Lecture 02 Class Notes

Environmental Accounting Identities: IPAT and its Children

IPAT is one (limited) approach – A tool for the toolbox.

IPAT Identity Origin
IPAT came about as a result of truce between two camps of environmentalists - Erlich and Erlich (too many people/overpopulation) vs. Commoner (destructive technology). There was a fight between population and technology as the main driving force of environmental problems. IPAT is the compromise (1960s to 1972 publication).

The Equation
Impact = Population*Affluence*Technology

How do you make Impact 0%? Or decline? You can’t have everything increasing.

1972 elaboration –
I = P*A*T -> Emissions (tons) = Pop*(GDP/Pop)*(tons/GDP)

How is it most commonly used?

  • Make and critique future trends in environmental impacts
    • Historical, recent, predictive
    • Manipulable
    • Can be measured using whatever metrics you’re interested in
    • Reduce intensity of use=dematerialization
    • Reduce impact/good=efficiency
  • Ex: Emissions CO2 (tons) = Pop*(GDP/pop)*(tons/$GDP)
    • Doesn’t have any content (just equals out to emissions) – only a method of understanding
  • Modern versions:
    • ImPACT model (Waggoner and Ausubel) - Adds consumption
      • Industrial ecology uses ImPACT
      • Im=Environmental impact
      • P=population
      • A=affluence (GDP/capita)
      • C=intensity of use/consumption
      • T=impact/good
    • Kaya identity (Raupach et al)
      • IPAT related to global CO2 emissions and climate change
      • CO2 = Pop*(GDP/Pop)*(Energy/GDP)*(CO2/Energy)
      • Energy intensity for generating goods and services versus how society gets it energy
      • Society that ran entirely on neutrals (nuke E, for ex) would be zero
      • Dominant framework (right now) for GHGs
    • Decomposing Rates of Change
      • Transform the identity to differential form
      • Using % rates of change, elasticities, logarithmic derivatives
      • IF… CO2=Pop*GDP/Pop*Energy/GDP*CO2/Energy, THEN… %ΔCO2 = %ΔPop + %ΔGDP/Pop + %ΔEnergy/GDP + %ΔCO2/Energy - see slide for example
      • Use for projecting future trends and/or scenarios

IPAT Limitations

  • Factors not independent, they can’t be separately manipulated.
  • Emissions don’t equal impact. There’s nothing about interactions and non-linearity of environmental effects. It could be more important to avoid certain increases in certain points of time and less important for others.
  • Aggregate representation ignores distributional issues. Differences in income, contribution, growth rates among sub-groups and individuals. Ex: lots of emissions from a very small number of cars.
  • Equity, types of consumption
  • Remember limitations and keep them in mind when using identities. Still overall useful.

Individual IPAT Factors


  • One of the earliest environmental concerns.
  • Malthus (1780), economics as the dismal science.
  • There was a population panic in the 1960s. 20th century population explosion as well as a food production collapse.
  • 1962-1963 was a peak in population growth
  • Now, the population is growing at 1.1% and declining.
  • This is a policy area that is usually avoided.

Does Earth have a carrying capacity?

  • Credible estimates from 1 billion to 1 trillion. Middle half is STILL 4-16 billion.
  • Very unhelpful
  • We’re just guessing how many people the Earth can support.


  • Affluence is a tricky term because it is measured mostly as consumption, but that’s not the same thing.
  • Flow of “value” or “affluence” (want it to grow) versus flow of “materials/energy” (decline); Is this possible?
  • Affluence has components of individual choice and value
  • Is identical to human welfare?
  • May be impossible to separate these two – no experience without burning the fuel
  • Working one good at a time doesn’t work – have lost the “aggregate” benefit
  • Limiting consumption problems
  • Legit? What about the poor? Is it feasible? Will be really, really hard to get people to give up what they love. → Polite conversation goes back to technology


  • “Evil technology” argument by Commoner has been refuted
  • Mixed and ambiguous – both increases AND decreases environmental burdens
  • Strongest driver of environmental trends
    • Direct effect reduces environmental burdens
    • Can manage environmental problems well using tech
  • Alt tech pathways drastically affect the future projections
  • Assumes, however: Tech can bring a future that is rich AND green.
  • Concerns:
    • Evidence of limits of our ability to control/predict technology
    • Lock-in effects and feedback loops
    • Decentralized actors are more effective than government
  • Linkage: rebound effects
    • Tech gain is offset by behavioral change. Jevons paradox – lose all you gain and more. Example – fuel economy, lighting.
    • Ex: increase fuel economy, people drive more
  • Can motivate tech change with policy?
    • Hard to control it

Closing questions:

  • For impacts:
    • What factors can be controlled?
    • By who? How?
    • What consequences?
  • How do we evaluate burdens/impacts?
    • Accounting identities are “value free,” except that we assume impacts are bad