Sustainability metrics and indices are measures of sustainability, using numbers to quantify environmental, social and economic aspects of the world. There are multiple perspectives on how to measure sustainability as there is no universal standard. Instead, different disciplines and international organizations have offered measures or indicators of how to measure the concept.

While sustainability indicators, indices and reporting systems gained growing popularity in both the public and private sectors, their effectiveness in influencing actual policy and practices often remains limited.

Metrics and indices

Various ways of operationalizing or measuring sustainability have been developed. Since the 2010s, there has been an expansion of interest in Sustainable Development Index (SDI) systems, both in industrialized and, albeit to a lesser extent, in developing countries. SDIs are seen as useful in a wide range of settings, by a wide range of actors: international and intergovernmental bodies; national governments and government departments; economic sectors; administrators of geographic or ecological regions; communities; nongovernmental organizations; and the private sector.

The driving force behind SDI processes is the demand for better, more frequent information with higher temporal and spatial resolution. It is also important to distinguish between relevant and irrelevant data within the context of different policy making decisions.

Alternative aggregate measures for sustainability offer a more detailed view of development than traditional indicators like Gross Domestic Product (GDP). Some examples include: the Ecological Footprint, the UN's Human Development Index (HDI), the Genuine Progress Index (GPI), and the Environmental Sustainability Index (ESI). Alongside these initiatives, there is political interest in producing a green GDP that accounts for resource depletion and pollution. However, due to technical and conceptual difficulties, adoption of a green GDP has been slow.

At the heart of the debate over different indicators are not only different disciplinary approaches but also different views of development. Some indicators reflect the ideology of globalization and urbanization that seek to define and measure progress on whether different countries or cultures agree to accept industrial technologies in their eco-systems. Other approaches, like those that start from international treaties on cultural rights of indigenous peoples to maintain traditional cultures, measure the ability of those cultures to maintain their traditions within their eco-systems at whatever level of productivity they choose.

The Lempert-Nguyen indicator, devised in 2008 for practitioners, starts with the standards for sustainable development that have been agreed upon by the international community and then looks at whether intergovernmental organizations such as the UNDP and other development actors are applying these principles in their projects and work as a whole.

In using sustainability indicators, it is important to distinguish between three types of sustainability that are often mentioned in international development: Sustainability of a culture (human system) within its resources and environment; Sustainability of a specific stream of benefits or productivity (usually just an economic measure); and Sustainability of a particular institution or project without additional assistance (institutionalization of an input).

The following list is not exhaustive but contains the major points of view:

"Daly Rules" approach University of Maryland School of Public Policy professor and former Chief Economist for the World Bank Herman E. Daly (working from theory initially developed by Romanian economist Nicholas Georgescu-Roegen and laid out in his 1971 opus "The Entropy Law and the Economic Process") suggested the following three operational rules defining the condition of ecological (thermodynamic) sustainability:

Renewable resources such as fish, soil, and groundwater must be used no faster than the rate at which they regenerate. Nonrenewable resources such as minerals and fossil fuels must be used no faster than renewable substitutes for them can be put into place. Pollution and wastes must be emitted no faster than natural systems can absorb them, recycle them, or render them harmless.

Some commentators have argued that the "Daly Rules", based on ecological theory and the Laws of Thermodynamics, should be considered implicit or foundational for the many other systems that are advocated, and are thus the most straightforward system for operationalization of the Bruntland Definition. In this view, the Bruntland Definition and the Daly Rules can be seen as complementary—Bruntland provides the ethical goal of non-depletion of natural capital, Daly details parsimoniously how this ethic is operationalized in physical terms. The system is rationally complete, and in agreement with physical laws. Other definitions may thus be superfluous, or mere glosses on the immutable thermodynamic reality.

There are numerous other definitions and systems of operationalization for sustainability, and there has been competition for influence between them, with the unfortunate result that, in the minds of some observers at least, sustainability has no agreed-upon definition.

Natural Step approach

Following the Brundtland Commission's report, one of the first initiatives to bring scientific principles to the assessment of sustainability was by Swedish cancer scientist Karl-Henrik Robèrt. Robèrt coordinated a consensus process to define and operationalize sustainability. At the core of the process lies a consensus on what Robèrt came to call the natural step framework. The framework is based on a definition of sustainability, described as the system conditions of sustainability (as derived from System theory). In the natural step framework, a sustainable society does not systematically increase concentrations of substances extracted from the Earth's crust, or substances produced by society; that does not degrade the environment and in which people have the capacity to meet their needs worldwide.

Ecological footprint approach

Ecological footprint accounting, based on the biological concept of carrying capacity, tracks the amount of land and water area a human population demands for producing the biological resources the population consumes, for absorbing its waste, and for accommodating its built infrastructure, all under prevailing technology. This amount then is compared to available biocapacity, in the world or in that region. The biocapacity represents the area able to regenerate resources and assimilate waste. Global Footprint Network publishes every year results for all nations captured in UN statistics.

The algorithms of ecological footprint accounts have been used in combination with the emergy methodology (S. Zhao, Z. Li and W. Li 2005), and a sustainability index has been derived from the latter. They have also been combined with a measure of quality of life, for instance through the "Happy Planet Index" (HPI) calculated for 178 nations (Marks et al., 2006). The Happy Planet Index calculates how many happy life years each country is able to generate per global hectare of ecological footprint.

One of the striking conclusions to emerge from ecological footprint accounting is that it would be necessary to have 4 or 5 back-up planets engaged in nothing but agriculture for all those alive today to live a western lifestyle. The Footprint analysis is closely related to the I = PAT equation that, itself, can be considered a metric.

Anthropological-cultural approach Though sustainable development has become a concept that biologists and ecologists have measured from an eco-system point of view and that the business community has measured from a perspective of energy and resource efficiencies and consumption, the discipline of anthropology is itself founded on the concept of sustainability of human groups within ecological systems. At the basis of the definition of culture is whether a human group is able to transmit its values and continue several aspects of that lifestyle for at least three generations. The measurement of culture, by anthropologists, is itself a measure of sustainability and it is also one that has been codified by international agreements and treaties like the Rio Declaration of 1992 and the United Nations Declaration on the Rights of Indigenous Peoples to maintain a cultural group's choice of lifestyles within their lands and ecosystems.

Terralingua, an organization of anthropologists and linguists working to protect biocultural diversity, with a focus on language, has devised a sert of measures with UNESCO for measuring the survivability of languages and cultures in given eco-systems.

The Lempert–Nguyen indicator of sustainable development, developed in 2008 by David Lempert and Hue Nhu Nguyen, is one that incorporates and integrates these cultural principles with international law.

Circles of Sustainability approach A number of agencies including the UN Global Compact Cities Programme, World Vision and Metropolis have since 2010 begun using the Circles of Sustainability approach that sets up a four-domain framework for choosing appropriate indicators. Rather than designating the indicators that have to be used like most other approaches, it provides a framework to guide decision-making on what indicators are most useful. The framework is arranged around four domains - economics, ecology, politics and culture - which are then subdivided into seven analytically derived sub-domains for each domain. Indicators are linked to each sub-domain. By choosing culture as one of its key domains, the approach takes into account the emphasis of the 'Anthropological' approach (above), but retains a comprehensive sense of sustainability. The approach can be used to map any other sustainability indicator set. This is foundationally different from the Global Reporting Initiative Index (below) which uses a triple-bottom-line organizing framework, and is most relevant to corporate reporting.

Global Reporting Initiative

In 1997 the Global Reporting Initiative (GRI) was started as a multi-stakeholder process and independent institution whose mission has been "to develop and disseminate globally applicable Sustainability Reporting Guidelines". The GRI uses ecological footprint analysis and became independent in 2002. It is an official collaborating centre of the United Nations Environment Programme (UNEP) and during the tenure of Kofi Annan, it cooperated with the UN Secretary-General's Global Compact.

Energy, Emergy and Sustainability Index

In 1956 Dr. Howard T. Odum of the University of Florida coined the term Emergy and devised the accounting system of embodied energy.

In 1997, systems ecologists M.T. Brown and S. Ulgiati published their formulation of a quantitative Sustainability Index (SI) as a ratio of the emergy (spelled with an "m", i.e. "embodied energy", not simply "energy") yield ratio (EYR) to the environmental loading ratio (ELR). Brown and Ulgiati also called the sustainability index the "Emergy Sustainability Index" (ESI), "an index that accounts for yield, renewability, and environmental load. It is the incremental emergy yield compared to the environmental load".

Sustainability Index =  = 

NOTE: The numerator is called "emergy" and is spelled with an "m". It is an abbreviation of the term, "embodied energy". The numerator is NOT "energy yield ratio", which is a different concept.

Writers like Leone (2005) and Yi et al. have also recently suggested that the emergy sustainability index has significant utility. In particular, Leone notes that while the GRI measures behavior, it fails to calculate supply constraints the emergy methodology aims to calculate.

Environmental Sustainability Index