What's nature worth? New computer models tell all

Breath in. The air is free. But we'd all agree it's not worthless. So, what's the price tag on benefits provided by nature? In 1997, the University of Vermont's Robert Costanza and his co-authors put the answer at $33 trillion per year in a now-famous paper in the journal Nature. In the decade following, the science of "ecosystem services" has bloomed. This young discipline studies how nature--through climate regulation, soil formation, crop pollination, flood protection, and so on--supports human welfare, and estimates its value in economic terms.