I was recently reminded of this beautifully simple yet power equation, that is a good basis for understanding the impact of all human action . Created by [Paul] Ehrlich and [John] Holden ( 1971 ) and mostly referred to as IPAT it connected impact to three variables :

I = P x A x T, or

Impact = Population x Affluence x Technology

Lets say if we try to understand the carbon emissions by cars travel by using this equation. Basically it says that total carbon emissions ( Impact ) will be product of total number of car owners ( population ) x number of car miles per owner ( affluence ) x emission per car mile ( technology )

This equation helps us understand the key variables and their relationship to the scale of the impact. So for the car example we know that while the technology for cars has improved a little in past, the affluence levels and car owners have grown rapidly thus greatly increasing the impact/volume of emissions from cars. Therefore perhaps electric cars and decreasing number of car owners via public transport incentives is a great idea.

Harsh