1:capable of being sustained2 a:of, relating to, or being a method of harvesting or using a resource so that the resource is not depleted or permanently damaged <sustainable techniques> <sustainable agriculture> b:of or relating to a lifestyle involving the use of sustainable methods <sustainable society>
— sus·tain·abil·i·ty noun — sus·tain·ably adverb Date: circa 1727 - - Merriam-Webster
Sustainability is 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, and fisheries, and human communities in general and the various systems on which they depend.
In recent years an academic and public discourse has led to this use of the word sustainability in reference to how long human ecological systems can be expected to be usefully productive. Observers point out that in the past, complex human societies have died out, sometimes as a result of their own growth and associated impacts on ecological support systems. The implication is that modern industrial society, which continues to grow in scale and complexity, might also collapse.
The implied preference would be for systems to be productive indefinitely, or be 'sustainable." For instance, "sustainable agriculture" would require agricultural systems expected to last indefinitely, "sustainable development" would be development of economic systems that last indefinitely, and so on. A side discourse relates the term sustainability to longevity of natural ecosystems and reserves (set aside for other-than-human species), but the greatest emphasis has been on human systems and anthropogenic problems, such as anthropogenic climate change, or the obviously anthropogenic depletion of fossil fuel reserves.
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 such as climate change and oil depletion. This discussion is an inherently useful activity. (What is even more useful, of course, is to then go out and find a way to make some unit of economic production -- a business firm, a family household, a farm -- more sustainable.) To help in this kind of discussion, 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. There also is some usefulness in talking 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, however, is that definitions of sustainability 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. (A list of the most important, with explanations, follows.) The often-uttered statement that there "is no agreed-upon definition of sustainability" results from this confusion. Utterers, however, overlook the fact that the word has a reasonably agreed-upon meaning, as a plain, English-language word, and in this particular environmental usage is used to refer to ecological longevity of human support systems. Definition is therefore implicit in usage. Human economic systems that are sustainable will be expected to last for a very long time; by definition, indefinitely, and will therefore be around for future generations to make use of, should they choose to do so. Those systems that are not sustainable, will not last as long, will be of less or no use to future generations, and may harm other systems that future generations will need.
One of the first and most oft-cited definitions of sustainability, and almost certainly the one that will survive for posterity (should there be 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."[1]
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, and is in the category of philosophical statements sometimes called "extension theories," by which certain rights, and duties of care, are extended to groups currently excluded from consideration for those rights and duties. In this case, usufruct rights to ecological goods and services are extended to future generations.
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.