STIRPAT, IPAT and ImPACT: analytic tools for unpacking the driving forces of environmental impacts

Despite the scientific consensus that humans have dramatically altered the global environment, we have a limited knowledge of the specific forces driving those impacts. One key limitation to a precise understanding of anthropogenic impacts is the absence of a set of refined analytic tools. Here we assess the analytic utility of the well-known IPAT identity, the newly developed ImPACT identity, and their stochastic cousin, the STIRPAT model. We discuss the relationship between these three formulations, their similar conceptual underpinnings and their divergent uses. We then refine the STIRPAT model by developing the concept of ecological elasticity (EE). To illustrate the application of STIRPAT and EE, we compute the ecological elasticities of population, affluence and other factors for cross-national emissions of carbon dioxide (CO2) from fossil fuel combustion and for the energy footprint, a composite measure comprising impacts from fossil fuel combustion, fuel wood, hydropower and nuclear power. Our findings suggest that population has a proportional effect (unitary elasticity) on CO2 emissions and the energy footprint. Affluence monotonically increases both CO2 emissions and the energy footprint. However, for the energy footprint the relationship between affluence and impact changes from inelastic to elastic as affluence increases, while for CO2 emissions the relationship changes from elastic to inelastic. Climate appears to affect both measures of impact, with tropical nations having considerably lower impact than non-tropical nations, controlling for other factors. Finally, indicators of modernization (urbanization and industrialization) are associated with high impacts. We conclude that the STIRPAT model, augmented with measures of ecological elasticity, allows for a more precise specification of the sensitivity of environmental impacts to the forces driving them. Such specifications not only inform the basic science of environmental change, but also point to the factors that may be most responsive to policy.