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When explaining why I don’t eat beef, pork, or chicken, despite the tempting aroma of a barbequed burger or bacon sizzling on the stove, I say that it’s better for the environment to abstain. But what, exactly, does that mean?

Of the approximately 300 million people in the U.S., 15 million are vegetarians, according to a 2011 Harris Interactive commissioned by The Vegetarian Resource Group, a pro-vegetarian non-profit. That’s up from 7.3 million people just three years before,  Vegetarian Times. Half of those, or 2.5 percent of the population, are vegan, meaning they consume no animal products, and the other half describe themselves as lactoovovegetarians, meaning they don’t eat meat, poultry, or fish, but do eat eggs and/or dairy. (Since the 2011 survey was only of 1,010 people, the margin of error is plus or minus three percentage points). Although the reasons for why someone might choose to eat only vegetarian fare are many, there are some environmental benefits to a face-free diet.

Fossil Fuels

Let’s start with the amount of fossil fuel energy required to get one kilocalorie (a unit of energy of 1,000 calories) of animal protein compared to that needed to produce a kilocalorie of grain protein. Although the amount varies depending on the type of meat we’re talking about, research shows that much more energy is needed to produce animal protein. According to a UNESCO , which sites work done by Cornell ecologist David Pimentel, beef requires the most. “On average, animal protein production in the U.S. requires 28 kilocalories (kcal) for every kcal of protein produced for human consumption,” he in 1997.

The meat-based average American diet, and the lactoovovegetarian diet, “require significant quantities of nonrenewable fossil energy to produce,” he and Marcia Pimentel wrote in a widely-cited (PDF) in 2003.

Neither are sustainable in the long term based on heavy fossil energy requirements, they go on to say. Still, “the lactoovovegetarian diet is more sustainable than the average American meat-based diet,” they conclude.

Water

The USGS has this nifty that lets you to guess how much water is needed to produce different types and amounts of food. They quote a by the Pacific Institute’s Peter Gleick, saying that it can take anywhere from 4,000 gallons to 18,000 gallons of water to produce a hamburger compared to 150 gallons of water per pound of wheat produced.

Land Use

Growing global demand for meat can also lead to negative environmental effects. Farmers need more land to grow crops to feed livestock and to manage the animals, and that can result in habitat fragmentation and biodiversity loss, researchers (PDF).

Carbon Emissions

The United Nations Food and Agriculture Organization in 2006 that meat production currently contribute about 18 percent of the greenhouse gas emissions—expressed as CO2 equivalent because it’s not always in the form of carbon dioxide—that the world produces annually. Yet another (PDF) complicates matters by stating that up to 51 percent of the greenhouse gas emissions can be attributed to livestock, as writer Robert Goodland this summer.

The numbers and images representing the environmental effects of meat production tell one side of the story; personal testimonials like in ���Ϲ��� tell another. (For more cool graphics, check out this --in PDF --from Scientific American). And my personal choice doesn’t mean that what I have decided to eat—or not to eat—is right for you, but eating less meat is one way to reduce your environmental footprint. Now if you’ll excuse me, I have to get back to my facon (read: fake bacon), lettuce, and tomato sandwich.