Why should we work at it? Because already, with over 7 billion people (in 2011), we're doing permanent damage to our land, water, and energy supplies -- and the amount of damage we do keeps increasing with our numbers.

If the Earth is unable to sustain approximately 7 billion people at an acceptable standard of living, how can we hope to sustain 9 billion or 12 billion? How can we hope to improve their standard of living when there's less of everything to go around?

The sooner we stop growing, the better chance we have of avoiding worldwide famines and other catastrophes.