The question addressed is whether it is possible to define working limits on environmental impacts from the built environment in terms of global carrying capacity. The main focus is on energy-related impacts, since these are global and relatively well-understood. Four possible approaches to defining limits are explored: static equilibrium, asymptotic, integral of excess and planned future. The conclusions that emerge from this exploration are that global environmental constraints are very tight, but also that they are dynamically and strongly influenced by the trajectory of social and technological development over the coming century. Their use as the basis for practical, quantitative metrics of sustainability, therefore, involves a large measure of subjectivity. A fifth approach - the developmental approach - is identified, which instead of focusing on long-term external constraints to human activity, focuses instead on the internal, short- to medium-term dynamics of the built environment itself. It appears likely that the developmental approach, guided by qualitative conclusions from the analysis of global carrying capacity, is likely to be most fruitful
