## The Commoner-Erhlich Equation

Further to my previous article, on the report from the Optimum Population Trust, I’ve been doing a bit of digging around on the I=PAT equation. Remember here that I is the measure of the impact of humanity on the environment and P is the population and A is a measure of affluence (or consumption). The question is what is T measuring? The OPT reports talks about T somehow measuring “technology”.

Anyway according to Wikipedia, T is in fact humanity’s ecological impact per unit of consumption. A is measured as consumption per capita. So by multiplying the population P by the consumption per capita A, you get total consumption, after which you multiply by T the total impact per unit consumption to get I, the total environmental impact of the population and its level of consumption.

Given this, it is clear Tim Worstall’s criticism of the I=PAT equation, saying that we should divide by T, not multiply by it, is mis-placed. Mr Worstall is treating T as if it measures technological sophistication. I agree with him that technological advancement reduces our environmental impact, at least for a given standard of living and population size, but that is not what T is measuring here. Technological advancement allows us to e.g. use less energy and resources and/or reduce pollution per unit of consumption. Thus such advancement reduces the value of T. The question then is whether the equation is an adequate description of what’s going on. It assumes independence of its variables and it also assumes the variables can be measured reasonably accurately. It seems to me both assumptions are questionable.

For example, there may be feedback loops between the variables that aren’t catered for and it’s not entirely clear how one would measure either “consumption” or “environmental impact” in a clear, accurate manner.

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