4.酒精和弹性燃料

    2007年，美国政府出台了《再生燃料法案》（Renewable Fuels），要求汽油中必须添加一定量的用再生能源制成的液体，这让用玉米加工的酒精得以进入美国的燃料供应体系。共有84种汽车或卡车使用了指定的“弹性燃料”，意味着它们的燃料最多可以包含85%的酒精。最近，针对酒精使用的批评正在抬头，因为大家逐渐认识到，酒精所含的能量比汽油少，导致每加仑燃油行驶的里程数减少，同时酒精生产却需要消耗大量的能量，而这又会导致二氧化碳排放增加。反对者还声称，使用酒精也是不道德的，因为它占用了食物供应中40%的玉米，同时推高了玉米的价格。

    4. Ethanol and flex fuel

    Thanks to the government's 2007 Renewable Fuels mandate requiring a certain amount of liquids made from renewable sources to be added to gasoline, ethanol made from corn has found its way into the nation's fuel supply. Some 84 car and truck models carry the designation "flex fuel," meaning they can run on mixtures that contain up to 85% ethanol. Lately, a backlash against ethanol use has developed as realization grows that ethanol contains less energy than gasoline, resulting in fewer miles per gallon, and takes a lot of energy to produce, which can lead to increased emissions of carbon dioxide. Opponents also argue that ethanol is unethical, because it diverts 40% of the corn grown from food supplies and drives up its cost.