(Reuters) - Corn-based ethanol, which for years has been mixed in huge quantities into gasoline sold at U.S. pumps, is likely a much bigger contributor to global warming than straight gasoline, according to a study published Monday.   Story here. An ethanol plant at Minnedosa, MB. A PinP photo   RELATED:                                                              BURSTING THE ETHANOL BUBBLE                                                                                       The case against food-based fuel