The Kaya identity is a concrete form of the more general I = PAT equation relating factors that determine the level of human impact on climate, in the form of emissions of the greenhouse gas carbon dioxide. This identity states that total emission level can be expressed as the product of four inputs: human population, GDP per capita, energy intensity (per unit of GDP), and carbon intensity (emissions per unit of energy consumed). This equation is both simple and tricky, as it can be reduced to only two terms, but it is developed so that the carbon emission calculation becomes easy, as per the available data, or generally in which format the data is available.
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).
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
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 climate change.
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 refuted the validity of his arguments, as well as clarifying the key idea behind the identity.
- Nakicenovic, Nebojsa; Swart, Rob, eds. (2000). "Chapter 3: Scenario Driving Forces, 3.1. Introduction". IPCC Special Report on Emissions Scenarios.
- Kaya, Yoichi; Yokoburi, Keiichi (1997). Environment, energy, and economy : strategies for sustainability. Tokyo [u.a.]: United Nations Univ. Press. ISBN 9280809113.
- Rehmeyer, Julie. "Yoichi Kaya's carbon fix formula". Wired. Retrieved 20 July 2012.
- "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" (PDF). PNAS. 99 (12): 7860–5. Bibcode:2002PNAS...99.7860W. doi:10.1073/pnas.122235999. PMC . PMID 12060732.
- Raupach, M.R.; et al. (May 22, 2007). "Global and regional drivers of accelerating CO2 emissions" (PDF). PNAS. 104 (24): 10288–10293. Bibcode:2007PNAS..10410288R. doi:10.1073/pnas.0700609104. PMC . PMID 17519334.
- Why Bill Gates’ Math Error About Climate Change Matters ThinkProgress May 2, 2016.