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RESEARCH ( RESEARCHARTICLE
<br />reversible. The challenge to the research commu-
<br />nity is to develop the knowledge base that allows
<br />the screening of chemicals, before they are re-
<br />leased into the environment, for properties that
<br />may predispose them toward becoming global
<br />problems.
<br />As a first step toward meeting this challenge,
<br />the three conditions outlined above have been
<br />used as the basis for identifying scenarios of
<br />chemical pollution that fulfill the conditions and
<br />as a next step for pinpointing chemical profiles
<br />that fit the scenarios (28). This proposal consti-
<br />tutes a first attempt at adding the Earth - system
<br />perspective when assessing hazard and risk of
<br />chemicals and offers a vision for a systematic ap-
<br />proach to a complex management situation with
<br />many unknowns.
<br />Despite this progress in developing an Earth -
<br />system- oriented approach, there is not yet an
<br />aggregate, global -level analysis of chemical pol-
<br />lution on which to base a control variable or a
<br />boundary value. It may also serve little purpose
<br />to define boundary values and control varia-
<br />bles for a planetary boundary of this complexity.
<br />Nevertheless, there is a potential threat from
<br />novel entities to disrupt the functioning of the
<br />Earth - system and society needs to learn how to
<br />mitigate these unknown risks and manage chem-
<br />icals under uncertainty (28, 73).
<br />Some precautionary and preventive actions
<br />can be considered. These may include a stronger
<br />focus on green chemistry (78), finding synergies
<br />with risk- reducing interventions in other fields
<br />such as occupational health (79), paying more
<br />attention to learning from earlier mistakes (80,
<br />81), and investing in science to better under-
<br />stand and monitor vital Earth- system processes
<br />in order to be able to detect disruptive effects
<br />from novel entities as early as possible.
<br />Hierarchy of boundaries
<br />An analysis of the many interactions among the
<br />boundaries (table S3 and fig. S10) suggests that
<br />two of them — climate change and biosphere
<br />integrity —are highly integrated, emergent system -
<br />level phenomena that are connected to all of the
<br />other PBs. They operate at the level of the whole
<br />Earth system (7) and have coevolved for nearly
<br />4 billion years (82). They are regulated by the
<br />other boundaries and, on the other hand, pro-
<br />vide the planetary -level overarching systems with-
<br />in which the other boundary processes operate.
<br />Furthermore, large changes in the climate or in
<br />biosphere integrity would likely, on their own,
<br />push the Earth system out of the Holocene state.
<br />In fact, transitions between time periods in Earth
<br />history have often been delineated by substantial
<br />shifts in climate, the biosphere, or both (82, 83).
<br />These observations suggest a two -level hierar-
<br />chy of boundaries, in which climate change and
<br />biosphere integrity should be recognized as core
<br />planetary boundaries through which the other
<br />boundaries operate. The crossing of one or more
<br />of the other boundaries may seriously affect hu-
<br />man well-being and may predispose the trans-
<br />gression of a core boundary(ies) but does not by
<br />itself lead to a new state of the Earth system. This
<br />hierarchical approach to classifying the bounda-
<br />ries becomes clearer by examining in more detail
<br />the roles of climate and biosphere integrity in the
<br />functioning of the Earth system.
<br />The climate system is a manifestation of the
<br />amount, distribution, and net balance of energy
<br />at Earth's surface. The total amount of energy
<br />sets the overall conditions for life. In Earth's cur-
<br />rent climate, a range of global surface temper-
<br />atures and atmospheric pressures allows the three
<br />phases of water to be present simultaneously,
<br />with ice and water vapor playing critical roles in
<br />the physical feedbacks of the climate system. The
<br />distribution of energy by latitude, over the land
<br />and sea surfaces, and within the ocean plays a
<br />major role in the circulation of the two great
<br />fluids, the ocean and the atmosphere. These sys-
<br />temic physical characteristics are key spatial de-
<br />terminants of the distribution of the biota and
<br />the structure and functioning of ecosystems and
<br />are controllers of biogeochemical flows.
<br />Biosphere integrity is also crucial to Earth -
<br />system functioning, where the biosphere is de-
<br />fined as the totality of all ecosystems (terrestrial,
<br />freshwater, and marine) on Earth and their biota
<br />(32). These ecosystems and biota play a critical
<br />role in determining the state of the Earth system,
<br />regulating its material and energy flows and its
<br />responses to abrupt and gradual change (7). Di-
<br />versity in the biosphere provides resilience to
<br />terrestrial and marine ecosystems (83, 84). The
<br />biosphere not only interacts with the other plan-
<br />etary boundaries but also increases the capacity
<br />of the Earth system to persist in a given state under
<br />changes in these other boundaries. The ultimate
<br />basis for the many roles that the biosphere plays
<br />in Earth - system dynamics is the genetic code of
<br />the biota, the basic information bank that de-
<br />fines the biosphere's functional role and its ca-
<br />pacity to innovate and persist into the future.
<br />Planetary boundaries in a
<br />societal context
<br />A proposed approach for sustainable develop-
<br />ment goals (SDGs) (85) argues that the stable
<br />functioning of the Earth system is a prereq-
<br />uisite for thriving societies around the world. This
<br />approach implies that the PB framework, or
<br />something like it, will need to be implemented
<br />alongside the achievement of targets aimed at
<br />more immediate human needs, such as provi-
<br />sion of clean, affordable, and accessible energy
<br />and the adequate supply of food. World devel-
<br />opment within the biophysical limits of a stable
<br />Earth system has always been a necessity [e.g.,
<br />(86, 87)]. However, only recently, for a number
<br />of reasons, has it become possible to identify,
<br />evaluate, and quantify risks of abrupt planetary -
<br />and biome -level shifts due to overshoot of key
<br />Earth - system parameters: (i) the emergence of
<br />global - change thinking and Earth - system think-
<br />ing (SS); (ii) the rise of "the Planetary" as a rel-
<br />evant level of complex system understanding
<br />(89 -92); and (iii) observable effects of the rapid
<br />increase in human pressures on the planet (I6).
<br />The PB approach is embedded in this emerg-
<br />ing social context, but it does not suggest how to
<br />maneuver within the safe operating space in the
<br />quest for global sustainability. For example, the
<br />PB framework does not as yet account for the re-
<br />gional distribution of the impact or its histor-
<br />ical patterns. Nor does the PB framework take
<br />into account the deeper issues of equity and cau-
<br />sation. The current levels of the boundary pro-
<br />cesses, and the transgressions of boundaries that
<br />have already occurred, are unevenly caused by
<br />different human societies and different social
<br />groups. The wealth benefits that these trans-
<br />gressions have brought are also unevenly distrib-
<br />uted socially and geographically. It is easy to
<br />foresee that uneven distribution of causation and
<br />benefits will continue, and these differentials
<br />must surely be addressed for a Holocene -like
<br />Earth - system state to be successfully legitimated
<br />and maintained. However, the PB framework as
<br />currently construed provides no guidance as to
<br />how this may be achieved [although some po-
<br />tential synergies have been noted (54)], and it
<br />cannot readily be used to make choices between
<br />pathways for piecemeal maneuvering within
<br />the safe operating space or more radical shifts of
<br />global governance (93).
<br />The nature of the PB framework implies that
<br />two important cautions should be observed when
<br />application of the framework to policy or man-
<br />agement is proposed: boundary interactions and
<br />scale.
<br />Boundary interactions
<br />The planetary boundaries framework arises from
<br />the scientific evidence that Earth is a single,
<br />complex, integrated system —that is, the bound-
<br />aries operate as an interdependent set [e.g.,
<br />(94)] (table S1 and fig. S10). Although a system-
<br />atic, quantitative analysis of interactions among
<br />all of the processes for which boundaries are
<br />proposed remains beyond the scope of current
<br />modeling and observational capacity, the Earth
<br />system clearly operates in well- defined states in
<br />which these processes and their interactions
<br />can create stabilizing or destabilizing feedbacks
<br />(16, 90, 95). This has profound implications for
<br />global sustainability, because it emphasizes the
<br />need to address multiple interacting environ-
<br />mental processes simultaneously (e.g., stabilizing
<br />the climate system requires sustainable forest
<br />management and stable ocean ecosystems).
<br />Scale
<br />The PB framework is not designed to be "down -
<br />scaled" or "disaggregated" to smaller levels, such
<br />as nations or local communities. That said, the
<br />PB framework recognizes the importance of
<br />changes at the level of subsystems in the Earth
<br />system (e.g., biomes or large river basins) on the
<br />functioning of the Earth system as a whole. Also,
<br />there are strong arguments for an integrated ap-
<br />proach coupling boundary definitions at region-
<br />al and global levels with development goals to
<br />enable the application of "PB thinking" at lev-
<br />els (nations, basins, and regions) where policy
<br />action most commonly occurs [e.g., (85, 96)].
<br />This update of the PB framework is one step on
<br />a longer -term evolution of scientific knowledge to
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