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雜志4月刊上發表的一篇報告文章指出，人們的飲食習慣對于溫室氣體排放量的影響與人們駕駛的汽車類型同樣重要。眾多科學家都已指出溫室氣體排放與全球變暖現象的直接關系。

    芝加哥大學的Gidon Eshel和Pamela Martin在該報告中寫道，在食品加工過程中的石化燃料的燃燒和牲畜動物排泄物中的非二氧化碳化合物的排放加劇了全球變暖現象。

    Eshel和Martin表示，傳統的美式飲食比素食排放出更多的包括二氧化碳、甲烷和其它溫室氣體在內的二氧化碳等價物。他們說，隨著4月22日世界地球日的臨近，只要簡單地減少每周雞蛋或者漢堡的食用量就能為減少溫室氣體排放做出貢獻。

    "在此我們不是做出價值評判或者嚴厲指責，"地球地理學助理研究員Eshel表示，"我們是說你的飲食習慣偏向素食遠離美國傳統飲食將對環保做出巨大貢獻。你也許無法成為嚴格的素食者。但只要你將每周2個漢堡減為一個，就已經是很大的改變了。"

    一個美國人平均每年駕車8322英里，考慮到汽車型號和耗油量的區別，大概會排放1.9到4.7噸二氧化碳。同時，美國人平均每天消耗3774卡路里的食物。

    在2002年，美國礦物燃料的17%用于食品生產。這些燃料的燃燒帶來了相當于每人4分之3噸的二氧化碳排放。

    這大概等于個人交通工具帶來的溫室氣體排放的三分之一。但是畜牧業和相關的動物產業資源浪費不僅排放溫室氣體，還包括甲烷和氮氧化物。

    "一個典型的例子就是大規模豬肉生產所需要的肥料湖。"Eshel 表示，"它釋放大量氮氧化物到大氣層中。"

    盡管甲烷和氮氧化物與二氧化碳相比排放量少得多，但是它們卻是比二氧化碳危害大得多的溫室氣體。一磅甲烷造成的溫室效應相當于50磅二氧化碳。

    在他們的研究中，Eshel 和Martin比較了5種飲食結構的能源消耗和溫室氣體排放情況，這5種飲食結構分別為：美國人的平均飲食結構，紅肉為主的飲食結構，海鮮魚類為主的飲食結構，家禽為主的飲食結構以及蛋奶型素食者的飲食結構，并假設每種飲食結構每天都消耗3774卡路里的熱量。

    研究結果表明蛋奶型素食者的飲食結構是最具有能源節約價值的，其次是家禽為主的飲食和美國人的平均飲食結構。以海鮮魚類為主和以紅肉為主的飲食是最缺乏效率的。

    作為地球物理科學的助理研究員，對于漁業的研究結果是很讓Martin吃驚的。"捕撈不同種類的魚所消耗的能源大不相同。"他說。捕撈在沿海地區產量豐富的沙丁魚和鳳尾魚只需要很小的能源消耗。但是捕撈旗魚和其它掠食性魚類則需要進行能源消耗很大的長距離航海。

    Eshel 和Martin的研究也證實對生態環境有益的飲食同樣對于人類也更加健康。

    "現已證實動物類脂肪攝入對心血管類疾病具有負面影響。當考慮肉類攝入量而非脂肪攝入量時，相似的負面影響同樣存在，"Eshel 和Martin在報告中寫到，"從我們現有的研究成果來看，并沒有可靠證據證明素食會對健康造成實質破壞；綜合現有的科研結論，我們可以說至少素食和雜食同樣安全，并且極有可能更安全。"

    在他們研究的下一階段，Eshel 和Martin將著手調查小型有機農場的能源消耗情況，看它們是否能為大型農業企業提供更加健康的商業模式選擇。這類農場一般可靠5到10英畝地提供足夠200到300個家庭消費的蔬菜。

    Martin說："我們正開始研究是否能在保持效率的同時縮減食品生產規模。"