A few years ago, the IPAT identity was all the rage when talking about the aggregate human impact on the global ecosystem. What is the IPAT identity exactly, and is it still relevant, or has it been superseded?

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The IPAT equation can be useful as a tool to describe the high-level factors involved in assessing sustainability world wide or for a particular good or service. The letters stem from the I=PAT equation which says that:

Impact = Population x Affluence x Technology

Here affluence is the consumption of goods or services of a person in the population, and technology is a measure of how resource intensive the production of consumed goods are. So a bit more verbose the formula says

Impact on the environment = Number of people x average consumption per person x impact per unit of consumption

which is sometimes rewritten as

I = P x C/P x I/C

However, the simplicity of the I=PAT formula is also it’s problem. The formula assumes that the terms P, A and T are independent, which in reality isn’t true. For example, when new technological developments allow us to use less resources to produce the same good or service, the formula says that the impact for this good or service goes down correspondingly. In reality, less resources means lower prices which results in more consumption. This effect, that increasing efficiency results in more consumption is also called ‘Jevons paradox’ or the ‘rebound effect’. Similarly, a doubling of the population doesn't necesarily mean a doubling of the impact, because the average consumption may actually go down.

The I=PAT formula was invented in the 70s and at the time it was used to emphasize the role of a growing global population on the environment, but also to indicate that multiple factors are involved in improving sustainability. The paper 'The IPAT equation and its variants' by Marian Chertow describes how this view has changed and the focus is now often on the T factor as that is the most easiest to modify. It seems IPAT in it's original form is hardly used in research and policy-making anymore, but revised forms of IPAT, such as the the Kaya equation are still being developed and used.

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