A framework for sustainability science: a renovated IPAT identity.

Learning actors' leverage for change along the journey to sustainability requires quantifying the component forces of environmental impact and integrating them. Population, income, consumers' behavior, and producers' efficiency jointly force impact. Here, we renovate the "IPAT Identity" to identify actors with the forces. Forcing impact I are P for population, A for income as gross domestic product (GDP) per capita, C for intensity of use as a good per GDP, and T for efficiency ratios as impact per good. In the "ImPACT Identity," parents modify P, workers modify A, consumers modify C, and producers modify T. Because annual percentage changes in component forces add to a change in national impact, actors' leverage is reflected transparently in consistent units of annual percentage changes that can be compared from force to force. Examples from energy and food, farming and manufacturing, and steel and water show that declining C, called dematerialization, can temper the sustainability challenge of growth (P x A), and that innovation or efficient technology that lowers T can counter rising consumption (P x A x C). Income elasticity can accommodate connections between income and other forces. From rates of change of forces, the identity can forecast impacts. Alternatively, by identifying the necessary change in forces to cause a projected impact, ImPACT can assay the likelihood and practicability of environmental targets and timetables. An annual 2-3% progress in consumption and technology over many decades and sectors provides a benchmark for sustainability.