Weak and strong sustainability are terms that have emerged from the field of environmental economics and describe different approaches to sustainability, specifically in relation to natural resource management and economic development. Weak sustainability is applicable when certain natural and human capital assets are assessed as interchangeable, meaning that the use or loss of, for example, a reduction in natural capital can be considered sustainable if the simultaneous change in human capital meets or exceeds the value of the change in natural capital. It assumes that different types of capital can be measured and given value in the same way. Strong sustainability is applicable when a specific capital asset, typically a natural capital asset, is assessed as incommensurable or so valuable that it should be maintained or enhanced independently of changes in other, typically human-made, capitals. It particularly considers that certain natural assets have critical ecological functions that cannot be substituted by human-made alternatives.
For example, according to weak sustainability, replacing a natural forest with a park or agricultural land can be considered sustainable if the recreational or economic value equal the value of the biodiversity lost and further environmental impact caused. According to strong sustainability, cutting down trees in a natural forest and planting new trees elsewhere might not be considered sustainable, when the value of biodiversity loss and wider ecological implications cannot be measured or offset.
