"According to a 2005 University of Chicago study, a lacto-ovo vegetarian emits far less greenhouse gas than a counterpart adhering to the standard, meat-rich American diet—the difference is equivalent to around 1.5 metric tons of carbon dioxide per year, assuming the same daily caloric intake. (The study's authors thus claim that going vegetarian has the same effect on carbon dioxide emissions as switching from a Chevrolet Suburban to a Toyota Camry.) The savings mostly come about because of the disparity between the fossil fuel required to produce a calorie's worth of grain vs. that needed to make a calorie's worth of beef; grain is nearly a dozen times more efficient in this regard. Cattle are also a huge source of methane, a particularly noxious greenhouse gas; it's estimated that bovines are responsible for roughly triple the methane emissions of the American coal industry.
But Eshel hastens to add...that your vegan acquaintance isn't necessarily some environmental saint. That's because direct carbon dioxide emissions are only part of the story when it comes to food's eco-impact. You also have to look at the issue of land use—specifically how much and what sort of land is required to sustain an agricultural enterprise. In a region with poor-to-mediocre soil, for example, it may be more efficient to operate a well-managed egg farm than to try growing vegetables that can't flourish under such conditions. And animals are handy at consuming low-quality grain that isn't necessarily fit for human consumption. (Rather than going to waste, that grain can help create nutrient-rich dairy products.) In fact, a recent Cornell University study concluded that modest carnivorousness may actually be better for the environment than outright vegetarianism, since cattle can graze on inferior land not suitable for crops. Squeezing more calories out of the land means that less food needs be imported from elsewhere, thereby reducing the burning of fossil fuels."
My friend who sent me this article added: "I ate steak tartare for supper last night and the night before."
Disclaimer: I am not a vegetarian