Matt Ridley has an excellent article in the Wall Street Journal about the different ways economists and ecologists look at the world.
How many times have you heard that we humans are "using up" the world's resources, "running out" of oil, "reaching the limits" of the atmosphere's capacity to cope with pollution or "approaching the carrying capacity" of the land's ability to support a greater population? The assumption behind all such statements is that there is a fixed amount of stuff—metals, oil, clean air, land—and that we risk exhausting it through our consumption.
"We are using 50% more resources than the Earth can sustainably produce, and unless we change course, that number will grow fast—by 2030, even two planets will not be enough," says Jim Leape, director general of the World Wide Fund for Nature International (formerly the World Wildlife Fund).