The Kaya identity is an identity stating that the total emission level of the greenhouse gas carbon dioxide can be expressed as the product of four factors: human population, GDP per capita, energy intensity (per unit of GDP), and carbon intensity (emissions per unit of energy consumed). It is a concrete form of the more general I = PAT equation relating factors that determine the level of human impact on climate. Although the terms in the Kaya identity would in theory cancel out, it is useful in practice to calculate emissions in terms of more readily available data, namely population, GDP per capita, energy per unit GDP, and emissions per unit energy. It furthermore highlights the elements of the global economy on which one could act to reduce emissions, notably the energy intensity per unit GDP and the emissions per unit energy.
The Kaya identity was developed by Japanese energy economist Yoichi Kaya. It is the subject of his book Environment, Energy, and Economy: strategies for sustainability co-authored with Keiichi Yokobori as the output of the Conference on Global Environment, Energy, and Economic Development (1993 : Tokyo, Japan). It is a mathematically more consistent variation of Paul R. Ehrlich & John Holdren's I=PAT formula that describes the factors of environmental impact.
Kaya identity is expressed in the form:
- F is global CO2 emissions from human sources
- P is global population
- G is world GDP
- E is global energy consumption
- G/P is the GDP per capita
- E/G is the energy intensity of the GDP
- F/E is the carbon footprint of energy
Use in IPCC reports
The Kaya identity plays a core role in the development of future emissions scenarios in the IPCC Special Report on Emissions Scenarios. The scenarios set out a range of assumed conditions for future development of each of the four inputs. Population growth projections are available independently from demographic research; GDP per capita trends are available from economic statistics and econometrics; similarly for energy intensity and emission levels. The projected carbon emissions can drive carbon cycle and climate models to predict future CO2 concentration and global warming.
Use in other scientific analysis
The Kaya identity is reviewed in a 2002 paper.
A 2007 article uses the Kaya Identity in its analysis of recent trends in carbon emissions, and finds:
... cessation or reversal of earlier declining trends in the energy intensity of gross domestic product (GDP) (energy/GDP) and the carbon intensity of energy (emissions/energy), coupled with continuing increases in population and per-capita GDP. Nearly constant or slightly increasing trends in the carbon intensity of energy have been recently observed in both developed and developing regions. No region is decarbonizing its energy supply.
- Bill Gates used a form of the Identity, without attribution, at a TED Talk called Innovating to zero!. Writing in ThinkProgress, Joseph J. Romm disputed the validity of Gates' arguments, as well as clarifying the key idea behind the identity.
It has been pointed out that the Kaya identity is a tautology because it is nothing but a rewrite of the identity: , i.e., "Carbon is carbon". This implies there are a number of alternative formulations for calculating net carbon emissions, which highlights the different possible ways of thinking about emissions reductions (e.g. Eco-sufficiency).
- Kaya, Yoichi; Yokoburi, Keiichi (1997). Environment, energy, and economy : strategies for sustainability. Tokyo [u.a.]: United Nations Univ. Press. ISBN 9280809113.
- Yamaji, Matsuhashi; Nagata, Kaya (1993). "A study on economic measures for CO2 reduction in Japan". Energy Policy. 21 (2): 123–132. doi:10.1016/0301-4215(93)90134-2.
- Nakicenovic, Nebojsa; Swart, Rob, eds. (2000). "Chapter 3: Scenario Driving Forces, 3.1. Introduction". IPCC Special Report on Emissions Scenarios.
- "The "Kaya Identity"". PennState Department of Meteorology. Meteo 469, From Meteorology to Mitigation: Understanding Global Warming. Retrieved 27 November 2015.
- Waggoner, P. E.; J. H. Ausubel (2002). "A framework for sustainability science: A renovated IPAT identity". PNAS. 99 (12): 7860–5. Bibcode:2002PNAS...99.7860W. doi:10.1073/pnas.122235999. PMC 122985. PMID 12060732.
- Raupach, M.R.; et al. (May 22, 2007). "Global and regional drivers of accelerating CO2 emissions". PNAS. 104 (24): 10288–10293. Bibcode:2007PNAS..10410288R. doi:10.1073/pnas.0700609104. PMC 1876160. PMID 17519334.
- Why Bill Gates’ Math Error About Climate Change Matters ThinkProgress May 2, 2016.
- Bunge, Mario (2012). Evaluating Philosophies. Springer Dordrecht Heidelberg New York London. pp. 57–59. ISBN 978-94-007-4407-3.