The Impact Formula is a useful tool for understanding humanitiy's relationship with the Earth. It goes like this: I=PxAxT.

I is human impact on the environment.
P is the size of the population.
A is the per capita “affluence,” measured by per capita rate of consumption.
T is a measure of the damage done by the technologies that are used in supplying the consumption.

As an example, let’s take the motor vehicles we drive.

Suppose there is a town in Maine with two-hundred families (P), and they are all wealthy families that drive hummers (A&T). These two-hundred families would, on average, be burning a gallon of gas (I) every 18 miles (1). Based on 15,000 miles of travel, each family would use 833 gallons of gas annually and the whole town would use 166,600 gallons per year.

However, if they suddenly got environmentally conscious and switched to driving a Prius they would each get about 45 miles to the gallon (2). At 15,000 miles of travel each family would use only 333 gallons of gas and the whole town would use 65,200 gallons.

By switching to hybrid technology, the environmentally conscious town would achieve a respectable 150% increase in fuel efficiency.

That sounds wonderful. But what would happen if the town then expanded its population by the percentage the global human population is projected to have to expanded from 1950 to 2050, that is to say -- by 263%. There would now be 536 families.

Even if all the 536 families, both new and old, continued to drive hybrid vehicles they would now be using 174,736 gallons of gas per year – over 8,000 gallons more than when they were 200 strong and driving the Hummers.

(1) Estimated highway mpg of the 2008 Hummer H3 5 cyclinder 3.7 liter engine
(2) Estimated highway mpg of the 2008 Toyota Prius