Despite its popularity, the term sustainability is used in a wide variety of ways with a plethora of meanings. At the 1992 United Nations-sponsored Earth Summit in Rio de Janeiro, sustainable development was discussed in the context of a 1987 report entitled Our Common Future, which defined sustainable development as "development that meets the needs of the present without compromising the ability of future generations to meet their own needs." However, challenges remain in understanding how this definition could be helpful in evaluating policy choices or business decisions, since avoiding impeding the "ability of future generations to meet their own needs" requires predicting both their needs and their abilities. More recently, industrial ecology has been referred to as the "science of sustainability", although industrial ecology itself is still emerging as a single recognizable construct.
Natural scientific perspectives
Ecological and biological perspectives on sustainability focus on resource consumption and replenishment. For example, ecologist C.S. Holling notes that organisms are "exuberantly over-productive" but inevitably encounter "limits set by time, space, and energy." Biodiversity is increasingly viewed as fundamental in providing ecosystems with stronger resilience to perturbations. Ecologists and agricultural economists have long studied the concepts of, and often attempt to quantify, "sustainable harvests" and "sustainable yields". These refer to the amount of resources that can be farmed or fished without limiting its the ecosystem's "carrying capacity" or, perhaps equivalently, damaging "natural capital". Both refer to the ecosystem's ability to regenerate resources to replace the harvested amount.
Sustainability is as much a construct in the social sciences as the natural sciences. Three popular frameworks that operationalize sustainability include the triple bottom line, The Natural Step, and the Ecological Footprint. In addition, several frameworks have emerged from peer-reviewed academic journals, including a method to calculate sustainable emissions and resource usage and the Sustainable Hierarchy, a meta-framework. The first framework emphasizes the need to balance economic, social, and ecological goals. The next three focus on measuring and reducing damage to the natural assets that provide ecosystem services necessary for human well-being. The fifth attempts to provide a meta-framework. While these frameworks avoid explicitly defining sustainability, they describe conditions, characteristics, and indicators of sustainability.
Triple bottom line
The Triple bottom line, initially proposed by John Elkington, suggests that organizations ought to manage not only their financial bottom line, but also their environmental and social "bottom lines" as well. The triple bottom line concept is being adopted by a growing number of multinational companies, despite challenges in determining appropriate environmental and social metrics, and in whether (and how) to aggregate these three "bottom lines". Critics have charged that the concept lacks methodologies and formulae to facilitate calculating the social or environmental "bottom lines".
The Natural Step
The Natural Step describes four "system conditions" for a society to be sustainable, "nature is not subject to systematically increasing (1) concentrations of substances extracted from the Earth's crust, (2) concentrations of substances produced by society, or (3) degradation by physical means; and, in that society, (4) human needs are met worldwide". According to this framework, the combustion of fossil fuels is not sustainable because it increases concentrations of atmospheric carbon dioxide. Nor are the emissions of persistent bioaccumulative chemicals, or the ongoing systematic loss of rainforests and wetlands. The Natural Step framework is being championed and implemented by affiliated nonprofit organizations in 10 countries. Several large companies including McDonalds (in Sweden) and Ikea have used The Natural Step framework.
The Ecological Footprint compares the environmental impact of particular actions – at the individual, city, or nationwide levels – to the limitations of the Earth's natural resources and ecosystem functionality. The Ecological Footprint calculates "how many Earths" would be required to provide enough biologically productive land area to maintain the flows of resources and wastes, if the particular activity were adopted by all the world's citizens.
Calculating sustainable emissions and resource usage
Thomas Graedel and Robert Klee offer a method to determine what levels of emissions and resource usage are sustainable. Their process for determining a sustainable rate of resource requires: (1) calculating the available supply of virgin material and recoverable stockpiles (e.g., amount in landfills); (2) allocating consumption of this supply among the global population over a particular time period (typically 50 years); (3) considering this rate to be the maximum sustainable consumption rate. They suggest that a maximum sustainable emission rate can be determined in a comparable manner, by (1) determining the total annual global human-caused emissions of a particular substance that meets a political target or that is below a threshold that would cause permanent environmental change; (2) dividing this total by the current global population and by 50 years to calculate an allocated emission rate (mass per person per year); and (3) accounting for recycle schemes such as sequestration and then update the allocated emission rate.
The Sustainability Hierarchy was proposed by Julian Marshall and Michael Toffel to provide an organizing framework around the wide variety of issues that have been associated with sustainability. It categorizes actions as unsustainable based on their direct or indirect potential to (1) endanger the survival of humans; (2) impair human health, (3) cause species extinction or violate human rights; or (4) reduce quality of life or have consequences that are inconsistent with other values, beliefs, or aesthetic preferences. Effects considered include impediments to the ecosystem functions that support human life, human health, and species viability. They argue that the concept of environmental sustainability should exclude the fourth category (values, beliefs, and aesthetic preferences).
Historical roots in economics
Some of the historical roots of sustainability can be traced to economics. In 1939, the economist J.R. Hicks defined income as "the maximum value which a man can consume during a week and still expect to be as well-off at the end of the week as at the beginning" and national wealth as the amount that can be consumed while maintaining a constant level of national wealth, including its natural capital. Ecological economics has developed the notion of sustainable consumption based in part on these Hicksian notions of living within limits that allow individuals or nations to remain "as well-off". A second historical notion of sustainability refers to intergenerational fairness and requires aggregate consumption levels to be maintained. This strand of thought led to the Solow-Hartwick sustainability rule, which argues that it is sustainable to consume at constant or even increasing levels, even while non-renewable resources decline, as long as the economic rents from these resources are reinvested in reproducible capital.
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