Increased food consumption

The diet of an average American is now 20 times the energy input of 200 years ago. A typical example of why: a head of iceberg lettuce is 95 percent water and contains just fifty calories of energy, but it takes 400 calories of energy to grow that head of lettuce in California's Central Valley, and another 1,800 to ship it.

Scientists in Vancouver calculated that although 1.7 million people lived on a million acres surrounding their city, those people required 21.5 million acres of land to support them, including wheat fields in Alberta, oil fields in Saudi Arabia, and tomato fields in California.