes are connected and dependent, within and across spatio-temporal scales. In 2015, Steffen et al (2015) published an updated analysis of the PBc, reviewing the state of the art in the respective research fields of individual boundaries, expanding the arguments and rationale for setting large-scale boundaries, and including original modelbased analysis.
'Planetary Boundary(ies)' and 'Safe Operating Space' from the keyword analyses, because they were the selection terms for all the articles in the first place. We filtered for words that appear at least twice. Font-size reflects absolute frequency of the terms (see supplementary materials). This analysis gives semi-quantitative confirmation that commentary articles prioritise different facets of the PBc than the articles focused on the further development and application of the concept (figures 2(a) and (b)).

Results-tracking the progress of PBc research
Most citations of the Rockström et al (2009aRockström et al ( , 2009b articles use the term 'planetary boundaries' as shorthand for issues of global unsustainability, whereas 6% of the literature that cites the concept engages explicitly and substantively with the framework. This is the fraction of the literature that we delve into here.
Commentaries: a new global sustainability debate 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 ( 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, 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. 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  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.

Scientific development of the PBc
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. 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).
Applications: the missing social dimensions 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
Needs look at how boundary values match against basic human necessities for life resonant with well-established conceptualizations of sustainable development in (UN 1987, Max-Neef 1991, O'Neill 2011. Water and food are the dominant topics in the needs perspective. Literature here highlights that projected human needs for water will overshoot the PB (e.g. Grafton et al 2015)-also for nitrogen-but see de Vries et al (2013) for a countering view. Rockström et al (2012) identify that freshwater availability does not suffice to feed humanity and to sequester carbon to curb climate change, and conclude that for water to be sufficient for human needs, humanity must limit climate change by reducing its carbon emissions. Bogardi et al (2013) use the freshwater boundary process to exemplify that a framework of planetary, ecosystem-based and social needs, is necessary to achieve sustainable resource use. Here humanity's purely functional freshwater needs are related to societal aspirations, which are seen as materialistic and currently unsustainable. de Vries et al (2013) reestimate the nitrogen boundary by adding a measure of per capita dietary nitrogen needed to feed humanity to the limit of nitrogen that the biosphere can process. Looking at nitrogen and phosphorus flows, Kahiluoto et al (2014) find nutrient uses exceed boundary values, but highlight the spatial disparity in nutrient excesses and needs, implying that local targets and resource redistribution, in addition to behavioural changes in diets, waste and recycling are necessary to implement sustainable resource use (Kahiluoto et al 2015). The needs perspective relates boundary processes and human use of resources in a functional and pragmatic way. For instance, O'Neill et al (2018) connect the basic needs approach, as framed in (Raworth 2017) with the planetary boundaries using 'provisioning systems' that represent links between resource use and social outcomes. In this way 'humanity' as seen through the needs perspective-even in scaled subglobal models-reflects a generalised global human. Indeed, resource distributions as well as societies' production and consumption patterns are revealed, but not explicitly addressed.

Aspirations
Framed as they are in the PBc and in the literature that builds on the concept, aspirations for humanity are global in scale, and are normative perspectives involving judgements about goals that people should collectively strive for, such as providing a resilient planetary system for future generations. In the literature analysed here, these aspirations for a 'global humanity' tacitly presuppose global policy; complementing social science perspectives of Okereke (2006) and Lockie (2016) that discuss wider implications of such a future-framing of global sustainability. Aspirations range from governments' social priorities (Raworth 2012, Dearing et al 2014), through Millennium Development Goals (Gerst et al 2013), to the Sustainable Development Goals that are now seen as the most up-to-date collective social targets and statements of humanity's aspirations (Hajer et al 2015). When aspirations are framed as positive levels to strive for, such as the safe and just operating space for humanity defined in 'Doughnut Economics' (Raworth 2012(Raworth , 2017, they reflect a social minimum standard. When they aspirations are framed as catastrophes to avoid, for example the 'Boundary Risks for Humanity and Nature' framework (Baum and Handoh 2014), they reflect a maximum limit.

Behaviours
In this categorisation, behaviours are the means by which humanity can reach shared global targets and/ or avoid catastrophes. The literature in this area adds a global dimension to existing sustainable behaviour research at the individual and community level (e.g. reviews in Barr andGilg 2006, Heiskanen et al 2010) and on governance through global actions (e.g. Bäckstrand 2008, Hale 2008, Bernstein and Cashore 2012. Some PBc research focuses on the behaviours needed to drive humanity away from all boundaries. For example, Robèrt et al (2013) outline a framework to define and reach-through sustainability principles and guidelines-a socio-ecological safe operating space. Other articles discuss staying within a specific combination of boundaries from a governance perspective (e.g. Nilsson and Persson 2012) or through bioengineering methods (Heck et al 2016). Yet others focus on managing humanity's sustainable development with reference to a single boundary, recommending such actions by state and business communities as 'green chemistry' to remain clear of chemical pollution limits (Tarasova et al 2015). We here see the option of achieving global sustainability through lifestyle transformations-i.e. driven from individuals up through social systems-that lead to low carbon futures within the climate change boundary (Neuvonen et al 2014), or of assessing the scale of social and cultural transformation needed to reach a social-ecological safe operating space (Gerst et al 2013). Behaviour perspectives reflect a conscious, enabled, self-determining nature of humanity, and the literature here describes humanity at many different resolutions: from differentiated cultural individuals, through social and governance systems to States and the generalised global Human.

Impacts
Impacts are the most common perspective we find in the literature (reflecting connections with well-established and diverse fields of environmental impact assessment and climate impacts research). Impacts are measures of the effects of human activities on the Earth system processes described in the PBc (e.g. Bringezu et al 2012, Heijungs et al 2014). They are mostly measured using either footprints or life cycle assessment approaches. A footprints approach consists of assessing the appropriation and use of a resource (for example carbon) by an individual, nation or globally (Hoekstra and Wiedmann 2014). A footprint is sustainable if the use of the resource enables it to regenerate at a rate sufficient to make it available and usable by future generations. Life cycle assessments focus on minimising the environmental impact of all different processes in the production of goods. When coupling these concepts with the PBc approach, footprints and life cycle assessments' maximum sustainable environmental impacts are derived from the boundary values of the relevant Earth system process. Whether these methods are suited to the PBc or not is still debated (Ryberg et al 2016)-as we discuss in the discussion section on scale. 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.

Discussion-challenges ahead
The context and challenges of the global sustainability discourse are clear: humanity is leaving an environmentally safe space while still trying to reach a socially just place, and this journey is happening during the Anthropocene, an epoch where humanity finds itself at the helm of global environmental change, yet also at the cusp of unprecedented shifts in Earth system dynamics.
In the following section, we discuss mismatches in the mandate, scope and applications of PBc science along three topics that emerge from the literature reviewed here: Who is the human? What is the goal? And Where is the action? We then propose a plan for the development of PBc science with a framework for the integration and implementation of resilience thinking, acknowledging that different fields of study must connect and inform each other in order to make the messages of global sustainability science more useable.

Who is the human?
In much of the literature analysed, humanity is seen as a globally uniform biological and/or economicopolitical entity-as in the 'Anthropocene' (Stubblefield 2018)  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 organisationand 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, industrialisedindustrialising-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 actionoriented 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.
What is the goal? 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 2012Raworth , 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(Raworth , 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).
Where is the action? 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 subglobal '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 bottomup perspectives that span from local to global scales.
Where is resilience? 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).

Conclusions
There has been considerable academic interest around the planetary boundaries concept. The body of literature engaging with the concept is developing along coherent themes, where social dimensions are coming into clearer focus, and the PBc is increasingly presented as the embodiment of the Anthropocene and global sustainability agenda. However, we find that the concept's scope and mandate are not always aligned. Indeed, our literature search on this growing field highlights a rift between the science that analyses the concept and the science that develops and uses it. Commentaries present explicit, dynamic and complex human dimensions and emphasize the importance of resilience thinking. Nonetheless, the core structure and elements of the PBs remain essentially the same: society is implicit, and resilience thinking purely theoretical. Uses rest in a middle ground, with an explicit but underspecified human, where resilience thinking is mostly ignored.
Through this review and analysis, we identify key avenues that SOS research should take in order for scope to match mandate, so it can stand out as an effective sustainability-informing concept. SOS research needs to explicitly take a human society on board its framework, joining forces with fields of humanities to better understand drivers of human behaviours in different cultural, historical and natural contexts. Our sustainability-assessing framework shows where this knowledge should be filled in, and this should happen at the smallest relevant action scale. Scaling-up from the action scale, connecting across and within scales and explicitly applying resilience thinking remain key scientific challenges. However, opportunities to address these challenges will arise when more human dimensions are integrated, as we learn to feed the global SOS vision with a plurality of societal realisations of sustainability.
Box 2. Challenges to combining multidisciplinary data.
Though the PBc mentions resilience and multi-scale interconnectedness, these facets are not applied in the concept itselfthe boundaries are presented as fixed and independent (de Vries et al 2013)-leaving resilience and multi-scale interconnectedness to be freely ignored in subsequent research. There are pragmatic reasons for these omissions. For example, different Earth system processes-and additional social values-are all expressed in different units that are often incompatible and incommensurate. Strategies to combine different fields of research tend to oversimplify the system being represented and seldom represent all fields on an equal footing (Stone-Jovicich 2015). So far, most common methods include converting all units to a single one, for example carbon or a currency. This approach excludes the comparison of continuous versus discrete processes, reduces the resolution of existing data, adds uncertainty with regards to fluctuations in exchange rates and hides the variability in conversion factors. Another common approach is to average out different indicators as one index, such as the Human Development Index or the Ocean Health Index, which aim to characterise complex social-ecological systems with a single number. Though these indices have a clear pragmatic aspect, they are not sufficient to characterize the multitude of diverse facets of complex social-ecological systems. Attempts to raise or lower such an index risk yielding short-sighted, singular solutions that have unintended consequences on the overall system.