The food that people eat is just as important as what kind of cars they drive when it comes to creating the greenhouse-gas emissions that many scientists have linked to global warming, according to a report accepted for publication in the April issue of the journal Earth Interactions.

    Both the burning of fossil fuels during food production and non-carbon dioxide emissions associated with livestock and animal waste contribute to the problem, the University of Chicago's Gidon Eshel and Pamela Martin wrote in the report.

    The average American diet requires the production of an extra ton and a half of carbon dioxide-equivalent, in the form of actual carbon dioxide as well as methane and other greenhouse gases compared to a strictly vegetarian diet, according to Eshel and Martin. And with Earth Day approaching on April 22, cutting down on just a few eggs or hamburgers each week is an easy way to reduce greenhouse-gas emissions, they said.

    "We neither make a value judgment nor do we make a categorical statement," said Eshel, an Assistant Professor in Geophysical Sciences. "We say that however close you can be to a vegan diet and further from the mean American diet, the better you are for the planet. It doesn't have to be all the way to the extreme end of vegan. If you simply cut down from two burgers a week to one, you've already made a substantial difference."

    The average American drives 8,322 miles by car annually, emitting 1.9 to 4.7 tons of carbon dioxide, depending on the vehicle model and fuel efficiency. Meanwhile, Americans also consume an average of 3,774 calories of food each day.

    In 2002, energy used for food production accounted for 17 percent of all fossil fuel use in the United States. And the burning of these fossil fuels emitted three-quarters of a ton of carbon dioxide per person.

    That alone amounts to approximately one-third the average greenhouse-gas emissions of personal transportation. But livestock production and associated animal waste also emit greenhouse gases not associated with fossil-fuel combustion, primarily methane and nitrous oxide.

    "An example would be manure lagoons that are associated with large-scale pork production," Eshel said. "Those emit a lot of nitrous oxide into the atmosphere."

    While methane and nitrous oxide are relatively rare compared with carbon dioxide, they are - molecule for molecule - far more powerful greenhouse gases than carbon dioxide. A single pound of methane, for example, has the same greenhouse effect as approximately 50 pounds of carbon dioxide.

    In their study, Eshel and Martin compared the energy consumption and greenhouse-gas emissions that underlie five diets: average American, red meat, fish, poultry and vegetarian （including eggs and dairy）, all equaling 3,774 calories per day.

    The vegetarian diet turned out to be the most energy-efficient, followed by poultry and the average American diet. Fish and red meat virtually tied as the least efficient.

    The impact of producing fish came as the study's biggest surprise to Martin, an Assistant Professor in Geophysical Sciences. "Fish can be from one extreme to the other," Martin said. Sardines and anchovies flourish near coastal areas and can be harvested with minimal energy expenditure. But swordfish and other large predatory species required energy-intensive long-distance voyages.

    Martin and Eshel's research indicated that plant-based diets are healthier for people as well as for the planet.

    "The adverse effects of dietary animal fat intake on cardiovascular diseases is by now well established. Similar effects are also seen when meat, rather than fat, intake is considered," Martin and Eshel wrote. "To our knowledge, there is currently no credible evidence that plant-based diets actually undermine health; the balance of available evidence suggests that plant-based diets are at the very least just as safe as mixed ones, and most likely safer."

    In their next phase of research, Eshel and Martin will examine the energy expenditures associated with small organic farms, to see if they offer a healthier planetary alternative to large agribusiness companies. Such farms typically provide the vegetables sufficient to support 200 to 300 families on plots of five to 10 acres.

    "We're starting to investigate whether you can downscale food production and be efficient that way," Martin said.

    Earth Interactions杂志4月刊上发表的一篇报告文章指出，人们的饮食习惯对于温室气体排放量的影响与人们驾驶的汽车类型同样重要。众多科学家都已指出温室气体排放与全球变暖现象的直接关系。

    芝加哥大学的Gidon Eshel和Pamela Martin在该报告中写道，在食品加工过程中的石化燃料的燃烧和牲畜动物排泄物中的非二氧化碳化合物的排放加剧了全球变暖现象。

    Eshel和Martin表示，传统的美式饮食比素食排放出更多的包括二氧化碳、甲烷和其它温室气体在内的二氧化碳等价物。他们说，随着4月22日世界地球日的临近，只要简单地减少每周鸡蛋或者汉堡的食用量就能为减少温室气体排放做出贡献。

    "在此我们不是做出价值评判或者严厉指责，"地球地理学助理研究员Eshel表示，"我们是说你的饮食习惯偏向素食远离美国传统饮食将对环保做出巨大贡献。你也许无法成为严格的素食者。但只要你将每周2个汉堡减为一个，就已经是很大的改变了。"

    一个美国人平均每年驾车8322英里，考虑到汽车型号和耗油量的区别，大概会排放1.9到4.7吨二氧化碳。同时，美国人平均每天消耗3774卡路里的食物。

    在2002年，美国矿物燃料的17%用于食品生产。这些燃料的燃烧带来了相当于每人4分之3吨的二氧化碳排放。

    这大概等于个人交通工具带来的温室气体排放的三分之一。但是畜牧业和相关的动物产业资源浪费不仅排放温室气体，还包括甲烷和氮氧化物。

    "一个典型的例子就是大规模猪肉生产所需要的肥料湖。"Eshel 表示，"它释放大量氮氧化物到大气层中。"

    尽管甲烷和氮氧化物与二氧化碳相比排放量少得多，但是它们却是比二氧化碳危害大得多的温室气体。一磅甲烷造成的温室效应相当于50磅二氧化碳。

    在他们的研究中，Eshel 和Martin比较了5种饮食结构的能源消耗和温室气体排放情况，这5种饮食结构分别为：美国人的平均饮食结构，红肉为主的饮食结构，海鲜鱼类为主的饮食结构，家禽为主的饮食结构以及蛋奶型素食者的饮食结构，并假设每种饮食结构每天都消耗3774卡路里的热量。

    研究结果表明蛋奶型素食者的饮食结构是最具有能源节约价值的，其次是家禽为主的饮食和美国人的平均饮食结构。以海鲜鱼类为主和以红肉为主的饮食是最缺乏效率的。

    作为地球物理科学的助理研究员，对于渔业的研究结果是很让Martin吃惊的。"捕捞不同种类的鱼所消耗的能源大不相同。"他说。捕捞在沿海地区产量丰富的沙丁鱼和凤尾鱼只需要很小的能源消耗。但是捕捞旗鱼和其它掠食性鱼类则需要进行能源消耗很大的长距离航海。

    Eshel 和Martin的研究也证实对生态环境有益的饮食同样对于人类也更加健康。

    "现已证实动物类脂肪摄入对心血管类疾病具有负面影响。当考虑肉类摄入量而非脂肪摄入量时，相似的负面影响同样存在，"Eshel 和Martin在报告中写到，"从我们现有的研究成果来看，并没有可靠证据证明素食会对健康造成实质破坏；综合现有的科研结论，我们可以说至少素食和杂食同样安全，并且极有可能更安全。"

    在他们研究的下一阶段，Eshel 和Martin将着手调查小型有机农场的能源消耗情况，看它们是否能为大型农业企业提供更加健康的商业模式选择。这类农场一般可靠5到10英亩地提供足够200到300个家庭消费的蔬菜。

    Martin说："我们正开始研究是否能在保持效率的同时缩减食品生产规模。"