Weak and strong sustainability

Although related, sustainable development and sustainability are two different concepts. Weak sustainability is an idea within environmental economics which states that 'human capital' can substitute 'natural capital'. It is based upon the work of Nobel laureate Robert Solow, and John Hartwick. Contrary to weak sustainability, strong sustainability assumes that 'human capital' and 'natural capital' are complementary, but not interchangeable. This idea received more political attention as sustainable development discussions evolved in the late 1980s and early 1990s. A key landmark was the Rio Summit in 1992 where the vast majority of nation-states committed themselves to sustainable development. This commitment was demonstrated by the signing of Agenda 21, a global action plan on sustainable development. Weak sustainability has been defined using concepts like human capital and natural capital. Human (or produced) capital incorporates resources such as infrastructure, labour and knowledge. Natural capital covers the stock of environmental assets such as fossil fuels, biodiversity and other ecosystem structures and functions relevant for ecosystem services. In very weak sustainability, the overall stock of man-made capital and natural capital remains constant over time. It is important to note that, unconditional substitution between the various kinds of capital is allowed within weak sustainability. This means that natural resources may decline as long as human capital is increased. Examples include the degradation of the ozone layer, tropical forests and coral reefs if accompanied by benefits to human capital. An example of the benefit to human capital could include increased financial profits. If capital is left constant over time intergenerational equity, and thus Sustainable Development, is achieved. An example of weak sustainability could be mining coal and using it for production of electricity. The natural resource coal, is replaced by a manufactured good which is electricity.