IPAT: Simple and Useful

Miles Kimball has steered me to Matt Ridley, who gets IPAT completely wrong.  Ridley says he used to be an ecologist, but if he was his training had some gaps.  Here’s what he says about IPAT:

In 1972, the ecologist Paul Ehrlich of Stanford University came up with a simple formula called IPAT, which stated that the impact of humankind was equal to population multiplied by affluence multiplied again by technology. In other words, the damage done to Earth increases the more people there are, the richer they get and the more technology they have.

Nope.  IPAT is simply a decomposition:

environmental impact = population x (real GDP/population) x (impact/real GDP)

The name comes from reading this as impact = population x affluence x technology

Old timers will remember that Barry Commoner used this framework to demonstrate that a number of critical environmental problems were due primarily to the latter ratio, not to the first two terms on right-hand side.  Rather than focusing on population growth (like Ehrlich did) or impoverishing ourselves, what we most needed to do was change the technologies we used to produce things.  Contra Ridley, it was not about “less” technology but different technology.

And it’s still a useful way to frame environmental issues.  Take carbon, for instance.  Us humans need to forego 60-80% of the available fossil fuel reserves if we are to avoid potentially catastrophic climate change.  How can we pull this off?  Simple arithmetic, mobilized by IPAT, shows us that neither population reduction nor shrinkage of GDP per capita can get us anywhere near this, short of some sort of nightmare scenario.  Our only option is to reduce the amount of fossil fuel use per unit GDP, and this we can do rather readily, if the IPCC can be believed.  (I’m not convinced personally, but I could be wrong.)  That’s the T part of IPAT.

I don’t like to spend my time correcting every mistake I find on the internet, but the IPAT formulation is worth preserving.