A complex system is a system composed of many components that interact with one another. Examples of complex systems are Earth's global climate, organisms, the human brain, infrastructure such as power grid, transportation or communication systems, complex software and electronic systems, social and economic organizations (like cities), an ecosystem, a living cell, and, ultimately, for some authors, the entire universe.
The behavior of a complex system is intrinsically difficult to
model due to the dependencies, competitions, relationships, and other
types of interactions between their parts or between a given system and
its environment. Systems that are "complex" have distinct properties that arise from these relationships, such as nonlinearity, emergence, spontaneous order, adaptation, and feedback loops, among others. Because such systems appear in a wide variety of fields, the
commonalities among them have become the topic of their independent area
of research. In many cases, it is useful to represent such a system as a
network (graph) where the nodes represent the components and links represent their interactions.
The term complex systems often refers to the study of
complex systems, which is an approach to science that investigates how
relationships between a system's parts give rise to its collective behaviors and how the system interacts and forms relationships with its environment. The study of complex systems regards collective, or system-wide,
behaviors as the fundamental object of study; for this reason, complex
systems can be understood as an alternative paradigm to reductionism, which attempts to explain systems in terms of their constituent parts and the individual interactions between them.
Polycentric systems "where many elements are capable of
making mutual adjustments for ordering their relationships with one
another within a general system of rules where each element acts with
independence of other elements",
Disorganised systems which involve localized interactions of multiple entities that do not form a coherent whole. Disorganised systems are linked to self-organisation processes.
Hierarchical systems which are analyzable into successive sets of subsystems. They can also be called nested or embedded systems.
A system is decomposable if the parts of the system (subsystems) are independent from each other, for example the model of a perfect gas consider the relations among molecules negligible.
In a nearly decomposable system, the interactions between subsystems are weak but not negligible, this is often the case in social systems. Conceptually, a system is nearly decomposable if the variables
composing it can be separated into classes and subclasses, if these
variables are independent for many functions but affect each other, and
if the whole system is greater than the parts.
Features
Complex systems may have the following features:
Complex systems may be open
Complex systems are usually open systems – that is, they exist in a thermodynamic gradient and dissipate energy. In other words, complex systems are frequently far from energetic equilibrium: but despite this flux, there may be pattern stability, see synergetics.
Complex systems may exhibit critical transitions
Graphical
representation of alternative stable states and the direction of
critical slowing down prior to a critical transition (taken from Lever
et al. 2020). Top panels (a) indicate stability landscapes at different conditions.
Middle panels (b) indicate the rates of change akin to the slope of the
stability landscapes, and bottom panels (c) indicate a recovery from a
perturbation towards the system's future state (c.I) and in another
direction (c.II).
Critical transitions are abrupt shifts in the state of ecosystems, the climate, financial and economic systems or other complex systems that may occur when changing conditions pass a critical or bifurcation point.The 'direction of critical slowing down' in a system's state space may
be indicative of a system's future state after such transitions when
delayed negative feedbacks leading to oscillatory or other complex dynamics are weak.
The components of a complex system may themselves be complex systems. For example, an economy is made up of organisations, which are made up of people, which are made up of cells
– all of which are complex systems. The arrangement of interactions
within complex bipartite networks may be nested as well. More
specifically, bipartite ecological and organisational networks of mutually beneficial interactions were found to have a nested structure. This structure promotes indirect facilitation and a system's capacity
to persist under increasingly harsh circumstances as well as the
potential for large-scale systemic regime shifts.
Dynamic network of multiplicity
As well as coupling rules, the dynamic network of a complex system is important. Small-world or scale-free networks which have many local interactions and a smaller number of inter-area
connections are often employed. Natural complex systems often exhibit
such topologies. In the human cortex for example, we see dense local connectivity and a few very long axon projections between regions inside the cortex and to other brain regions.
Complex systems may exhibit behaviors that are emergent,
which is to say that while the results may be sufficiently determined
by the activity of the systems' basic constituents, they may have
properties that can only be studied at a higher level. For example,
empirical food webs display regular, scale-invariant features across aquatic and terrestrial ecosystems when studied at the level of clustered 'trophic' species. Another example is offered by the termites in a mound which have physiology, biochemistry and biological development at one level of analysis, whereas their social behavior and mound building is a property that emerges from the collection of termites and needs to be analyzed at a different level.
Relationships are non-linear
In practical terms, this means a small perturbation may cause a large effect (see butterfly effect), a proportional effect, or even no effect at all. In linear systems, the effect is always directly proportional to cause. See nonlinearity.
Relationships contain feedback loops
Both negative (damping) and positive (amplifying) feedback
are always found in complex systems. The effects of an element's
behavior are fed back in such a way that the element itself is altered.
History
In 1948, Dr. Warren Weaver published an essay on "Science and Complexity", exploring the diversity of problem types by contrasting problems of
simplicity, disorganized complexity, and organized complexity. Weaver
described these as "problems which involve dealing simultaneously with a
sizable number of factors which are interrelated into an organic
whole."
While the explicit study of complex systems dates at least to the 1970s, the first research institute focused on complex systems, the Santa Fe Institute, was founded in 1984. Early Santa Fe Institute participants included physics Nobel laureates Murray Gell-Mann and Philip Anderson, economics Nobel laureate Kenneth Arrow, and Manhattan Project scientists George Cowan and Herb Anderson. Today, there are over 50 institutes and research centers focusing on complex systems.
Since the late 1990s, the interest of mathematical physicists in
researching economic phenomena has been on the rise. The proliferation
of cross-disciplinary research with the application of solutions
originated from the physics epistemology has entailed a gradual paradigm
shift in the theoretical articulations and methodological approaches in
economics, primarily in financial economics. The development has
resulted in the emergence of a new branch of discipline, namely
"econophysics", which is broadly defined as a cross-discipline that
applies statistical physics methodologies which are mostly based on the
complex systems theory and the chaos theory for economics analysis.
The traditional approach to dealing with complexity is to reduce or
constrain it. Typically, this involves compartmentalization: dividing a
large system into separate parts. Organizations, for instance, divide
their work into departments that each deal with separate issues.
Engineering systems are often designed using modular components.
However, modular designs become susceptible to failure when issues arise
that bridge the divisions.
Complexity of cities
Jane Jacobs described cities as being a problem in organized complexity in 1961, citing Dr. Weaver's 1948 essay. As an example, she explains how an abundance of factors interplay into
how various urban spaces lead to a diversity of interactions, and how
changing those factors can change how the space is used, and how well
the space supports the functions of the city. She further illustrates
how cities have been severely damaged when approached as a problem in
simplicity by replacing organized complexity with simple and predictable
spaces, such as Le Corbusier's "Radiant City" and Ebenezer Howard's
"Garden City". Since then, others have written at length on the
complexity of cities.
Recurrence quantification analysis has been employed to detect the characteristic of business cycles and economic development. To this end, Orlando et al. developed the so-called recurrence quantification correlation index
(RQCI) to test correlations of RQA on a sample signal and then
investigated the application to business time series. The said index has
been proven to detect hidden changes in time series. Further, Orlando
et al., over an extensive dataset, shown that recurrence quantification
analysis may help in anticipating transitions from laminar (i.e.
regular) to turbulent (i.e. chaotic) phases such as USA GDP in 1949,
1953, etc. Last but not least, it has been demonstrated that recurrence
quantification analysis can detect differences between macroeconomic
variables and highlight hidden features of economic dynamics.
Complexity and education
Focusing on issues of student persistence with their studies,
Forsman, Moll and Linder explore the "viability of using complexity
science as a frame to extend methodological applications for physics
education research", finding that "framing a social network analysis
within a complexity science perspective offers a new and powerful
applicability across a broad range of PER topics".
Complexity in healthcare research and practice
Healthcare systems are prime examples of complex systems,
characterized by interactions among diverse stakeholders, such as
patients, providers, policymakers, and researchers, across various
sectors like health, government, community, and education. These systems
demonstrate properties like non-linearity, emergence, adaptation, and
feedback loops. Complexity science in healthcare frames knowledge translation
as a dynamic and interconnected network of processes—problem
identification, knowledge creation, synthesis, implementation, and
evaluation—rather than a linear or cyclical sequence. Such approaches
emphasize the importance of understanding and leveraging the
interactions within and between these processes and stakeholders to
optimize the creation and movement of knowledge. By acknowledging the
complex, adaptive nature of healthcare systems, complexity science advocates for continuous stakeholder engagement, transdisciplinary collaboration, and flexible strategies to effectively translate research into practice.
Complexity and biology
Complexity science has been applied to living organisms, and in particular to biological systems. Within the emerging field of fractal physiology, bodily signals, such as heart rate or brain activity, are characterized using entropy
or fractal indices. The goal is often to assess the state and the
health of the underlying system, and diagnose potential disorders and
illnesses.
Complexity and chaos theory
Complex systems theory is related to chaos theory, which in turn has its origins more than a century ago in the work of the French mathematician Henri Poincaré. Chaos is sometimes viewed as extremely complicated information, rather than as an absence of order. Chaotic systems remain deterministic, though their long-term behavior
can be difficult to predict with any accuracy. With perfect knowledge of
the initial conditions and the relevant equations describing the
chaotic system's behavior, one can theoretically make perfectly accurate
predictions of the system, though in practice this is impossible to do
with arbitrary accuracy.
The emergence of complex systems theory shows a domain between deterministic order and randomness which is complex. This is referred to as the "edge of chaos".
When one analyzes complex systems, sensitivity to initial conditions,
for example, is not an issue as important as it is within chaos theory,
in which it prevails. As stated by Colander, the study of complexity is the opposite of the study of chaos.
Complexity is about how a huge number of extremely complicated and
dynamic sets of relationships can generate some simple behavioral
patterns, whereas chaotic behavior, in the sense of deterministic chaos,
is the result of a relatively small number of non-linear interactions. For recent examples in economics and business see Stoop et al. who discussed Android's market position, Orlando who explained the corporate dynamics in terms of mutual synchronization
and chaos regularization of bursts in a group of chaotically bursting
cells and Orlando et al. who modelled financial data (Financial Stress Index, swap and equity,
emerging and developed, corporate and government, short and long
maturity) with a low-dimensional deterministic model.
Therefore, the main difference between chaotic systems and complex systems is their history. Chaotic systems do not rely on their history as complex ones do.
Chaotic behavior pushes a system in equilibrium into chaotic order,
which means, in other words, out of what we traditionally define as
'order'. On the other hand, complex systems evolve far from equilibrium at the
edge of chaos. They evolve at a critical state built up by a history of
irreversible and unexpected events, which physicist Murray Gell-Mann called "an accumulation of frozen accidents". In a sense chaotic systems can be regarded as a subset of complex
systems distinguished precisely by this absence of historical
dependence. Many real complex systems are, in practice and over long but
finite periods, robust. However, they do possess the potential for
radical qualitative change of kind whilst retaining systemic integrity.
Metamorphosis serves as perhaps more than a metaphor for such
transformations.
Complexity and network science
A complex system is usually composed of many components and their
interactions. Such a system can be represented by a network where nodes
represent the components and links represent their interactions. For example, the Internet
can be represented as a network composed of nodes (computers) and links
(direct connections between computers). Other examples of complex
networks include social networks, financial institution
interdependencies, airline networks, and biological networks.
The
"Planetary Boundaries (PBs) diagram visually represents the current
status of the nine PB processes that regulate our planet's health. Each
process is quantified by one or more control variables based on
observational data, model simulations and expert opinions." 2025.
Planetary boundaries are a framework to describe limits to the impacts of human activities on the Earth system.
Beyond these limits, the environment may not be able to continue to
self-regulate. This would mean the Earth system would leave the period
of stability of the Holocene, in which human society developed.
The framework is based on scientific evidence that human actions, especially those of industrialized societies since the Industrial Revolution,
have become the main driver of global environmental change. According
to the framework, "transgressing one or more planetary boundaries may be
deleterious or even catastrophic due to the risk of crossing thresholds
that will trigger non-linear, abrupt environmental change within
continental-scale to planetary-scale systems."
The normative component of the framework is that human societies have been able to thrive under the comparatively stable climatic and ecological conditions of the Holocene.
To the extent that these Earth system process boundaries have not been
crossed, they mark the "safe zone" for human societies on the planet. Proponents of the planetary boundary framework propose returning to
this environmental and climatic system; as opposed to human science and
technology deliberately creating a more beneficial climate. The concept
doesn't address how humans have massively altered ecological conditions
to better suit themselves. The climatic and ecological Holocene this
framework considers as a "safe zone" doesn't involve massive industrial
farming. So this framework begs a reassessment of how to feed modern
populations.
The concept has since become influential in the international community (e.g. United Nations Conference on Sustainable Development), including governments at all levels, international organizations, civil society and the scientific community. The framework consists of nine global change processes. In 2009, according to Rockström
and others, three boundaries were already crossed (biodiversity loss,
climate change and nitrogen cycle), while others were in imminent danger
of being crossed.
In 2015, several of the scientists in the original group
published an update, bringing in new co-authors and new model-based
analysis. According to this update, four of the boundaries were crossed:
climate change, loss of biosphere integrity, land-system change,
altered biogeochemical cycles (phosphorus and nitrogen). The scientists also changed the name of the boundary "Loss of biodiversity"
to "Change in biosphere integrity" to emphasize that not only the
number of species but also the functioning of the biosphere as a whole
is important for Earth system stability. Similarly, the "Chemical
pollution" boundary was renamed to "Introduction of novel entities",
widening the scope to consider different kinds of human-generated
materials that disrupt Earth system processes.
In 2022, based on the available literature, the introduction of novel
entities was concluded to be the 5th transgressed planetary boundary. Freshwater change was concluded to be the 6th transgressed planetary boundary in 2023 before ocean acidification was documented to be the 7th crossed limit in 2025.
Framework overview and principles
The basic idea of the Planetary Boundaries framework is that maintaining the observed resilience of the Earth system in the Holocene is a precondition for humanity's pursuit of long-term social and economic development. The Planetary Boundaries framework contributes to an understanding of global sustainability because it brings a planetary scale and a long timeframe into focus.
The framework described nine "planetary life support systems" essential for maintaining a "desired Holocene state", and attempted to quantify how far seven of these systems had been pushed already. Boundaries were defined to help define a "safe space for human
development", which was an improvement on approaches aiming at
minimizing human impacts on the planet.
The framework is based on scientific evidence that human actions, especially those of industrialized societies since the Industrial Revolution,
have become the main driver of global environmental change. According
to the framework, "transgressing one or more planetary boundaries may be
deleterious or even catastrophic due to the risk of crossing thresholds
that will trigger non-linear, abrupt environmental change within continental-scale to planetary-scale systems." The framework consists of nine global change processes. In 2009, two
boundaries were already crossed, while others were in imminent danger of
being crossed. Later estimates indicated that three of these boundaries—climate change, biodiversity loss, and the biogeochemical flow boundary—appear to have been crossed.
The scientists outlined how breaching the boundaries increases
the threat of functional disruption, even collapse, in Earth's
biophysical systems in ways that could be catastrophic for human
wellbeing. While they highlighted scientific uncertainty, they indicated
that breaching boundaries could "trigger feedbacks that may result in
crossing thresholds that drastically reduce the ability to return within
safe levels". The boundaries were "rough, first estimates only,
surrounded by large uncertainties and knowledge gaps" which interact in
complex ways that are not yet well understood.
The planetary boundaries framework lays the groundwork for a
shifting approach to governance and management, away from the
essentially sectoral analyses of limits to growth aimed at minimizing
negative externalities, toward the estimation of the safe space for human development. Planetary boundaries demarcate, as it were, the "planetary playing field" for humanity if major human-induced environmental change on a global scale is to be avoided.
Most of the contributing scientists were involved in strategy-setting for the Earth System Science Partnership, the precursor to the international global change research network Future Earth. The group wanted to define a "safe operating space for humanity" for the wider scientific community, as a precondition for sustainable development.
Nine boundaries
Thresholds and tipping points
The 2009 study identified nine planetary boundaries with
quantifications for seven of them, eight of them are now being
quantified in 2025. These are :
climate change (CO2 concentration in the atmosphere < 350 ppm and/or a maximum change of +1 W/m2 in radiative forcing);
change in biosphere integrity (an annual rate of loss of biological diversity of < 10 extinctions per million species).
land system change (< 15% of the ice-free land surface under cropland);
freshwater change (< 4000 km3/yr of consumptive use of runoff resources);
increase in atmospheric aerosol loading
(for this one process in the planetary boundaries framework, the
scientists have not specified a global boundary quantification);
stratospheric ozone depletion (less than 5% reduction in total atmospheric O3 from a pre-industrial level of 290 Dobson Units);
The quantification of individual planetary boundaries is based on the
observed dynamics of the interacting Earth system processes included in
the framework. The control variables were chosen because together they
provide an effective way to track the human-caused shift away from
Holocene conditions.
For some of Earth's dynamic processes, historic data display clear thresholds between comparatively stable conditions. For example, past ice-ages show that during peak glacial conditions, the atmospheric concentration of CO2 was ~180-200 ppm. In interglacial periods (including the Holocene), CO2 concentration has fluctuated around 280 ppm. To know what past climate conditions were like with an atmosphere with over 350 ppm CO2, scientists need to look back about 3 million years. The paleo record of climatic, ecological and biogeochemical changes shows that the Earth system has experienced tipping points, when a very small increment for a control variable (like CO2) triggers a larger, possibly catastrophic, change in the response variable (global warming) through feedbacks in the natural Earth System itself.
For several of the processes in the planetary boundaries
framework, it is difficult to locate individual points that mark the
threshold shift away from Holocene-like conditions. This is because the
Earth system is complex and the scientific evidence base is still
partial and fragmented. Instead, the planetary boundaries framework
identifies many Earth system thresholds at multiple scales that will be
influenced by increases in the control variables. Examples include shifts in monsoon behavior linked to the aerosol loading and freshwater use planetary boundaries.
Planetary Boundaries (as defined in 2025)
Earth-system process
Control variable
Boundary value in 2025
"Current" value
(i.e. for the year provided in the source)
Boundary now exceeded beyond the 2025 values? (based on "current" value)
The planetary boundaries framework proposes a range of values for its
control variables. This range is supposed to span the threshold between
a 'safe operating space' where Holocene-like dynamics can be maintained
and a highly uncertain, poorly predictable world where Earth system
changes likely increase risks to societies. The boundary is
defined as the lower end of that range. If the boundaries are
persistently crossed, the world goes further into a danger zone.
It is difficult to restore a 'safe operating space' for humanity
that is described by the planetary boundary concept. Even if past
biophysical changes could be mitigated, the predominant paradigms of
social and economic development appear largely indifferent to the
looming possibilities of large scale environmental disasters triggered
by human actions. Legal boundaries can help keep human activities in check, but are only
as effective as the political will to make and enforce them.
Interaction among boundaries
Understanding the Earth system is fundamentally about understanding
interactions among environmental change processes. The planetary
boundaries are defined with reference to dynamic conditions of the Earth
system, but scientific discussions about how different planetary
boundaries relate to each other are often philosophically and
analytically muddled. Clearer definitions of the basic concepts and
terms might help give clarity.
There are many many interactions among the processes in the planetary boundaries framework.While these interactions can create both stabilizing and destabilizing
feedbacks in the Earth system, the authors suggested that a transgressed
planetary boundary will reduce the safe operating space for other
processes in the framework rather than expand it from the proposed
boundary levels. They give the example that the land use
boundary could "shift downward" if the freshwater boundary is breached,
causing lands to become arid and unavailable for agriculture. At a
regional level, water resources may decline in Asia if deforestation continues in the Amazon.
That way of framing the interactions shifts from the framework's
biophysical definition of boundaries based on Holocene-like conditions
to an anthropocentric definition (demand for agricultural land). Despite
this conceptual slippage, considerations of known Earth system
interactions across scales suggest the need for "extreme caution in
approaching or transgressing any individual planetary boundaries."
Another example has to do with coral reefs and marine ecosystems: In 2009, researchers showed that, since 1990, calcification in the reefs of the Great Barrier that they examined decreased at a rate unprecedented over the last 400 years (14% in less than 20 years). Their evidence suggests that the increasing temperature stress and the declining ocean saturation state of aragonite is making it difficult for reef corals to deposit calcium carbonate. Multiple stressors, such as increased nutrient loads and fishing pressure, moves corals into less desirable ecosystem states. Ocean acidification will significantly change the distribution and
abundance of a whole range of marine life, particularly species "that
build skeletons, shells, and tests of biogenic calcium carbonate.
Increasing temperatures, surface UV radiation levels and ocean acidity all stress marine biota,
and the combination of these stresses may well cause perturbations in
the abundance and diversity of marine biological systems that go well
beyond the effects of a single stressor acting alone."
Proposed new or expanded boundaries since 2012
In 2012, Steven Running suggested a tenth boundary, the annual net global primary production of all terrestrial plants, as an easily determinable measure integrating many variables that will give "a clear signal about the health of ecosystems".
In 2015, a second paper was published in Science to update the Planetary Boundaries concept. The update concluded four boundaries had now been transgressed:
climate, biodiversity, land use and biogeochemical cycles. The 2015
paper emphasized interactions of the nine boundaries and identified
climate change and loss of biodiversity
integrity as 'core boundaries' of central importance to the framework
because the interactions of climate and the biosphere are what
scientifically defines Earth system conditions.
In 2017, some authors argued that marine systems are
underrepresented in the framework. Their proposed remedy was to include
the seabed
as a component of the earth surface change boundary. They also wrote
that the framework should account for "changes in vertical mixing and ocean circulation patterns".
Subsequent work on planetary boundaries begins to relate these thresholds at the regional scale.
A 2018 study calls into question the adequacy of efforts to limit
warming to 2 °C above pre-industrial temperatures, as set out in the Paris Agreement. The scientists raise the possibility that even if greenhouse gas emissions are substantially reduced to limit warming to 2 °C, that might exceed the "threshold" at which self-reinforcing climate feedbacks add additional warming until the climate system stabilizes in a hothouse climate state.
This would make parts of the world uninhabitable for people, raise sea
levels by up to 60 metres (200 feet), and raise temperatures by 4–5 °C
(7.2–9.0 °F) to levels that are higher than any interglacial period in the past 1.2 million years.
According to the biologist Cristián Samper,
a "boundary that expresses the probability of families of species
disappearing over time would better reflect our potential impacts on the
future of life on Earth." The biodiversity boundary has also been criticized for framing
biodiversity solely in terms of the extinction rate. The global
extinction rate has been highly variable over the Earth's history, and
thus using it as the only biodiversity variable can be of limited
usefulness.
Nitrogen and phosphorus
The biogeochemist William Schlesinger
thinks waiting until we near some suggested limit for nitrogen
deposition and other pollutions will just permit us to continue to a
point where it is too late. He says the boundary suggested for
phosphorus is not sustainable, and would exhaust the known phosphorus
reserves in less than 200 years.
The ocean chemist
Peter Brewer queries whether it is "truly useful to create a list of
environmental limits without serious plans for how they may be achieved
... they may become just another stick to beat citizens with. Disruption
of the global nitrogen cycle is one clear example: it is likely that a
large fraction of people on Earth would not be alive today without the
artificial production of fertilizer. How can such ethical and economic
issues be matched with a simple call to set limits? ... food is not
optional."
Peak phosphorus is a concept to describe the point in time at which the maximum global phosphorus
production rate is reached. Phosphorus is a scarce finite resource on
earth and means of production other than mining are unavailable because
of its non-gaseous environmental cycle. According to some researchers, Earth's phosphorus reserves are expected
to be completely depleted in 50–100 years and peak phosphorus to be
reached by approximately 2030. However, recent evidence shows that if phosphorus applications to soil
are matched to the agronomic optimum for crop yield, it would take
>500 years to exhaust currently econimically viable phosphorus
reserves.
Ocean acidification
Surface ocean acidity
is clearly interconnected with the climate change boundaries, since the
concentration of carbon dioxide in the atmosphere is also the
underlying control variable for the ocean acidification boundary.
The ocean chemist Peter Brewer thinks "ocean acidification has
impacts other than simple changes in pH, and these may need boundaries
too."
Land-system change
Across the planet, forests, wetlands and other vegetation types are being converted to agricultural and other land uses, impacting freshwater, carbon and other cycles, and reducing biodiversity. In the year 2015 the boundary was defined as 75% of forests rested intact, including 85% of boreal forests, 50% of temperate forests and 85% of tropical forests. The boundary is crossed because only 62% of forests rested intact as of the year 2015.
The boundary for land use has been criticized as follows: "The boundary of 15 per cent land-use change
is, in practice, a premature policy guideline that dilutes the authors'
overall scientific proposition. Instead, the authors might want to
consider a limit on soil degradation or soil loss. This would be a more valid and useful indicator of the state of terrestrial health."
Freshwater
The freshwater cycle is another boundary significantly affected by climate change. Overexploitation of freshwater occurs if a water resource is mined or extracted at a rate that exceeds the recharge rate. Water pollution and saltwater intrusion can also turn much of the world's underground water and lakes into finite resources with "peak water" usage debates similar to oil.
The hydrologist David Molden stated in 2009 that planetary boundaries are a welcome new approach in the "limits to growth"
debate but said "a global limit on water consumption is necessary, but
the suggested planetary boundary of 4,000 cubic kilometres per year is
too generous."
Green and blue water
A study concludes that the 'Freshwater use' boundary should be
renamed to the 'Freshwater change', composed of "green" and "blue" water
components. 'Green water' refers to disturbances of terrestrial precipitation, evaporation and soil moisture. Water scarcity can have substantial effects in agriculture.When measuring and projecting water scarcity in agriculturefor climate change scenarios, both "green water" and "blue water" are of relevance.
The stratospheric ozone layer protectively filters ultraviolet radiation (UV) from the Sun, which would otherwise damage biological systems. The actions taken after the Montreal Protocol appeared to be keeping the planet within a safe boundary.
The Nobel laureate in chemistry Mario Molina says "five per cent is a reasonable limit for acceptable ozone depletion, but it doesn't represent a tipping point".
Atmospheric aerosols
Worldwide each year, aerosol particles result in about 800,000 premature deaths from air pollution. Aerosol loading is sufficiently important to be included among the
planetary boundaries, but it is not yet clear whether an appropriate
safe threshold measure can be identified.
A Bayesian emulator for persistent organic pollutants has been
developed which can potentially be used to quantify the boundaries for
chemical pollution. To date, critical exposure levels of polychlorinated biphenyls (PCBs)
above which mass mortality events of marine mammals are likely to occur,
have been proposed as a chemical pollution planetary boundary.
There are at least 350,000 artificial chemicals in the world. They are coming from "plastics, pesticides, industrial chemicals, chemicals in consumer products, antibiotics and other pharmaceuticals". They have mostly "negative effects on planetary health".
Their production increased 50 times since 1950 and is expected to
increase 3 times more by 2050. Plastics alone contain more than 10,000
chemicals and create large problems. The researchers are calling for
limit on chemical production and shift to circular economy, meaning to products that can be reused and recycled.
In January 2022 a group of scientists concluded that this
planetary boundary is already exceeded, which puts in risk the stability
of the Earth system. They integrated the literature information on how production and release of a number of novel entities, including plastics and hazardous chemicals, have rapidly increased in the last decades with significant impact on the planetary processes.
In August 2022, scientists concluded that the (overall
transgressed) boundary is a placeholder for multiple different
boundaries for NEs that may emerge, reporting that PFAS pollution is one such new boundary. They show that levels of these so-called "forever chemicals" in rainwater are ubiquitously, and often greatly, above guideline safe levels worldwide. There are some moves to restrict and replace their use.
In 2024 a report by Planet Tracker, dedicated to this planetary boundary stipulated that The Chemical Abstracts Service (CAS)
registered more than 204,000,000 chemicals since the 19 century. From
them, over 350,000 chemicals are allowed for production and use in North
America and Europe and from those 350,000, most are untested and 14%
are unknown because companies does not disclose their composition. Their
negative impacts on health is more than a tenth of the global GDP and
they can impact more than 1 planetary boundary: Chlorofluorocarbons, for example, can impact 3 at the same time.
Planetary integrity is also called earth's life-support systems or ecological integrity. Scholars have pointed out that planetary integrity "needs to be maintained for long-term sustainability". The term integrity refers to ecological health in this context. The concept of planetary integrity is interlinked within the concept of planetary boundaries.
An Expert Panel on Ecological Integrity in 1998 has defined ecological integrity
as follows: "Ecosystems have integrity when they have their native
components (plants, animals and other organisms) and processes (such as
growth and reproduction) intact."
There are many negative human impacts on the environment that are causing a reduction in planetary integrity. For example, the current biodiversity loss is threatening ecological integrity on a global scale. The Sustainable Development Goals might be able to act as a steering mechanism to address the current loss of planetary integrity.
The "Limits to Growth" (1972) and Gaia theory
The idea that there are limits to the burden placed upon our planet
by human activities has been around for a long time. The Planetary
Boundaries framework acknowledges the influence of the 1972 study, The Limits to Growth, that presented a model in which exponential growth in world population, industrialization, pollution, food production, and resources depletion outstrip the ability of technology to increase resources availability. Subsequently, the report was widely dismissed, particularly by economists and business people, and it has often been claimed that history has proved the projections to be incorrect. In 2008, Graham Turner from the Commonwealth Scientific and Industrial Research Organisation (CSIRO) published "A comparison of The Limits to Growth with thirty years of reality". The Limits to Growth has been widely discussed, both by critics of the modelling approach and its conclusions and by analysts who argue that the insight that societies do not live
in an unlimited world and that historical data since the 1970s support
the report's findings. The Limits to Growth
approach explores how the socio-technical dynamics of the world economy
may limit humanity's opportunities and introduce risks of collapse. In
contrast, the Planetary Boundaries framework focuses on the biophysical
dynamics of the Earth system.
Our Common Future was published in 1987 by United Nations' World Commission on Environment and Development. It tried to recapture the spirit of the Stockholm Conference.
Its aim was to interlock the concepts of development and environment
for future political discussions. It introduced the famous definition
for sustainable development:
"Development that meets the needs of the present without compromising
the ability of future generations to meet their own needs."
Another key idea influencing the Planetary Boundaries framework is the Gaia theory or hypothesis. In the 1970s, James Lovelock and microbiologistLynn Margulis presented the idea that all organisms and their inorganic surroundings on Earth are integrated into a single self-regulating system. The system has the ability to react to perturbations or deviations,
much like a living organism adjusts its regulation mechanisms to
accommodate environmental changes such as temperature (homeostasis).
Nevertheless, this capacity has limits. For instance, when a living
organism is subjected to a temperature that is lower or higher than its
living range, it can perish because its regulating mechanism cannot make
the necessary adjustments. Similarly the Earth may not be able to react
to large deviations in critical parameters. In Lovelock's book The Revenge of Gaia, he suggests that the destruction of rainforests and biodiversity, compounded with global warming resulting from the increase of greenhouse gases
made by humans, could shift feedbacks in the Earth system away from a
self-regulating balance to a positive (intensifying) feedback loop.
Science indicates that we are transgressing planetary boundaries that
have kept civilization safe for the past 10,000 years. Evidence is
growing that human pressures are starting to overwhelm the Earth's
buffering capacity. Humans are now the most significant driver of global change, propelling the planet into a new geological epoch, the Anthropocene.
We can no longer exclude the possibility that our collective actions
will trigger tipping points, risking abrupt and irreversible
consequences for human communities and ecological systems.
Scientists have affirmed that the planet has entered a new epoch, the Anthropocene. In the Anthropocene, humans have become the main agents of not only change to the Earth System but also the driver of Earth System rupture, disruption of the Earth System's ability to be resilient and recover from that change, potentially ultimately threatening planetary habitability. The previous geological epoch, the Holocene began about 10,000 years ago. It is the current interglacial
period, and was a relatively stable environment of the Earth. There
have been natural environmental fluctuations during the Holocene, but
the key atmospheric and biogeochemical parameters have remained within
relatively narrow bounds. This stability has allowed societies to thrive worldwide, developing
agriculture, large-scale settlements and complex networks of trade.
According to Rockström et al., we "have now become so
dependent on those investments for our way of life, and how we have
organized society, technologies, and economies around them, that we must
take the range within which Earth System processes varied in the
Holocene as a scientific reference point for a desirable planetary
state."
Various biophysical processes that are important in maintaining the resilience of the Earth system are also undergoing large and rapid change because of human actions. For example, since the advent of the Anthropocene, the rate at which species are going extinct has increased over 100 times, and humans are now the driving force altering global river flows as well as water vapor flows from the land surface. Continuing perturbation of Earth system processes by human activities
raises the possibility that further pressure could be destabilizing,
leading to non-linear, abrupt, large-scale or irreversible environmental change responses by the Earth system within continental- to planetary-scale systems.
In summary, the planetary boundary concept is a very important one,
and its proposal should now be followed by discussions of the
connections between the various boundaries and of their association with
other concepts such as the 'limits to growth'.
Importantly, this novel concept highlights the risk of reaching
thresholds or tipping points for non-linear or abrupt changes in
Earth-system processes. As such, it can help society to reach the
agreements required for dealing effectively with existing global
environmental threats, such as climate change.
The 2009 report was presented to the General Assembly of the Club of Rome in Amsterdam. An edited summary of the report was published as the featured article in a special 2009 edition of Nature alongside invited critical commentary from leading academics like Nobel laureate Mario J. Molina and biologist Cristián Samper.
Development studies scholars have been critical of aspects of the framework and constraints that its adoption could place on the Global South.
Proposals to conserve a certain proportion of Earth's remaining forests
can be seen as rewarding the countries such as those in Europe that
have already economically benefited from exhausting their forests and
converting land for agriculture. In contrast, countries that have yet to
industrialize are asked to make sacrifices for global environmental
damage they may have had little role in creating.
The biogeochemist William Schlesinger
queries whether thresholds are a good idea for pollutions at all. He
thinks waiting until we near some suggested limit will just permit us to
continue to a point where it is too late. "Management based on
thresholds, although attractive in its simplicity, allows pernicious,
slow and diffuse degradation to persist nearly indefinitely."
In a global empirical study, researchers investigated how
students of environmental and sustainability studies in 35 countries
assessed the planetary boundaries. It was found that there are
substantial global differences in the perception of planetary
boundaries.
Subsequent developments
The "safe and just space" doughnut
Doughnut (economic model)
The Doughnut, or Doughnut economics, is a visual framework for sustainable development – shaped like a doughnut or lifebelt – combining the concept of planetary boundaries with the complementary concept of social boundaries. The name derives from the shape of the diagram, i.e. a disc with a hole
in the middle. The centre hole of the model depicts the proportion of
people that lack access to life's essentials (healthcare, education, equity
and so on) while the crust represents the ecological ceilings
(planetary boundaries) that life depends on and must not be overshot. The diagram was developed by University of Oxford economist Kate Raworth in her 2012 Oxfam paper A Safe and Just Space for Humanity and elaborated upon in her 2017 book Doughnut Economics: Seven Ways to Think Like a 21st-Century Economist and paper.
National environmental footprints
Several studies have assessed environmental footprints of nations based on planetary boundaries: for Portugal, Sweden, Switzerland, the Netherlands, the European Union, India, many of Belt and Road Initiative countries as well as for the world's most important economies. While the metrics and allocation approaches applied varied, there is a
converging outcome that resource use of wealthier nations – if
extrapolated to world population – is not compatible with planetary
boundaries.
Boundaries related to agriculture and food consumption
Visualization of the planetary boundaries related to agriculture and nutrition
Human activities related to agriculture and nutrition globally
contribute to the transgression of four out of nine planetary
boundaries. Surplus nutrient flows (N, P) into aquatic and terrestrial
ecosystems are of highest importance, followed by excessive land-system
change and biodiversity loss. Whereas in the case of biodiversity loss, P
cycle and land-system change, the transgression is in the zone of
uncertainty—indicating an increasing risk (yellow circle in the figure),
the N boundary related to agriculture is more than 200%
transgressed—indicating a high risk (red marked circle in the figure).
Here, nutrition includes food processing and trade as well as food
consumption (preparation of food in households and gastronomy).
Consumption-related environmental impacts are not quantified at the
global level for the planetary boundaries of freshwater use, atmospheric
aerosol loading (air pollution) and stratospheric ozone depletion.
Individual and collective allowances
Approaches based on a general framework of ecological limits include (transferable) personal carbon allowances and "legislated" national greenhouse gas emissions limits. Consumers would have freedom in their (informed) choice within (the collective) boundaries.
Usage at international policy level
United Nations
The United Nations secretary general Ban Ki-moon
endorsed the concept of planetary boundaries on 16 March 2012, when he
presented the key points of the report of his High Level Panel on Global
Sustainability to an informal plenary of the UN General Assembly. Ban stated: "The Panel's vision is to eradicate poverty and reduce
inequality, to make growth inclusive and production and consumption more
sustainable, while combating climate change and respecting a range of
other planetary boundaries." The concept was incorporated into the so-called "zero draft" of the outcome of the United Nations Conference on Sustainable Development to be convened in Rio de Janeiro 20–22 June 2012. However, the use of the concept was subsequently withdrawn from the
text of the conference, "partly due to concerns from some poorer
countries that its adoption could lead to the sidelining of poverty
reduction and economic development. It is also, say observers, because
the idea is simply too new to be officially adopted, and needed to be
challenged, weathered and chewed over to test its robustness before
standing a chance of being internationally accepted at UN negotiations."
In 2011, at their second meeting, the High-level Panel on Global
Sustainability of the United Nations had incorporated the concept of
planetary boundaries into their framework, stating that their goal was:
"To eradicate poverty and reduce inequality, make growth inclusive, and
production and consumption more sustainable while combating climate
change and respecting the range of other planetary boundaries."
Elsewhere in their proceedings, panel members have expressed
reservations about the political effectiveness of using the concept of
"planetary boundaries": "Planetary boundaries are still an evolving
concept that should be used with caution [...] The planetary boundaries
question can be divisive as it can be perceived as a tool of the "North"
to tell the "South" not to follow the resource intensive and
environmentally destructive development pathway that rich countries took
themselves... This language is unacceptable to most of the developing
countries as they fear that an emphasis on boundaries would place
unacceptable brakes on poor countries."
However, the concept is routinely used in the proceedings of the United Nations, and in the UN Daily News. For example, the United Nations Environment Programme (UNEP) Executive Director Achim Steiner states that the challenge of agriculture is to "feed a growing global population without pushing humanity's footprint beyond planetary boundaries." The UNEP Yearbook 2010 also repeated Rockström's message, conceptually linking it with ecosystem management and environmental governance indicators.
In their 2012 report entitled "Resilient People, Resilient
Planet: A future worth choosing", The High-level Panel on Global
Sustainability called for bold global efforts, "including launching a
major global scientific initiative, to strengthen the interface between
science and policy. We must define, through science, what scientists
refer to as "planetary boundaries", "environmental thresholds" and
"tipping points"".
European Commission
The planetary boundaries concept is also used in proceedings by the European Commission, and was referred to in the European Environment Agency synthesis report The European environment – state and outlook 2010.
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