Washington Post: It sounds like alchemy, an act of bureaucratic magic. Under the climate-change bill just approved by a House committee, the U.S. government would literally make a commodity — as tradable as a Pontiac or a pork belly — out of thin air. The bill would require polluters to obtain "allowances" — permits allowing them to emit a given amount of a greenhouse gas such as carbon dioxide or methane. Today, these gases are invisible, free and floating all around us. This bill would put a …