美国2024年玉米产量中约37%被转化为燃料乙醇。1 2024年美国农民收获149亿蒲式耳玉米，其中5.5亿蒲式耳用于乙醇生产，相当于3070万英亩土地的农业产能。1 按每英亩1500万卡路里计算，这些用于乙醇的玉米可满足约5.7亿人的年度热量需求。1
这一大规模转向乙醇燃料源于2005年《能源政策法案》和2007年《能源独立与安全法案》建立的可再生燃料标准（RFS），这是全球最大的生物燃料强制要求。1 在政策刺激下，美国玉米种植面积在2006-2007年间实现历年最大年度增幅，从7830万英亩扩至9350万英亩。1
科学研究揭示了这一政策的隐性成本。根据2022年《美国国家科学院院刊》的分析，考虑土地转换、肥料使用增加和作物位移影响后，玉米乙醇的碳强度可能至少比汽油高24%。1 2019年研究估计，美国玉米生产相关污染每年导致约4300人过早死亡，其中肥料中的氨是主要贡献者。1
与此同时，替代能源方案已显现潜力。工业甘薯在阿拉巴马州和马里兰州的试验表明，其乙醇产量可达玉米全国平均水平的两倍多。1
The U.S. corn ethanol policy, established through the 2005 Energy Policy Act and the 2007 Energy Independence and Security Act, which created the Renewable Fuel Standard (RFS)—the world's largest mandatory biofuel requirement—carries substantial environmental and health consequences that extend far beyond energy production goals.1
In 2024, American farmers harvested 14.9 billion bushels of corn, with approximately 5.5 billion bushels, or 37 percent of the total crop, directed toward ethanol production.1 This volume represents agricultural output equivalent to 30.7 million acres of land.1 According to a 2022 analysis published in the Proceedings of the National Academy of Sciences, when accounting for land conversion, increased fertilizer use, and crop displacement effects, corn ethanol carries a carbon intensity at least 24 percent higher than gasoline.1 The production of corn for fuel also creates significant public health burdens: a 2019 study estimated that pollution associated with U.S. corn production causes approximately 4,300 premature deaths annually, with ammonia in fertilizers identified as a principal contributor.1
The rapid expansion of corn cultivation for ethanol production has reshaped American agriculture. Between 2006 and 2007, corn acreage increased from 78.3 million acres to 93.5 million acres, marking the largest annual expansion on record.1 Meanwhile, alternative feedstock technologies show promise; pilot programs in Alabama and Maryland demonstrated that industrial sweetpotatoes can yield more than double the ethanol per acre compared to the national corn average.1
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