Note that the linear relationship between (logged) emissions and (logged) population implies that we estimate the direct effect of population on emissions only, but not the indirect effects that might work via the impact of population on either A or T. This has been critically noted by a number of participants in the ongoing discussion about the usefulness of IPAT (., MacKellar, Lutz, Prinz & Goujon, 1995). Such complex inter- action effects are beyond what can be achieved in this paper and are left for future research. Cramer (2001) is concerned about potential feedbacks of pollution on population. Of course, the direct effect is likely to be small as.
