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
1,2
1
Department of Earth and Environmental Sciences, University of Leuven, Celestijnenlaan 200E box 2411,
3001 Leuven, Belgium; E-Mail: [email protected]; Tel.: +32 16329726; Fax: +32 16329760
2
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 (http://creativecommons.org/licenses/by/3.0/).
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
biosphere.
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
42
2. Background and Scientific Basis for a New
Definition
2.1. Fundamental Changes in the Social-Ecological
System
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, Schrödinger 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
order).
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
bufferworld
.
43
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
SO
= incoming solar
energy; P
P
= primary production of plant
biomass in the ecosystem; P
H
= production of
herbivores in the ecosystem; P
C
= production of
carnivores in the ecosystem; E
PS
= energy needs
of the primitive society; P
A
= primary production
in the agricultural ecosystem; P
AH
= production
of herbivores in the agricultural ecosystem; E
AS
=
energy needs of the agrarian society; E
NR
= non
renewable energy sources; P
I
= industrial
production; E
US
= 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.
44
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
source
Solar exergy Ecosystems, fossil
fuels
Exergy
storage
1
Biomass, genetic
diversity, diaspores,
foodwebs and other
ecosystem structures
Food reserves,
houses, money, social
and institutional
structures, other
capital and assets
Memory and
information
transfer
2
DNA DNA, oral and written
information, bits and
bytes
Exergy
dissipation
(Buffer
function)
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
1
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.
2
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
buffering.
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
45
Figure 3. Conceptual scheme showing the relationships between ecosystems and social systems as
closely interlinked subsystems of the overarching social-ecological systems occurring in the biosphere.
Both subsystems develop a structural/compositional component (exergy content), which provides exergy
to their functional component (buffering). The composition, structure and function of the ecosystem offer
a potential source of ecosystem services to human society [39,40], which may use the ecosystem service
benefits to increase prosperity (the economic pillar of development) and well-being (the social pillar of
development). The feedback arrow at the bottom illustrates that ecoystems are heavily shaped by
deliberate and unintended human influences.
Figure 4. Sustainable development as an optimization exercise between human goal functions and
ecosystem goal functions. a) The Pareto front is the set of efficient solutions (it means solutions where
further human development would unavoidably lead to ecosystem damage, and vice versa). Trajectories
from a present non Pareto-efficient situation A to B and C show the main development options. Trajectory
AB evokes the challenge of sustainable development, increasing human goals without decreasing
ecosystem goals. All trajectories between As and As' are Pareto efficient and therefore sustainable.
Trajectory AC shows the current development trend, which is outside the trajectory range between As and
As’ and therefore unsustainable, given that it increases human goals while decreasing ecosystem goals. b)
As a consequence of anthropogenic environmental degradation the current Pareto front may shrink to a
future Pareto front with lower potential exergy buffering for both sub-systems. Under such a regime shift
(cf. Section 5.2) an effort of sustainable development AB will end up as an inferior adjusted sustainable
development AB' (cf. [41]). Lovelock [42] considers that we are now already in a situation where further
development without environmental damage is no-longer possible (this means that we are on or above
the Pareto front) and where the shrinking resource base urges for a so-called sustainable retreat to a
lower future Pareto front, which can be visualized by the trajectory DE.
46
related impact on the structure and function of the
ecosystem), the replacement of overconsumption by a
more frugal lifestyle (decreasing ecosystem impacts
caused by prosperity that does not contribute much to
well-being), and setting safeguards on vital ecosystem
structures and functions (implementing caps on
human development where it directly affects vital
ecosystem structures and functions).
This definition is transparent and functional.
Anchored in the laws of thermodynamics it allows the
selection of quantitative indicators (see Section 4).
4. Indicators and Application
Rather than presenting a concrete indicator set, some
guiding principles are formulated as to the selection
and processing of indicators based on the new
definition of SD. Sustainability is evaluated for a given
social-ecological system with defined system
boundaries over a certain period of time, by
comparing human development (change of prosperity
and well-being over this time period) with ecosystem
development (change in structure and function over
this time period), which can be written as:
(1)
where I
PW
is the selected indicator for human
prosperity and human well-being (remember that,
thermodynamically speaking, this corresponds
respectively to the exergy content and exergy
buffering of the human system), I
ESF
the selected
indicator for ecosystem structure and ecosystem
function (remember that these are respectively the
exergy content and exergy buffering of the
ecosystem), t
0
and t
1
the start and end of the
considered evaluation period.
Basically the sustainability check of equation (1)
has four possible outcomes:
• Increase in numerator and denominator: sustainable
development;
• Increase in numerator and decrease in denominator:
unsustainable development;
• Decrease in numerator and increase in denominator:
sustainable retreat;
• Decrease in numerator and decrease in denom-
inator: unsustainable stagnation.
The magnitude of the obtained ratio in comparison
with values from other regions or time periods allows
interpretation of how sustainable or unsustainable the
observed development is. This is possible for
contemporary studies (e.g. annual monitoring of
human prosperity and well-being and ecosystem
structure and function in a Brazilian catchment after
inaugurating a new dam), retrospective analysis of
human development in the past (e.g., calculating
sustainability in Roman and Byzantine periods based
on forest resource modeling modulated by population
density estimates from archaeological evidence for an
ancient city excavated in Turkey), or prospective
analysis of human development in the future (e.g.,
modeling the biodiversity loss caused by climate
change for different IPCC SRES scenarios based on
scenarios of human development, see [43]).
As such, this approach sets a framework for
continuous monitoring and improvement, rather than
proposing fixed sustainability thresholds. This makes
sense, because as a consequence of regime shifts,
which are inherent to complex social-ecological
systems (Figure 4b and Section 5.2), sustainable
development is a moving target. But Rockström et al.
[6] argue that to avoid unwanted regime shifts in the
biosphere, thresholds must be placed on vital
biophysical conditions that determine the safe space
within which humans can operate. Putting a minimum
threshold level on ecosystem exergy content and
dissipation is perfectly possible within the here
proposed framework.
Note that numerator and denominator are not
necessarily in the same units, unless exergy analysis
would be applied. In practice analysts may want to
work with proxy indicators having different units, e.g.,
Gross Domestic Product (GDP per capita in monetary
units, $ per capita per annum) for human prosperity
and well-being (in fact , the GDP per capita, also
called the standard of living is not a proxy of the
exergy content but of the annual exergy inflow in the
human system, which can be used for maintenance,
increase in prosperity, and increase in well-being) and
e.g., protected area (in km
2
) or free Net Primary
Production (fNPP, in ton per ha per annum, cf.
[44,45]) for ecosystem structure and function (in fact,
the fNPP is not a state indicator but measures the
fraction of the annual increase in ecosystem exergy
content that is not extracted by humans), or may
want to work with dimensionless composite indicators
like e.g. Inequality Adjusted Human Development
Index (cf. [46]), Genuine Progress Indicator (cf. [47])
or Gross National Happiness (cf. [48]) for human
prosperity and well-being (although the latter already
includes ecosystem fitness).
A large remaining challenge is the development of
indicators for ecosystem structure and function. In
fact, the effects of human activity on ecosystem
composition, structure and function are, thus far,
poorly understood. As a consequence, indicators and
monitoring instruments for ecosystem structure and
function are still largely underdeveloped, and
multitemporal information of ecosystem trends are
hardly available. According to Rosen [49] it is a big
asset of ecosystem exergy analysis that it can
measure the increase in disorder in ecosystems
associated with human environmental impact. Odum
[27] was one of the first to propose an indicator set
47
I
PW , t
1
− I
PW ,t
0
I
ESF , t
1
− I
ESF ,t
0
for measuring ecosystem maturity based on eco-
system thermodynamics. Several other indicators
measuring the degree of self-organization, integrity or
naturalness of ecosystems have been proposed ever
since. Bendoricchio and Jørgensen [50] came up with
an elegant formula to calculate the exergy content of
ecosystems including the exergy content included in
its biodiversity. This formula was criticized as
thermodynamically incorrect (e.g., [51]), but was later
recycled as a calculation of eco-exergy, a proxy for
ecosystem exergy content useful for accounting
purposes. Others developed indicators for solar exergy
dissipation by ecosystems, based on the evaluation of
their energy balance (e.g., [30,52]). There is also a
long tradition of trying to give monetary value to
ecosystems and ecosystem services, which is
potentially a good proxy of ecosystem exergy content
and buffering. But it is important to recognize the
important limitations of economic valuing, including
the poor methodological development of valuing
biodiversity and biodiversity function, and the serious
limitations of the ceteris paribus principle of partial
equilibrium when upscaling value to the global level
(cf. the criticized global valuing of ecosystem services
like pollination by [53]). For the time being, end point
indicators of changes in ecosystem state and function
in the denominator of equation (1) can be replaced by
mid-point indicators of human input-related (re-
sources use) and output-related (emissions) impacts,
or inversely, of human efforts towards sustainability,
like efficiency indicators. Another complication of
selecting indicators is the problem of spillover and
double counting. Spill-over happens when a selected
indicator does not include all aspects of human or
ecosystem development and, as a consequence, shows
externalities. A concrete example is the use of forest
transition [54] as a sign of sustainable development.
The forest index of countries typically evolves from a
trend of more people, less trees in the early stages of
development (positive numerator and negative denom-
inator in our formula = unsustainable development) to
a trend of more people, more trees in later stages of
development. The explanation of this geographical
theory is however leakage and spillover: in later
stages of development countries increasingly thrive on
imported carrying capacity (wood imports from
neighboring countries with a lower standard of living
is exporting the deforestation problem, see [55], and
on converting the energy system from wood-fuel to
fossil fuel turning the input-related environmental
impact into an output-related environmental impact.
Double counting is a typical problem of using indicator
baskets. In the land use impact method used in
Garcia et al. [56] for example, Leaf Area Index is used
as an indicator of ecosystems structure (exergy
content) and soil erosion is used as an indicator of
ecosystem function (exergy dissipation), but the soil
erosion buffer is a direct consequence of the presence
of a large leaf area.
5. Discussion
5.1. Focus and Functional Strength of the New
Definition
The revised definition of SD has a more solid scientific
background than earlier ones, which facilitates the
selection of indicators that are not arbitrary, but that
quantify the exergy content and exergy dissipation of
both human and ecosystem subsystems of the social-
ecological system.
The system boundaries for global SD assessment are
set to the biosphere, the vital space for life on earth (or
to part of the biosphere for SD assessment at a smaller
geographical level). The geosphere is excluded, which
means that in contradiction to some impact methods (cf.
[57]) the use of fossil fuels or ores is not considered an
environmental burden, but obviously the impact on the
ecosystem of careless extraction and emissions as a
consequence of its use are considered a burden.
Different from the Brundtland definition, the revised
definition does not focus on the trade-off between
present and future generations of humans, but rather on
present, past and future trade-offs between humans and
ecosystems. This is similar to the definition recently
published by [5]. One could say that this approach is
less anthropocentric than the Brundtland definition and
other definitions along the line of the pillar discourse, as
it proposes equal interests for humans and ecosystems.
Since humans depend on ecosystems, the state of the
ecosystem partly reveals the fate of future generations
of humans. But only two of the nine planetary thresholds
that [6] use to determine the safe operating space of
humans to avoid a catastrophic shift in the planetary
metabolism are directly related to ecosystem structure
and function (biodiversity loss, change in land use),
while the seven others (climate change, ocean
acidification, stratospheric ozone depletion, nitrogen and
phosphorus cycles change, global freshwater use,
atmospheric aerosol loading and chemical pollution) are,
albeit interlinked with ecosystems, physical-chemical
state and rate variables that will affect both humans and
ecosystems of the future. This means that planetary
stewardship (see [58]) needs to consider effects of
present development on both present ecosystem
structure and function and future human and ecosystem
development. In that sense, the indicator set linked to
the denominator of equation [1] should not be limited to
the structure and function of the ecosystem, but it is
recommended that it includes also physical/chemical
state indicators of the overall social-ecological system.
5.2. Determinism versus Stochasticity
The exergy concept shows several parallels with the
ecosystem succession theory of Odum (1969) [26],
which has been criticized for being unidirectional and
deterministic. In reality stochastic phenomena make the
behavior of social-ecological systems largely unpre-
48
dictable, and disturbances have to be considered
inherent to the existence of ecosystems. Kay [59]
showed that the ecosystem exergy concept is not in
contradiction with chaos theory and the occurrence of
alternative stable states [60]. The panarchy model for
the ecological and social systems of Gundersen & Holling
[61] very satisfactorily links the deterministic com-
ponents of ecosystem thermodynamics with the
stochastic aspects of chaos theory into one single theory.
By adding an extra dimension of resilience to the trend
of exergy increase during a process of self-organization,
they are able to clarify how disturbance is inherent to
complex systems: increasing order and fine-tuning the
bufferwork to the small recurrent disturbances, the
system is losing resilience, and becomes fragile and
sensitive for catastrophic shift to a different state. Figure
5 illustrates how the existence of such stable states in
both human systems and ecosystems complicates the
goal setting of sustainable development.
Figure 5. Example based on observations from
[62] of an extremely non-linear response of
ecosystems to pressures caused by human
development, giving existence to alternative stable
states and making sustainable development a
moving target. The Afromontane Forest of semi-
arid northern Ethiopia is well buffered against the
effects of human development, but beyond a
certain threshold the forest collapses and changes
into degraded grazing land. If forest restoration
efforts are made, it appears that restoration only
becomes possible at much lower pressures than
the collapse occurred, and restoration does not
directly result in the recuperation of the original
vegetation but in a bush state with lower
ecosystem services than the original forest. This
phenomenon of non-reversibility is called
hysteresis, and is a typical indication of alternative
stable states.
The panarchy theory is a good basis to explain the
efforts needed for a transition towards sustainable
development. The loss of resilience in mature complex
systems is congruent with the so-called institutional
lock-ins described in transition theory [63]. Transition
only boosts when innovation niches are created
through institutional reforms focusing on the increased
resilience of society and ecosystems [64-66]. This
means that the transition pathway towards sustainable
development could pass through phases where the
order or buffer capacity of the human society
temporarily decreases, while the resilience increases. In
order to evaluate the success of a transition process, it
is therefore recommended that monitor resilience
indicators of the social-ecological system as a whole is
also carried out, in addition to equation (1).
6. Conclusion
The proposed definition of sustainable development
completes the nested systems discourse on
sustainability, which considers that socio-economic
development needs to operate within the safe
operating space defined by planetary boundaries. It
is a science-based functional definition, which
facilitates the selection of indicators, and the
development of simple measuring tools for the
evaluation of complex social-ecological systems. It can
serve as an operational support to assess the progress
along the transition pathway towards a sustainable
society. It hopes to contribute to moving sustainable
development away from a fuzzy contradiction in terms
towards an objective optimization problem between
the human system and the ecosystem, two strongly
interlinked sub-systems, nested in the overall social-
ecological system, and showing fundamentally similar
patterns and processes of structures and functions for
buffering. It finally holds an active invitation for
human society to make a transition to more
harmonious development as part of the social-
ecological system rather than autonomous devel-
opment at the expense of the ecosystem.
Acknowledgements
The author wrote this article in the context of the
project GOA/13/004 "Approaching patterns of
nature−society interactions in regional development.
An interdisciplinary dialogue between past and
present in the region of Sagalassos" financed by the
KU Leuven Research Fundand with support of the
KLIMOS R&D platform on climate and development,
financed by VLIR and Belgian Development
Cooperation.
49
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