System Sustainability
Federico M Pulselli, University of Siena, Siena, Italy
r 2019 Elsevier B.V. All rights reserved.
Sustainability
Sustainability is a word that is used—sometimes overused—in many fields to indicate many things. Sometimes it refers to the
environmental sphere and sometimes it has to do with the human sphere, considering its social, economic, financial and political
aspects at macro and micro levels. It is therefore difficult to arrive at a univocal definition of the concept, although a strong
scientific basis for it can be found in the literature that has evolved in the last 30–40 years.
Nature is the quintessence of sustainability; a translation of the concept for humans would define a virtuous path for evolution
of the human system. Sustainability is important for humans and their actions for many reasons, not least the survival of our
species. Learning from nature can enable us to translate its winning strategies into virtuous behavior and avoid mistakes and risks
for the future.
The first step to characterize sustainability consists in the identification of its biophysical foundations as necessary condition to
limit the elusive aspects of the concept (Pulselli et al., 2008).
Time
If we look at a pianist, we see that he plays a chord by striking keys on the piano. When he removes his hands from the keys, the
chord dies. If the pianist presses a special pedal, a mechanism enables the chord to continue to vibrate even after the fingers have
been removed. The pedal maintains the notes for a while and its name is “sustain.” Time is therefore intrinsic to the meaning of
sustainability. Something that exists or survives in time is sustainable. This essence of the term “sustainability” and the adjective
“sustainable” is important to avoid misuse. It also characterizes the difference between the expressions “carrying capacity” and
“sustainable development” because the verb “to carry” means to hold, contain or support something, whereas “to sustain” means
to maintain in time. So if we consider time as a crucial element of sustainability it means that we view the dynamics of ecosystems
and human activity not as a simple sequence of changes of state and modifications, but as evolution. This helps clarify which
actions are virtuous and which should be avoided in order to maintain a system indefinitely in time.
Biophysical Limits
Nature has been surviving for about 4 billion years, during which time biological evolution proceeded through winning strategies
under the inevitable condition of limited resources. Nature diversifies: biodiversity is a strong general attribute for survival. Nature
exploits the most abundant and reliable energy source, solar energy, finding ways of using it to feed vital mechanisms. Nature
optimizes resources and disposes of wastes and degraded energy (e.g., by closure of cycles and dissipation of heat into space).
Nature has not only been successful in surviving in time but also in thriving and flourishing within geobiophysical constraints
(Jørgensen et al., 2015). Human activities, which are always fed by resource extraction and consumption and generate flows of
degraded energy and matter, should be consistent with the finite and constrained ability of the planet to regenerate resources and
absorb wastes.
Relations
Thermodynamics gives us the tools to understand these constraints. For instance, its fundamental laws define the limits to overall
availability and our capacity to exploit energy: the law of conservation of energy (first law of thermodynamics) states that energy
cannot be created or destroyed, and the law of energy dissipation (second law of thermodynamics) states that every activity or
conversion degrades energy to heat which is unable to do work. Biological systems are open systems (they exchange matter and
energy with their environment) that involve extremely ordered structures and evolve in the direction of increasing order by
processing the flow of energy and resources they capture from the environment. They discharge their wastes as degraded energy
(heat) and matter (e.g., emissions, pollutants). A continuous process of transformations, cycles and feedbacks demonstrates the
importance of relationships for living systems and the dependence of these systems on the context in which they live. This is valid
for a cell, a tree, a human being, an ecosystem, but also for a city, a production process, an urban or regional system. All these
systems survive by exploiting flows of energy and matter, releasing wastes, emissions and heat into their surroundings (see, among
others, Schrödinger, 1944; Prigogine, 1954; Tiezzi, 2003).
Time, biophysical limits and relations are three cornerstones of the concept of sustainability. A system or project cannot be
called sustainable if it does not rest on these foundations: it must be durable, it must develop within the limits imposed by natural
Encyclopedia of Ecology, 2nd edition, Volume 4 doi:10.1016/B978-0-12-409548-9.10610-4432
laws (such as the laws of thermodynamics) and it must maintain relationships with other systems and the surrounding environment.
Without considering these pillars, the use of the word sustainable is misleading, inappropriate, or even false.
Representing Sustainability
Human development involves three spheres: economic, social and environmental. According to Barbier (1987), sustainable
development is defined by simultaneous interaction of the three intersecting spheres. The intersection is where sustainable
development can occur, suggesting that converging goals from the different spheres must be achieved together (Fig. 1A). This view,
however, places the three circles on the same level, and excluding their intersection, it considers the ecological, social and
economic elements of sustainability to be interchangeable or substitutable (which amounts to the condition of weak sustainability).
An alternative interpretation is that of concentric circles, which implies an environmental basis for the social sphere that
contains the economic expression of society (Fig. 1B). Another view depicts sustainability as a pyramid, highlighting the relative
sizes of the three spheres (Fig. 1C). These rather conventional ways of representing the concept of sustainability underline the need
for an organic view of human presence and activity on the planet. Study of the relationship between the human species and the
Earth system is urgently needed. Researchers have been proposing ideas in this field for several decades, highlighting the risk and
consequences of reaching or passing critical points or thresholds. This was one of the aims of the well-known and influential report
entitled “The Limits to Growth” (Meadows et al., 1972), which took an organic, comprehensive and global view of the planet.
System Sustainability
The basic concept of system is expressed by the well-known adage of systems science: “the whole is greater than the sum of its
parts.” This can be explained by the fact that a system, namely “things that stay together,” consists of components that cooperate
with each other, giving rise to emerging novelties and new properties. For instance, a human being has a body made of 1014
cells.
Every cell has biochemical, metabolic and anabolic characteristics and properties determined by genetic and environmental
conditions but they do not explain the characteristics and properties of the organism as a whole (knowledge, feelings, emotions,
pain, etc.). Similarly, an ecosystem is composed of organisms linked in a complex cooperative synergistic relationship that results
in properties like adaptation, evolution, self-organization, resistance, flexibility and ultimately beauty (Jørgensen et al., 2015).
According to Jørgensen (2012), “systems with emerging properties cannot be described by listing the components and their
properties, but it is necessary to capture, understand and describe the emerging system properties.” A fundamental overview of
general systems theory was proposed by Von Bertalanffy (1969). Since then many authors have investigated the question which is
relevant to our life on the planet and especially sustainability. In a comprehensive survey of the systems approach, Capra (1996)
suggested that the essential properties of an organism or living system are properties of the whole system, which no single
component has per se. These properties emerge from interactions and relations among the parts of the system. There have been
major advances and novelties in many areas of systems science. Ecology, for example, is an area permeated by the systems view.
According to Capra, if we consider the world as an integrated system rather than a series of separate components, we take a holistic
or even ecological view in keeping with the term “ecological,” namely something characterized by relations among its parts and
included in a larger context. One fundamental contribution to the advancement of ecology was by Eugene Odum. In a major
paper, he emphasized the role of ecology as a discipline that connects and advanced various ideas about the concept of sustainability:
“[…] cell level science will contribute very little to the well-being or survival of human civilization if our understanding
of supraindividual levels of organization is so inadequate that we can find no solutions to population over-growth, social disorder,
pollution, and other forms of societal and environmental cancer. […] It is in the properties of the large-scale, integrated system
that hold solutions to most of the long-range problems of society” (Odum, 1977). The paper is actually a precursor of sustainability
science because it also traces the link between ecology on the one hand, and social sciences, technology, politics and
economics on the other. Jørgensen (2012) subsequently published a textbook on Systems Ecology that deals with
Fig. 1 Representations of sustainability. The usual representation of sustainability shows the different (A–C) contributions and interactions
between the environmental (Env.), social (Soc.) and economic (Econ.) spheres of human life.
Human Ecology and Sustainability: System Sustainability 433
thermodynamics- and biochemistry-based foundations, ecological laws, ecosystem properties and ultimately environmental
management. He stresses the need for a holistic view to problem solving and touches on the progressive emergence of metadisciplines,
particularly ecological modeling and ecological engineering, that tend to unify different approaches. Another interesting
example of adoption of a systems approach can be found in systems chemistry, that is “the study of complex systems, or
networks, of molecules [to] investigate how interactions between members propagate through networks allowing complex
behavior to emerge” (Nitschke, 2009).
The concept of sustainability can only be considered from a systemic viewpoint. The study of human sustainable development
is concerned with relations between the individual and collective expressions of humankind and the multiplicity of contexts in
which the human action develops. The context can be physical, environmental, social, economic, political, institutional, urban,
legal and so forth. Pulselli et al. (2016) derive at least three key points from this: (a) the organic picture of reality (i.e., what should
be sustainable?) requires a transdisciplinary approach in order to encompass the many dimensions of the context in which we live;
(b) the purpose (i.e., why should we be sustainable?) is to create and maintain the conditions for durably living better and in
harmony with nature and other individuals; and (c) the critical assessment of how we can reach these conditions (i.e., how can we
be sustainable?) demands new frameworks in which to evaluate progress toward the desired change. Sustainability therefore
connotes a system within its context. By adapting a statement from Jørgensen (2012), originally referring to ecosystems, we can say
that if we can understand how ecosystems (as well as human systems) work as systems, we will be able to develop an ecosystem
(as well as sustainability) theory that can be used to predict the effects on an ecosystem (as well as a human system) of welldefined
changes of forcing function.
Various approaches and methodologies have been developed in recent years to implement a systemic view. Regarding system
sustainability, attempts have been made to identify, categorize, quantify and manage the relationships between human life and
activity and the context(s) in which they develop, with particular emphasis on environmental context.
An important approach is that of ecosystem service (ES) identification and evaluation. Ecosystem services are the portion of
ecosystem functions that directly or indirectly contributes to human welfare. Ecosystem functions occur independently of humans,
however humans exploit the existence of ecosystems and their dynamics. These services are usually classified in four categories:
provisioning services (e.g., food, water, timber, fiber), regulating services (regarding climate, floods, disease, wastes, water quality
etc.), cultural services (that provide recreational, esthetic and spiritual benefits) and supporting services (e.g., soil formation,
photosynthesis, nutrient cycling) (see MA, 2005, and the corresponding entry in this encyclopedia). The benefits people obtain
from these ecological services can be assessed in economic terms, and the results are surprising. As Costanza et al. (1997)
demonstrated, at global level, nature provides humans with more resources, and more efficiently, than does world economic
infrastructure (which is designed to do just that). Assessment of ecosystem services is therefore a way to estimate and express the
importance of ecological dynamics for human life and its sustainability.
Sustainability research has always paid great attention to the determination of thresholds. These are measured or estimated
levels of an entity or parameter, beyond which conditions may change unexpectedly, unpredictably and/or irreversibly. An
important approach to human activity thresholds at global level is the so-called planetary boundaries framework that aims to
identify a safe global operating space for the survival and prosperity of humanity (see Rockström et al., 2009, and the corresponding
entry in this encyclopedia). The approach measures selected control variables to determine the risk of dangerous
consequences for human well-being in nine Earth processes: climate change, biogeochemical flows, biodiversity loss, land system
changes, ocean acidification, stratospheric ozone depletion, freshwater use, atmospheric aerosol loading and chemical pollution.
Research in this field is demonstrating that the first four processes involve risk factors for humanity. A major aspect of this
approach is the concept of the Earth as an integrated system to be considered, preserved and managed as a whole. The approach is
also inspiring research in other fields, such as legislation. For example, a multidisciplinary group of researchers are promoting a
systemic legal framework applicable to the Earth as a whole, with a set of assessment and regulatory tools that can be condensed
into a safe operating space treaty (SOS Treaty). The treaty is proposed as a new global guideline for legal framework researchers
because the Earth is still formally an unidentified legal object (Magalhães et al., 2016).
A further system sustainability approach is the so-called food-energy-water nexus. Used in many studies by various organizations
(e.g., the Food and Agriculture Organization of the United Nations, FAO), it offers a way to first identify and then solve
problems that a large portion of the world population is facing. By combining different fields of investigation and action it
confirms the importance of seeing the world in which we live as a set of cooperative interrelated units and systems. In this case,
study of the three sectors—food, energy and water—, to which the climatic system is often added, helps us recognize the many
links between them and the fact that problems in one may trigger serious crises in the others. The three-sided approach highlights
the main conditions for achieving essential tasks, such as poverty reduction, human well-being and sustainable development at
global level, that depend on the context in which human development occurs.
The Sustainable Development Goals, promoted by the United Nations in 2015, are an enlargement of this view. Countries
adopted these goals “to end poverty, protect the planet, and ensure prosperity for all as part of a new sustainable development
agenda. Each goal has specific targets to be achieved by 2030” (for an overview, see UN, 2015). The set of goals is so wide that it
offers an encompassing vision of the human condition on the planet. It includes the following 17 items: no poverty, zero hunger,
good health and well-being, quality education, gender equality, clean water and sanitation, affordable and clean energy, decent
work and economic growth, industry, innovation and infrastructure, reduced inequalities, sustainable cities and communities,
responsible consumption and production, climate action, life below water, life on land, peace, justice and strong institutions and
434 Human Ecology and Sustainability: System Sustainability
partnerships for the goals. Again all these aspects are linked and the set works as a system. The challenge is that the purpose of
system sustainability is only achieved if the goals are largely achieved together at the same time.
Problems do not observe disciplinary or geographical boundaries but arise almost everywhere, in many ways and with different
dimensions and scales. The approaches outlined above can help solve these problems by adopting a systemic view that facilitates
understanding of the connections between different components of a given system and identification of emerging properties and
problems of the system.
Less Usual Representation of Sustainability and a Possible Interpretative Framework
The scientific community recognizes the environmental, social and economic spheres as components of the concept of sustainability.
However, we have seen that they cannot be considered interchangeable.
Fig. 1C shows the relationships between the three spheres of sustainability in the form of a pyramid: the base of the pyramid
represents natural assets, crucial inputs into the system; the middle section represents society with its organization and structure,
and can be viewed as the state of the system; the point of the pyramid is the real economy that produces the system's output. The
pyramid can be considered an evolution of the concentric-circles representation (Fig. 1B) that also combines the environmental,
social and economic spheres.
Pulselli et al. (2015) proposed a slightly different version of this three-stage approach in order to represent sustainability and
facilitate investigation. This less usual representation of sustainability rotates the pyramid 90 degree clockwise to capture the
succession of the stages (Fig. 2) from left to right. “A flow of material and energy inputs generated by the available stock of Natural
Capital feeds (is captured by) the system. These resources are necessary for the elements of the system (i.e., society and its
organizational units) to operate (act, live, survive); the level of organization of the society influences the degree of utility/
satisfaction derived from processing/using/consuming resources. An organized society is supposed to be able to achieve better
economic results, providing outputs from its productive processes” (Pulselli et al., 2015).
This version of the scheme aims to represent the physical, relational and thermodynamic order (environment-society-economy)
of the succession of stages. It offers a more logical/consequential approach to combining and evaluating different indicators,
starting from the economy's dependence on societal organization and environmental resources and including feedback produced
by the socioeconomic system.
A general systemic approach emerging from this new version of the sustainability scheme could be the Input-State-Output
(I-S-O) framework, useful for assessing system sustainability. In Sustainability section, we saw that living biological systems
(including human-driven ones like cities and production processes) are fed by a continuous flow of energy and matter that is
processed internally (e.g., self-organization) to give useful outputs (in terms of services, feedback flows, products, etc.). These vital
evolutionary steps are therefore simplified by this linear but organic scheme (Fig. 3) that also helps select suitable methods and
tools for quantifying phenomena. Here different combinations of indicators can be used to count inputs of energy and matter into
a system, describe organizational state and quantify outputs. The I-S-O framework orientates the use of well-defined triads of
systems indicators representing links between the three spheres of sustainability. This process, consisting of a succession of stages
in a series, must be viewed as the integrated way in which the system works. Nevertheless, identifying the three stages makes it
possible to select corresponding indicators and compose multidimensional data, often expressed in different units, without losing
information. Indeed, the framework can be investigated using a diagram with three axes representing the input, state and output
indicators, and the information gained by different indicators is not lost in final aggregations. This framework represents relationships
between indicators and helps classify the systems in question in terms of their sustainability.
The choice of indicators is crucial for implementing this method. If we consider the correspondence between the I-S-O scheme
and the environmental-social-economic spheres of sustainability, we can use physical measures for input, social indicators for state
and economic parameters for output. Thus the triads are composed of groups of indicators/measures representative of each
dimension: measures of resource flows (such as ecological footprint, emergy flow, gross emission of CO2 equivalent, or more
Fig. 2 A less usual representation of sustainability with a more logical/consequential series of stages ordered from left to right.
Human Ecology and Sustainability: System Sustainability 435
generically, energy, matter and pollutant flows) to represent the environmental contribution to the system; measures representative
of the state or health of a society or its inequality level (such as employment level or an index of income distribution); economic
measures representative of output (such as GDP or value added, or the welfare/happiness of a population).
Each triad gives different information, depending on the indicators chosen. The point representing combination of the three
indicator values can be identified in the 3D space enclosed by the three axes. Every system can therefore be identified by a point on
the three-axis diagram and its position can be discussed in a static way, determining, for example, the main components that
characterize the results obtained (environmental, social or economic), or in a dynamic way, when a time series of indicators is
available, signaling development trajectories for that system.
According to the authors, “this application is a rational solution for the study of system sustainability, because it incorporates
consistency with traditional sectors proposed in sustainability research and is feasible because it is limited to small number of
(already available) data. [The proposal also] maintains the informative capacity of every aspect of system behavior, but it also
provides a synthetic picture of the reality. […] This new way of considering these fundamental aspects of our life can be the basis
for assessment of long term system sustainability and helps identify the contradictions, hypocrisies, and nonsustainable nature of
many of our current behaviors” (Pulselli et al., 2015).
Conclusion
System sustainability designates a condition that characterizes a given system. A system is sustainable if it has durable prospects
with respect to constraints and preserves relations. Sustainability is a property of the whole system with all its elements, actors,
connections and properties. The concept of sustainability applies especially to human systems, for which a multidimensional/
systemic approach makes it possible to consider man-driven systems and actions within their context and limits. Humanity is
continuously faced with major problems regarding the health of the planet and ecosystems as life supporting systems, relationships
between nations and populations, and ultimately the survival of the species. Improving our capacity to measure/observe the
world in which we live and widening the set of investigation tools in this field are urgent tasks. Instruments that enable holistic
understanding of the environmental, social and economic view of human life are the most suitable for achieving sustainable
systems. In particular, we need investigation and management tools for the environmental and social sphere that complement the
primacy of the economic dimension. We have seen that systemic approaches, methods, interpretation proposals and solutions
proposed in the last 30 years aim to make the multidimensionality of our world more explicit. To conclude this survey, it is worth
mentioning two books which are fundamental references for a systemic view of sustainability. The first is “A prosperous way
down” (Odum and Odum, 2001), an innovative contribution that anticipated more modern tendencies like the so-called degrowth
movements and the EU Beyond GDP initiative. The importance of the book lies in the fact that it associates relevant aspects
of natural and physical phenomena and energy-based dynamics with economic, social, political and demographic problems. The
second is “Flourishing within limits to growth. Following nature's way” (Jørgensen et al., 2015): inspired by “Limits to growth”
(Meadows et al., 1972), it acknowledges the existence of constraints and limits, but stresses that like ecosystems, human systems
must find a way to flourish while observing these limits, seeking wealth but preserving the environment and quality of life.
See also: Human Ecology and Sustainability: Emergy and Sustainability; Ecological Footprint; Ecological Economics 1
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