When people talk about reducing their “carbon footprint,” transportation and energy use in the home usually get all the attention. Diet deserves to be a part of that conversation, too, however. The global agricultural system is complex, and not all food choices are created equal in terms of their impact on climate and their use of resources.

Agriculture accounts for roughly 12 percent of human-caused greenhouse gas emissions. Population growth obviously increases the demand for agricultural production, but there’s another important trend as well—the rising consumption of meat. People in many developing nations are eating more meat as they gain the means to afford it. This is significant, as meat is a sort of demand multiplier because of the crops needed to feed livestock. A field of corn, for example, may be able to feed x number of people, but it can feed far fewer if it’s used to raise cattle.

The animal part of our diet is a significant portion of the agricultural system. Animal feed requires the output of 40 percent of US cropland—and if you include pastureland for grazing, it accounts for 40 percent of all US land. Feed also uses 27 percent of total irrigation and half of the nitrogen fertilizer used, and it contributes about five percent of total greenhouse gas emissions in the US.

A lot goes into raising livestock and getting meat to market, and a lot of effort has been put into quantifying the environmental impacts of different types of food. This isn’t easy given the number of factors involved and the difficulty of obtaining real data from real farms. Some people try to attack the question from the bottom up—gathering information from individual farms and generalizing to the big picture—while others take the big picture data that is available and try to estimate the farm-level details.

A new study by researchers led by Bard College’s Gidon Eshel takes the latter, “top-down” approach to compare the land, water, greenhouse gas, and fertilizer footprints of beef, chicken, pork, eggs, and dairy. Their results show that one of these foods is not like the others.

They used a national database containing information on land use, irrigation water, and fertilizer application between 2000 and 2010 in the United States. Based on the number of livestock produced, they analyzed what each type of animal ate, how much manure was produced, and other details. They calculated the impact in each category per calorie of food produced, as well as per gram of protein in that food.

Pork, chicken, eggs, and dairy were all pretty similar, but beef was a different story. Compared to the average of the other four, beef required 28 times the area of land per calorie, 11 times the water for irrigation, six times the nitrogen fertilizer, and it resulted in five times the greenhouse gas emissions.

For reference, they calculated similar impacts for potatoes, wheat, and rice. Compared to the average of those, beef’s footprint ballooned to 160 times the land, eight times the water, 19 times the nitrogen fertilizer, and 11 times the greenhouse gas emissions. Per gram of protein, the story is largely the same except that potatoes, wheat, and rice have less of an advantage due to their lower protein content.

Why the difference? First, basic biology dictates that eating higher up the food chain is less efficient than eating plants. The general rule is that about 90 percent of the calories eaten by an animal are lost before it makes it to your dinner plate. But given the way livestock are raised in the US, beef’s footprint is considerably worse than dairy, pork, chicken, or eggs. In the simplest terms, we put almost four times as many calories into beef to get a calorie out as we do for chicken, pork, eggs, and dairy.

But despite the fact that beef is the least efficient of these foods, it’s extremely popular in the US—partly due to agricultural subsidies that keep the price down—and it accounts for seven percent of all calories consumed. Between its inefficiency and its popularity, beef accounts for 60 to 90 percent of the total footprint of all animal-based foods in each of the four categories.

When it comes to your dinnertime options, beef has about the biggest resource footprint you can find. As climate scientist Ken Caldeira put it to several news outlets following a similar study, “[E]ating a pound of beef causes more greenhouse warming than burning a gallon of gasoline.”

PNAS, 2014. DOI: 10.1073/pnas.1402183111 (About DOIs).