Sustainability, in a broad sense, is the capacity to endure. It can be defined as the ability of an ecosystem to maintain ecological processes, functions, biodiversity and productivity into the future. In ecology, the word describes how biological systems remain diverse and productive over time. Negative effects due to human interaction in the natural world in regard to responsible use of natural resources has brought about dire warnings in regard to sustainability issues by a variety of science based groups.
Sustainability has become a wide-ranging term that can be applied to almost every facet of life on Earth, from a local to a global scale and over various time periods. Long-lived and healthy wetlands and forests are examples of sustainable biological systems. Invisible chemical cycles redistribute water, oxygen, nitrogen and carbon through the world's living and non-living systems, and have sustained life for millions of years. As the earth’s human population has increased, natural ecosystems have declined and changes in the balance of natural cycles has had a negative impact on both humans and other living systems.
There is now abundant scientific evidence that humanity is living unsustainably. Returning human use of natural resources to within sustainable limits will require a major collective effort. Since the 1980s, human sustainability has implied the integration of economic, social and environmental spheres to: “meet the needs of the present without compromising the ability of future generations to meet their own needs.”
Efforts to live more sustainably can take many forms from reorganising living conditions (e.g., ecovillages, eco-municipalities and sustainable cities), reappraising economic sectors (green building, sustainable agriculture), or work practices (sustainable architecture), using science to develop new technologies (green technologies, renewable energy), to adjustments in individual lifestyles.
Although the definition of sustainable development (above), given by the Brundtland Commission, is frequently quoted, it is not universally accepted and has undergone various interpretations. Definitions of sustainability may be expressed as statements of fact, intent, or value with sustainability treated as either a "journey" or "destination". Where we are now, where we need to be going, and how we are to get there are all open to interpretation and will depend on the particular context under consideration. What can meaningfully be described as sustainable will depend on the scale of space and time that is appropriate to the item under consideration. For example, if time criteria have not been met, then assertions of sustainability are more like predictions than definitions. This difficult mix has been described as a "dialogue of values that defies consensual definition." Sustainability has been regarded as both an important but unfocused concept like "liberty" or "justice" and as a feel-good buzzword with little meaning or substance. The idea of sustainable development is sometimes viewed as an oxymoron because development inevitably depletes and degrades the environment. Consequently some definitions either avoid the word development and use the term sustainability exclusively, or emphasise the environmental component, as in "environmentally sustainable development."
Another representation of sustainability showing how both economy and society are constrained by environmental limits
The dimensions of sustainability are often taken to be: environmental, social and economic, known as the "three pillars". These can be depicted as three overlapping circles (or ellipses), to show that they are not mutually exclusive and can be mutually reinforcing. While this model initially improved the standing of environmental concerns, it has since been criticised for not adequately showing that societies and economies are fundamentally reliant on the natural world. According to English environmentalist and author Jonathon Porritt, "The economy is, in the first instance, a subsystem of human society ... which is itself, in the second instance, a subsystem of the totality of life on Earth (the biosphere). And no subsystem can expand beyond the capacity of the total system of which it is a part." For this reason a second diagram shows economy as a component of society, both bounded by, and dependent upon, the environment. As the American World Bank ecological economist Herman Daly famously asked, "what use is a sawmill without a forest?" The concept of living within environmental constraints yields another definition of sustainability: "improving the quality of human life while living within the carrying capacity of supporting eco-systems."
The Earth Charter goes beyond defining what sustainability is, and seeks to establish the values and direction needed to achieve it: "We must join together to bring forth a sustainable global society founded on respect for nature, universal human rights, economic justice, and a culture of peace. Towards this end, it is imperative that we, the peoples of Earth, declare our responsibility to one another, to the greater community of life, and to future generations."