The Kaya Identity

Japanese energy economist Yoichi Kaya developed the Kaya identity as a function relating factors that determine the level of human impact on climate, in the form of emissions of the greenhouse gas, carbon dioxide.¹Kaya, Yoichi; Yokoburi, Keiichi (1997). Environment, energy, and economy : strategies for sustainability. Tokyo [u.a.]: United Nations Univ. Press. This identity states that total emissions can be expressed as the product of four inputs.

\[F = P \times \frac{G}{P} \times \frac{E}{G} \times \frac{F}{E}\]

F = global CO₂ emissions from human sources

P = global population

G = global gross domestic product

E = global energy consumption

Policy experts and scientists focus on the ratios which make up this identity, which are crucial to understanding the relationship between emissions, population, gross domestic product (GDP), and energy consumption. 

G/P = affluence (GDP per person)

E/G = energy intensity (Energy per $ of economic activity)

F/E = cleanliness of energy (emissions per BTU of energy consumed)

This equation is both simple and tricky, as it can be reduced to only two terms. However, as the data requested are generally available, this identity straightforwardly relates how many of us there are, how rich we are, how much energy we use, and how the energy we use impacts the world around us.

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