Challenges in Sustainability | 2013 | Volume 1 | Issue 1 | Pages 41–52
DOI: 10.12924/cis2013.01010041
Research Article
Sustainable Development within Planetary Boundaries:
A Functional Revision of the Definition Based on the
Thermodynamics of Complex Social-Ecological Systems
Bart Muys
Department of Earth and Environmental Sciences, University of Leuven, Celestijnenlaan 200E box 2411,
3001 Leuven, Belgium; E-Mail:; Tel.: +32 16329726; Fax: +32 16329760
European Forest Institute, Mediterranean Regional Office, Sant Pau Historic Site, Santa Victoria Pavilion,
St. Antoni M. Claret 167, 08025 Barcelona, Spain
Submitted: 13 February 2013 | In revised form: 13 June 2013 | Accepted: 19 June 2013 |
Published: 12 July 2013
Abstract: The dominant paradigm of sustainable development (SD) where the environment is
just the third pillar of SD has proven inadequate to keep humanity within the safe operational
space determined by biophysical planetary boundaries. This implies the need for a revised
definition compatible with a nested model of sustainable development, where humanity forms
part of the overall social-ecological system, and that would allow more effective sustainable
development goals and indicators. In this paper an alternative definition is proposed based on
the thermodynamics of open systems applied to ecosystems and human systems. Both sub-
systems of the global social-ecological system show in common an increased capability of
buffering against disturbances as a consequence of an internal increase of order. Sustainable
development is considered an optimization exercise at different scales in time and space based
on monitoring the change in the exergy content and exergy dissipation of these two sub-
systems of the social-ecological system. In common language it is the increase of human
prosperity and well-being without loss of the structure and functioning of the ecosystem. This
definition is functional as it allows the straightforward selection of quantitative indicators,
discerning sustainable development from unsustainable development, unsustainable stagnation
and sustainable retreat. The paper shows that the new definition is compatible with state of the
art thinking on ecosystem services, the existence of regime shifts and societal transitions, and
resilience. One of the largest challenges in applying the definition is our insufficient
understanding of the change in ecosystem structure and function as an endpoint indicator of
human action, and its effect on human prosperity and well-being. This implies the continued
need to use midpoint indicators of human impact and related thresholds defining the safe
operating space of the present generation with respect to future generations. The proposed
definition can be considered a valuable complement to the recently emerged nested system
© 2013 by the authors; licensee Librello, Switzerland. This open access article was published
under a Creative Commons Attribution License (
discourse of sustainable development, by offering a more quantitative tool to monitor and guide
the transition of human society towards a harmonious relationship with the rest of the
Keywords: anthropocene; Brundtland; dissipation; ecocrisis; entropy; exergy; pareto; resilience;
self-organization; transition
1. Introduction
Sustainable Development as defined by the Brundt-
land Commission [1]—development that meets the
needs of the present without compromising the
ability of future generations to meet their own needs
was the hopeful but paradoxical concept which
made the 1992 United Nations Conference on
Environment and Development (UNCED) in Rio de
Janeiro an unprecedented success in global coop-
eration. Hopeful, because it held the promise of
developing the world's majority of people living in
poverty. Paradoxical, because it aimed at reconciling
the right of development of every world citizen with
the global environmental burdens associated with
the current development model.
The Brundtland definition was a milestone on the
long trajectory of humanity's quest to increase and
sustain prosperity in the long term without disrupting
the natural resource base on which it has developed
(cf. [2]). New to this definition was however the
notion of (intergenerational) solidarity between
people (e.g., [3]), which adds a social dimension to
the economic and environmental dimensions of
sustainable development (further denoted as SD).
Unfortunately, this commonly adopted three-pillar
model of sustainable development [4] (Figure 1) has
not shown sufficient effectiveness for acting within
planetary boundaries [5]. The observation that the
thresholds for several planetary equilibria have been
passed (see e.g., [6]) illustrates the failure of the pil-
lar model, and implies the adoption of an alternative
sustainability model. There is increasing under-
standing that global environmental quality is a non-
negotiable boundary condition for the economic
system [7]. Obviously, something more fundamental
has to change in the overall strategies of production,
consumption and organizing markets [8]. Therefore,
a nested sustainability model considering human
society and its economy as a subsystem nested in
the planetary ecosystem [5,9] (Figure 1) seems a
more adequate basis for initiating and implementing
a transition towards planetary stewardship [10].
The success of the currently dominating pillar
discourse is in its vagueness [11]. Worldwide
sustainable development acquired a common
connotation of being something important and
positive, while leaving large flexibility of attributing
very different meanings to it among different
2. Background and Scientific Basis for a New
2.1. Fundamental Changes in the Social-Ecological
The relationship of humanity with nature changed
fundamentally between the onset of the Holocene period
about 12,000 years ago and today. Human societies
developed from small groups of hunter-gatherers
through larger farming/agricultural communities to
global urban-industrial society [8,20]. Larger complex
societies led to a more efficient buffering of external and
internal disturbances and thus to more prosperity and
well-being. From an energetic point of view, this
evolution of mankind from a modest role in the food
web to the prominent ecosystem engineer was
characterized by a regime shift in metabolic profile,
characterized by an increase in per capita daily caloric
energy consumption by more than a factor of 50 [8,21].
This was the consequence of agricultural and industrial
revolutions, which complemented manpower with
horsepower and later machine power. This improved the
human condition to such an extent that an increase in
human population with by a factor of more than 10,000
occurred. The resource needs to sustain such a large
complex system have grown far beyond what nature or
agricultural production can provide, and non-renewable
resources external to the biosphere (e.g. petrol and
uranium from the geosphere) have been discovered and
are being exploited to meet these needs [22]. These
fundamental differences in the energetic relationships to
nature between hunter/gatherer, agrarian, and
industrial-urban societies are visualized in Figure 2.
2.2. The Ecosystem Exergy Concept
In section 2.1 the crucial role of energy flows to sustain
complex human societies was explained. Ther-
modynamics is therefore a suitable discipline to describe
the macroscopic behavior of complex living systems.
Early scholars including Lotka, Schdinger and
Prigogine have developed the basics for thermodynamics
of such open systems. Schneider and Kay [23]
formulated the ecosystem exergy concept (exergy is
useful energy able to do work; it can be consumed in
contrast to energy; it is often what people mean when
using the word energy; see [24] for a review) as a
holistic descriptive model of the behavior of complex
living systems far from thermodynamic equilibrium. It
basically comprises four essential elements: a)
Ecoystems are open systems exposed to exergy fluxes
(mainly solar radiation). b) Like a dam in a river,
ecosystems accumulate part of that incoming exergy to
increase their own exergy content (Schrödinger's order
from disorder premise, [25]). c) Ecosystems with higher
exergy content are more effective dissipative structures,
i.e. dispose of a larger buffering capacity against
destructive exergy fluxes such as radiation, wind, rain,
and nutrient and sediment loss. Buffering is defined here
as any physical or chemical activity at the disposal of a
system to reduce a gradient imposed on it (see [23]).
Forest ecosystems for example buffer against sunlight
and destructive rains with their canopy structure, and
against the leaching of nutrients and erosion with their
root network; the buffer capacity depends on the quality
of the filter, i.e. the density and equal distribution of
leaves and roots. d) It is crucial to understand that
improved buffering in an ecosystem leads to improved
chances on survival and thus to evolutionary advantage,
and is as such a motor of evolution: ecosystems improve
and keep their capacity to create order and dissipate
exergy by Darwinian selection and transfer of genetic
information to subsequent generations (order from
In this model, exergy maximization is considered a
goal function of ecosystem development, which leads,
in the absence of large disturbances, to increased
control over energy and matter flows. This model
concurs with the ecosystem succession model of
Odum [26] and Bormann & Likens [27], and is
supported by thermal remote sensing observations
[28-30]. It does not conflict with the second law of
thermodynamics, because the local increase of exergy
in open dissipative systems leads to more effective
dissipation and, as a matter of fact, to an increase of
entropy of the global system, in which the ecosystem
is embedded [31].
Social scientists (e.g., [20,32]) independently came
to a similar insight that the thread of human evolution
is towards larger societies with more complex
institutional organizations leading to stronger
collective protection against human suffering of all
kinds. This remarkable parallel in structure and
function between ecosystems and human systems is
illustrated in Table 1. The ecosystem exergy concept
proves to be a powerful model to describe the
relationship between the structure and function that
ecosystems and human societies have in common
with Carnot's law for closed systems: the higher the
exergy availability of a system, the higher its potential
to perform work. Complex systems can basically: 1)
store exergy and keep it available for one or more of
the following uses (storage also implies a risk of loss,
e.g. forest biomass accumulation leading to increased
fire loss); 2) use it for maintenance (as survival
depends on it, it is typically a priority allocation); 3)
for buffering (as it offers collective long-term survival
perspectives, it is an important driver of co-evolution
for the different elements of the system); or 4) for
luxury consumption (this is exergy consumption not
leading to one of the former two outcomes, and that
in an evolutionary perspective will ultimately get
eliminated by selection pressure). Buffering leads to
better fitness of the system and is therefore a
fundamental principle of self-organization. We
therefore name our world where ecoystems and
human systems co-exist
Figure 2. A simplified representation of the
energetic relationships between mankind and
nature in a) a primitive society; b) an agrarian
society; and c) an industrial-urban society (after
[33]). Legend of symbols: E
= incoming solar
energy; P
= primary production of plant
biomass in the ecosystem; P
= production of
herbivores in the ecosystem; P
= production of
carnivores in the ecosystem; E
= energy needs
of the primitive society; P
= primary production
in the agricultural ecosystem; P
= production
of herbivores in the agricultural ecosystem; E
energy needs of the agrarian society; E
= non
renewable energy sources; P
= industrial
production; E
= energy needs of the urban
society. Dashed, resp. dotted lines indicate fluxes
of relatively decreasing importance, which in
absolute terms may be increasing. Note that
through the evolution from primitive over
agrarian to industrial-urban society the human
population increases, the area of (semi-)natural
systems decreases in favor of agricultural and
urban land; wildlife decreases and large
predators become extinct.
3. Proposal for a New Definition
3.1. The Anthropocene
Human communities form part of the biosphere and have
always been heavily dependent on resources extracted
from the ecosystem for their exergy provision, and on
other ecosystem services for their buffering (Figure 2). In
recent history humans discovered and used extensively
more concentrated exergy sources exogenous to the
biosphere (coal, petrol, natural gas and uranium from the
geosphere). Apart from the risk of depletion, their
consumption causes toxic or climate forcing emissions,
which provoke disturbances in the biosphere. Meanwhile
their use greatly increases the power of humans to modify
the biosphere. As a consequence, increasing amounts of
land gradually or abruptly change from the sphere of
nature dominion to increasing human dominion
[17,34,35]. It makes the protective vegetation canopy
thinner and scarcer, undermining its buffering capacity for
light, heat, wind, rain and dust. Human efforts to
concentrate solar exergy in useful target crops through
intensive agriculture, forestry and biocide use are leading
to an overall simplification of the biosphere (see e.g.,
[36]). Human development-induced changes in biogeo-
chemistry and atmospheric composition at planetary scale
are large enough to consider the onset of a new
geological era, called anthropocene [37].
A thermodynamic interpretation of the anthropocene
would be that the human society is increasingly behaving
as a separate system, which means that it increases its
order at the expense of the order in the biosphere. The
current development of human society is causing a trade-
off with entropy production in its environment, which is
threatening the buffering capacity of the biosphere in the
long term. Anthropogenic entropization of the biosphere is
the essence of the ecocrisis in bufferworld. Considering
the heterotrophic metabolism of humans and the large
dependence of human society on ecosystem services [38]
(Figure 3), it must be emphasized that human society is a
subsystem nested in the biosphere. It is not viable
without the ecosystem, while the ecosystem is viable
without human society. As a consequence, this evolution
seems more worrisome for mankind than for the
biosphere in general.
Table 1. The ecosystem exergy model of Schneider
& Kay [24] as the universal goal function of complex
self-organizing systems, here applied to ecosystems
and human society, illustrating the analogy in
structure and function between the two systems.
Ecosystems Human Society
Goal function Max[buffer exergy
flows] through
max[exergy content]
Max[buffer exergy
flows] through
max[exergy content]
Main exergy
Solar exergy Ecosystems, fossil
Biomass, genetic
diversity, diaspores,
foodwebs and other
ecosystem structures
Food reserves,
houses, money, social
and institutional
structures, other
capital and assets
Memory and
DNA DNA, oral and written
information, bits and
Buffering against
sunlight, temperature
change, nutrient loss,
water runoff, sediment
loss, wind damage
Shelter against
climatic extremes,
internal and external
threats in terms of
conflict, hunger,
disease, natural and
technical disasters
See [25], supplementary material S4 for a discussion on the
exergy content of information. A tree seed has much lower
exergy content than an adult tree weighing 5 tonnes, but it holds
the potential to accumulate a similar amount. Also, money is an
important carrier of exergy, which can be exchanged at any time
against exergy for maintenance or to perform buffer work.
Memory and information transfer are essential to share
successful experiences of exergy accumulation and exergy
buffering with conspecifics of the next generation. Plants transfer
information mainly through DNA, while vertebrate animals show
plenty of learning methods in addition to genetic transfer.
Although the hereditary intelligence of humans is not very much
higher than that of apes, the revolutions of non-genetic
information transfer through oral and written communication
have boosted their progress in exergy capture and exergy
3.2. The Definition
Based on the former, we define Sustainable
Development as the increase of the exergy content
and exergy buffering of human society, not provoking
a measurable decrease of exergy content and exergy
buffering of the ecosystem. This scientific definition is
valid and applicable for social-ecological systems at
different scales of time and space, e.g. over a decade
at the level of a local community with its surrounding
landscape, or on an annual basis at the level of the
world community with its global natural resources.
This can be easily translated into everyday language
as the increase of human prosperity (exergy content)
and human well-being (exergy buffering) without the
loss of ecosystem structure (exergy content) and
ecosystem functioning (exergy buffering). In short it is
development that does not degrade the biosphere. It
is important to observe that both the human and the
ecosystem side of the definition have a structural and
a functional component: human prosperity and
ecosystem structure and composition as the structural
component (exergy content, order); human well-being
and ecosystem function as the functional component
(exergy dissipation, bufferwork). As mentioned earlier
exergy content and exergy dissipation are related but
not linearly: exergy content is a necessary condition
to perform bufferwork (no well-being without capital),
but inversely, exergy content has many options, as it
can be used as a reserve, maintenance, luxury
consumption or buffering. Buffering can also be the
mere consequence of the presence of dissipative
structures. Especially on the human side, the build-up
of capital with a limited increase in overall societal
buffering capacity has been common in the history of
mankind, and has been extensively debated in
classical socio-economic literature. Indicator selection
should therefore include both prosperity (economic
pillar) and well-being (social pillar) aspects to
measure human development.
The foregoing has made clear that increasing the
prosperity and well-being of human society often
implies the extraction of resources from ecosystems,
emissions into ecosystems, and competition for space,
and will thus often be at the expense of their
structure and function. These trade-offs between
human society and ecosystems suggest the existence
of a set of optimal solutions as a compromise
between human development and ecosystem
development. Technically speaking, the new definition
is the result of an optimization exercise, that is
searching for efficient solutions along a Pareto front
formed by the trade-off between human prosperity
and well-being and ecosystem structure and function
(Figure 4). In Figure 4 we can see how sustainable
development can move the system to improved
human prosperity and well-being under a status quo
or an improvement of the ecosystem structure and
function, until it reaches a new state (the Pareto
efficient solution) where further human development
would unavoidably lead to ecosystem degradation. It
becomes obvious that sustainable development
(development without the loss of ecosystem structure
and function) is a difficult challenge, and does not
seem achievable with technical measures alone or
isolated project-wise actions within the current
institutional context, but would need a large societal
transition accompanied by a global institutional reform
[8,41]. Such a transition should lay the basis for a
more harmonious co-evolution between humans and
ecoystems as a unified social-ecological system
inhabiting the biosphere. Possible elements of such a
transition are captured by the proposed definition: an
increase of resource efficiency (creating more
prosperity and well-being with less input or output