Sustainability
Sustainability can be defined as humanitys investment in a system of living, projected to be viable on an ongoing basis that provides quality of life for all individuals of sentient species and preserves natural ecosystems.  
Sustainability in its simplest form describes a characteristic of a process or state that can be maintained at a certain level indefinitely. The term, in its environmental usage, refers to the potential longevity of vital human ecological support systems, such as the planet's climatic system, systems of agriculture, industry, forestry, fisheries, and the systems on which they depend. In recent years, public discourse has led to a use of "sustainability" in reference to how long human ecological systems can be expected to be usefully productive. In the past, complex human societies have died out, sometimes as a result of their own growth-associated impacts on ecological support systems. The implication is that modern industrial society, which continues to grow in scale and complexity, will also collapse.
The implied preference would be for systems to be productive indefinitely, or be "sustainable." For example, "sustainable agriculture" would develop agricultural systems to last indefinitely; "sustainable development" can be a development of economic systems that last indefinitely, etc. A side discourse relates the term sustainability to longevity of natural ecosystems and reserves (set aside for other-than-human species), but the challenging emphasis has been on human systems and anthropogenic problems, such as anthropogenic climate change, or the depletion of fossil fuel reserves.

# Definitions
Though relatively new, the term "sustainability" has already proved useful. Sustainability discourse is discussion of how to make human economic systems last longer and have less impact on ecological systems, and particularly relates to concern over major global problems relating to climate change and oil depletion. More useful than discussion, however, is to find ways to make some unit of economic production  a business firm, a family household, a farm  more sustainable. To assist in this, it is meaningful, and pragmatic, to speak of some practices being "more sustainable" or "less sustainable." Thus energy-saving compact fluorescent light bulbs might be considered more sustainable than incandescent ones, and so on. Given the science, it is more apt to talk of moving "towards sustainability," or away from it. Sustainability advocates would argue that this kind of discourse helps inform debate about human impacts on planet Earth.
One reason many commentators consider sustainability hard to define, is the sheer number of meanings of sustainability that abound. The popularity of the term, and the many isolated attempts on the part of governments and other agents to begin sustainability programs, have led to these competing definitions, and much confusion. The often-uttered statement that there "is no agreed-upon definition of sustainability" results from this confusion.
One of the first and most oft-cited definitions of sustainability, and almost certainly the one that will survive for posterity, is the one created by the Brundtland Commission, led by the former Norwegian Prime Minister Gro Harlem Brundtland. The Commission defined sustainable development as development that "meets the needs of the present without compromising the ability of future generations to meet their own needs." The Brundtland definition thus implicitly argues for the rights of future generations to raw materials and vital ecosystem services to be taken into account in decision making.
Sustainability can be defined both qualitatively in words, as an ethical/ecological proposition such as the Brundtland definition above, and quantitatively in terms of system life expectancy and the trajectory of certain factors or terms in the system. Operationalization of the term obviously raises the question of a quantitative definition; in order to set sustainability goals and achieve them, communities have to know whether their efforts are successful or not, so they have to know what to measure. Most recently, the leading attempts at operationalization have given metrics of climate emissions, and their reduction, some level of priority above other metrics. Since the factor of fossil fuel use is necessarily embedded in any meaningful climate emissions metric, climate neutrality (or the state of being climate neutral) is not an unreasonable partial proxy metric for overall sustainability, and is also relatively easy to measure. Many institutional sustainability programs have placed becoming climate neutral at the top of their list of sustainability goals, although the social and deliberative processes by which this prioritization took place is generally unexamined, or only partially examined a priori.
Other sustainability concerns might be harder to account for because of the complexity of their cycles and systems. Quantitative analysis in sustainability thinking typically uses system dynamics modeling, because systems are often non-linear and so-called feedback loops are key factors. So, for instance, important human ecological sub-systems that could be analyzed or modeled in this way might include the nitrogen cycle, and cycles of other important nutrients, in sustainable agriculture, or the depletion of oil reserves and other fossil energy reserves. One of the key problems in communicating the quantitative impacts of many sustainability issues, such as climate change, oil depletion, or population growth, is that feedback effects often create exponential change. Because the mathematics of exponentiality is not well-understood by ordinary people, and since human nature seems to be to expect linear change, if any, people are often surprised by the speed and rate of change of sustainability phenomena. This has led to recommendations that understanding feedback in dynamic systems be a primary goal of basic environmental education.
According to Organisation for Economic Co-operation and Development (OECD), it stated that in 1980, the term sustainable development was introduced, and it popularised in the 1987 report of the World Commission on Environment and Development (the Brundtland Commission). In the Brundtland Commission, sustainable development was defined as development that meets the needs of the present without compromising the ability of future generations to meet their own needs. From this definition, it has been noted by the Commission that it contained two key concepts which are needs and limitations. These can be described as following:
needs, meaning in particular the essential needs of the worlds poor, and limitations, meaning limitations imposed by the state of technology and social organization on the environments ability to meet present and future needs

# Conceptual issues in sustainability thinking

## Values, purpose, the focus on outcomes
For what purpose are we conserving natural capital? Is the society supported by this capital just and decent, worthy of preservation? Obviously, the work of sustaining a society raises the question of the moral worth of that society. This is clearly a question of ethics or values.
Values vary greatly in detail within and between cultures, as well as between academic disciplines (e.g., between economists and ecologists).   The introduction of social values to sustainability goals implies a much more complex and contentious debate, and those focused on ecological impacts tend to strongly resist non-ecological interpretations.
Others see at the heart of the concept of sustainability a fundamental, immutable value set that is best stated as 'parallel care and respect for the ecosystem and for the people within'. From this value set emerges the goal of sustainability: to achieve human and ecosystem longevity and well-being together. Seen in this way, the concept of sustainability is much more than environmental protection in another guise. It is a positive concept that has as much to do with achieving well-being for people and ecosystems as it has to do with reducing ecological stress or environmental impacts. This kind of vision is of course much more debatable or subjective than the simpler definitions such as the Bruntland Definition or the "Daly Rules."
At its least, sustainability implies paying attention to comprehensive outcomes of events and actions insofar as they can be anticipated at present. This is known as full cost accounting, or Environmental accounting.  This kind of accounting assumes that all aspects of a system can be measured and audited (Environmental audits).
Environmental accounting can be a limited biological interpretation as in ecological footprint analysis, or may include social factors as in the ICLEITriple Bottom Line standards for urban and community accounts. Obviously, sustainability definitions and metrics that focus on accounting are often less prescriptive of economic systems or of political, philosophical, or religious values.
At most, sustainability is clearly intended by some advocates as a means of configuring civilization and human activity so that society, its members and its economies are able to meet their needs and express their greatest potential in the present, while preserving biodiversity and natural ecosystems, and planning and acting for the ability to maintain these ideals in a very long term. It can easily be seen that the definitions and metrices that might result are prescriptive of political, philosophical or religious values.
The evolution of the concept of Sustainability in engineering design is modern, where it is a fundamental change in philosophy, it is a non linear process driven by internal values instead of compliance in response to imposed requirements. This requires a multi-disciplinary approach to decision making, consideration of long term sustainability over short-term benefits (Hasna, 2007)

## Common principles
Despite differences, a number of common principles are embedded in most charters or action programmes to achieve sustainable development, sustainability or sustainable prosperity. These include (Hargroves & Smith 2005, see bibliography):
- Dealing transparently and systemically with risk, uncertainty and irreversibility.
- Ensuring appropriate valuation, appreciation and restoration of nature.
- Integration of environmental, social, human and economic goals in policies and activities.
- Equal opportunity and community participation/Sustainable community.
- Conservation of biodiversity and ecological integrity.
- Ensuring inter-generational equity.
- Recognizing the global integration of localities.
- A commitment to best practice.
- No net loss of human capital or natural capital.
- The principle of continuous improvement.
- The need for good governance.

## Weak versus strong sustainability
However, a distinction between different 'degrees' of sustainability should be made. The debate currently focuses on the sustainability between economy and the environment which can in other words be considered as between 'natural capital' and 'manufactured/man-made capital'. This is also captured in the 'weak' versus 'strong' sustainability discussions, which began as a debate between conservative British economist Wilfred Beckerman and sustainability founder Herman E. Daly.
Weak sustainability is advocated by the Hartwick's Rule, which states that as long as TOTAL capital stays constant, sustainable development can be achieved.  As long as the diminishing natural capital stocks are being replaced by gains in the man-made stock, total capital will stay constant and the current level of consumption can continue. The proponents believe that economic growth is beneficial as increased levels of income lead to increased levels of environmental protectionism. This is also known as the 'substitutability paradigm'.
Conversely, strong sustainability, as supported by Herman Daly, holds the view that natural capital and man-made capital are only complementary at best. In order for sustainable development to be achieved, natural capital has to be kept constant independently from man-made capital. This is known as the 'non-substitutability paradigm'. Advocates of weak sustainability thus make a categorical error. So, for instance, and according to Daly, it makes no sense to substitute man-made capital, in the form of fishing boats, for natural capital, in the form of fish stocks, and the attempt to do so usually ends in ecological disaster.

## Population growth and consumption
One of the critical issues in sustainability is that of human overpopulation combined with current lifestyle patterns.  Some studies have suggested that the current population of the Earth exceeding six billion, is too great to support sustainably, others, such as the book The Improving State of the World, argues that this is sustainable. At current material consumption levels, this challenge for sustainability is distributed unevenly. According to calculations of the ecological footprint, the ecological pressure of a US resident is 12 times that of a resident of India and 24 times that of a Somali resident. Obviously, were the total human population to be reduced, it would be easier to achieve sustainability in most human systems. Equally, reduction of levels of consumption by those nations with large per-capita footprints could have an equal or greater impact.  The inclusion of discussion of the factor of population in the overall sustainability debate has led to the accusation, typically from conservative or libertarian economists such as Julian Simon, that sustainability advocates are neo-malthusians.
With the world population continuing to grow, there is increasing pressure on arable land, water, energy, and biological resources to provide enough food while supporting viable ecosystems. World Bank and United Nations studies show that there are over 854 million people who are undernourished. This is due to a combination of lack of food, low incomes, and poor food distribution. The world population is projected to grow from the current 6 billion and peak at 9.2 billion in 2075 due to the demographic transition.. With expanding population, the food problem will worsen.
Critics of efforts to reduce population rather than consumption fear that efforts to reduce population growth may lead to human rights violations such as involuntary sterilization and the abandoning of infants to die. Some human-rights watchers report that this is already taking place in China, as a result of its one child per family policy.
It appears inevitable to some commentators that human population numbers will be constrained and brought into some form of equilibrium by the Malthusian limit and in accordance with the Logistic function.  In his book Collapse, author Jared Diamond presents several societies where population growth mixed with unsustainable consumption levels have led to collapses in population numbers.

# The phenomenon of change resistance
The above concepts focus primarily on the proper practices required to live sustainably. However, there is also the need to consider why there is such strong resistance to adopting sustainable practices.

## Barriers to achieving ecological sustainability
There has been long-standing and widespread public awareness of the seriousness of the consequence of overpopulation (e.g., Nelson, 1986; Yankelovitch, et al., 1983; Diamond, Jared (2005) ).
Unruh (2000, 2002) has argued that numerous barriers to sustainability arise because today's technological systems and governing institutions were designed and built for permanence and reliability, not change. In the case of fossil fuel-based systems this is termed "carbon lock-in" and inhibits many change efforts.
Others, particularly Thwink.org,  argue that if enough members of the environmental movement adopted a problem solving process that fit the problem, the movement would make the astonishing discovery that the crux of the problem is not what it thought it was. It is not the proper practices or technical side of the problem after all. Any number of these practices would be adequate. Instead the real issue is why is it so difficult to persuade social agents (such as people, corporations, and nations) to adopt the proper practices needed to live sustainably? Thus the heart of the matter is the  change resistance or social side of the problem.
This is generally attributed to change resistance (see, e.g., Thwink.org), viewed as involving change in individual values, whether at personal, corporate, or collective levels (see e.g., Stafford Beer). Unfortunately, it has been frequently demonstrated, e.g., in the studies cited, that peoples values are, in general, in the right place. The problem is to enact them. This has led to the preparation of numerous wish listssuch as that compiled by Shah, H., & Marks, N. (2004)drawing together many recommendations for government action.
Government and individual failure to act on the available information is widely attributed to personal greed (deemed to be inherent in human nature) especially on the part of international capitalists. But even Karl Marx did not suggest this, instead highlighting sociological processes which have been in operation for thousands of years. Murray Bookchin likewise documents this process over millennia, describing, in detail, the factors that were operational at each transition point. If fault is to be found with Marx's work it can be argued that it lies elsewhere. Because he believed that the collapse of capitalism was imminent, he never discussed how to run society in an innovative way in the long term public interest. Strangely, Bookchin, in the end, does not suggest how to intervene in and harness the sociocybernetic processes he has identified but contents himself with an account of requisite features of a sustainable society derived from his analysis of organic (primordial) societies.
Two things seem to follow from this brief discussion.
- It is vital to follow up the study of the sociocybernetic, or systems (see also systems theory), processes which, it seems, primarily control what happens in society.
- We should use the social-science-based insights already available to evolve forms of Public management that will act on information in an innovative way in the long term public interest.

# Precautionary principle
The precautionary principle states that if there is a risk that an action could cause harm, and there is a lack of scientific consensus on the matter, the burden of proof is on those who would support taking the action.
When competing "experts" recommend diametrically opposing paths of action regarding resources, carrying capacity, sustainability, and the future, we serve the cause of sustainability by choosing the conservative path, which is defined as the path that would leave society in the less precarious position if the chosen path turns out to be the wrong path.
Cleaner Production aims at applying the precautionary principle to industrial processes. The objective is to minimize waste, emissions, energy consumption by optimizing the organization and technology of production, and increasing the use of renewable resources.