green is in. Very in. But green policies and decisions need to be based on more than a vague desire to save the planet. The principles of the natural sciences and economics must play an essential role -- a part of policy-making that often eludes politicians. The latest examples are the federal government's efforts to reduce the United States's dependence on imported oil (now more than 60 percent) by shifting a big share of the nation's largest crop, corn, to the production of ethanol for fueling automobiles.

Good goal, bad policy. In fact, in the short- and medium-term, ethanol can do little to reduce the vast amount of oil that is imported, and the ethanol policy will have widespread and profound ripple effects on other commodity markets.

These biofuel goals are propped up by a generous federal subsidy -- via tax credits -- of 51 cents a gallon for blending ethanol into gasoline, and a tariff of 54 cents a gallon on most imported ethanol, to keep out cheap imports from Brazil.