IIASA RepositoryPopulation, Households, and CO2 EmissionsF.L.MacKellarauthorW.LutzauthorC.PrinzauthorA.GoujonauthorIn this paper, the Income-Population-Affluence-Technology, or I=PAT, model is reformulated in terms of households (i.e., I=HAT) as opposed to persons. Such an approach is preferable in the case of environmental impacts, such as CO2 emissions, which are caused by activities such as residential heating and automobile transport, for which there exist significant household-level economies of scale. Because of historical changes in average household size, the IHAT identity gives rise to a very different decomposition of the sources of growth in environmental impacts than does the IPAT identity. Taking growth in commercial energy consumption as an example, we find that the IPAT identity attributes 18.2% of the annual increase (in absolute terms) over the period 1970-90 to industrialized-country demographic increase, whereas the IHAT model attributes 41.5% because of the significant decrease of mean household sizes. The difference between the two decompositions is simply that, in the first, it is the individual who is the demographic unit of account, whereas in the latter, it is the household. Based on the IIASA world population projections and derived projections of households, the IPAT and IHAT identities are employed to project the level of CO2 emissions in the year 2100. In the case where the level of emissions per demographic unit is assumed to remain constant at 1990 levels, the IHAT projection for 2100 is over 40% higher than the IPAT projection; in the case where emissions per demographic unit are projected to increase over rime, tine MAT projection is over 80% higher. We conclude that decomposition and projection exercises are very sensitive to the unit of account chosen. Should the unit of analysis be the individual, the household, the community, or what? Until more is known about the nature of the many activities which give rise to environmental impacts -- specifically, the role of economies of scale -- it is unwise to draw far-reaching conclusions from one specific choice of model without a substantive justification of that choice.1995-08WP-95-081Monograph

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