"Much research is being conducted around the world to find good ways to measure natural capital. In 1997, an American economist named Robert Costanza headed a team of 13 economists and ecologists that gained international attention when it published an article in Nature magazine (volume 387) estimating that the Earth's natural ecosystems provided US$33 trillion in economic value annually by providing fresh water, regulating climate, pollinating crops, controlling pests, and performing other natural processes. That was nearly twice the value of the world's gross national product at that time (US$18 trillion)."

                                                       7 Perspectives On Sustainability, page 45