One of the issues that Michael Finus discussed last Tuesday night in his I-SEE seminar was the Kaya Identity, which is an equation relating to human impact on climate. I had not come across it before – such a sheltered life!

The idea is that total emission levels can be set out as the product of four inputs: population, GDP per capita, energy intensity (per unit of GDP), and carbon intensity (emissions per unit of energy). It's a version of the I = PAT equation (which I had heard of).

It can be written: **F** = **P ** x **G**/**P** x **E**/**G ** x **F**/**E** where:

**F**is CO2 emissions**P**is population**G**is GDP, and**E**is energy consumption

and where

**G/P**is a measure of affluence**E/G**is energy intensity, and**F/E**is carbon intensity

It can apply to nations and globally, and I wrote about carbon intensity last October. This was one of the less gloomy parts of the talk.

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