Which factors are the most important in determining the impact of…

Concept

The environmental impact of human (anthropogenic) activity is standardly expressed by the IPAT identity: I = P × A × T. Total impact (I) is the product of three multipliers — population (P), the goods and services consumed per person, that is affluence (A), and the environmental burden generated per unit of output, that is technology (T). Because the three terms multiply, holding any two constant makes total impact rise in direct proportion to the third; a quantity that is not one of these multipliers cannot by itself scale total impact.

Applying the identity here

• Population (P) fixes how many consumers there are.

• Affluence per person (A) fixes how much each consumer consumes.

• The technology used for exploiting resources (T) fixes how much environmental burden — resource draw and waste — each unit of that consumption generates.

The set that carries all three multipliers is "Population, affluence per person and the technology used for exploiting resources", which is exactly I = P × A × T.

Cross-check with the other sets

• "Population, affluence per person, land available per person": land available per person is total area divided by head-count, a distribution ratio; it carries no information about the burden generated per unit of output, so the T multiplier is absent.

• "Atmospheric conditions, population and forest cover": atmospheric conditions and forest cover are observed states of the environment — part of the impact being measured — rather than drivers of human activity, so both the A and the T multipliers are absent.

• "Population, forest cover and land available per person": forest cover and land per person are both area-based stock measures of the natural endowment; neither supplies consumption per person nor burden per unit of output.

Hence the determining factors are population, affluence per person and the technology used for exploiting resources — the three terms of I = P × A × T (Ehrlich and Holdren, 1971).