Ask most Americans what causes global warming, and they’ll point to a coal plant’s smokestack or a car’s tailpipe. But two other images should be granted similar iconic status: the front and rear ends of a cow.

According to a little-known 2006 United Nations (UN) report, “Livestock’s Long Shadow,” livestock is a “major player” in climate change, accounting for 18 percent of all greenhouse gas emissions. That’s more than the worldwide transportation system.

The U.S. Department of Agriculture (USDA) reports that the American meat industry produced more than 1.4 billion tons of combined wastes in 1997—five tons for every citizen and 130 times the volume of human waste. Methane, a global warming gas 23 times more potent than CO2, comes from many human sources, but livestock account for 37 percent of its total. And waste is just one of meat’s many harmful and far-reaching environmental side effects.

Livestock production consumes 8 percent of the world’s water, mainly to irrigate animal feed; causes 55 percent of land erosion and sediment; uses 37 percent of all pesticides; directly or indirectly results in 50 percent of all antibiotic use; and dumps a third of all nitrogen and phosphorous into global freshwater supplies.

Raising livestock is the single largest human-related use of land. Grazing occupies 26 percent of the ice-free and water-free surface of our planet, and cropland to feed those animals devours 33 percent of Earth’s arable land. All that grazing leaves an impact.

The UN reports that 20 percent of the world’s pastures and rangelands have been degraded through overgrazing, soil compaction and erosion. Meat production leads to deforestation, too—grazing now occupies 70 percent of previously forested land in the Amazon region.

Worse, the food grown to feed animals could be feeding more people. For example, livestock eat 90 percent of the U.S. soy crop, 80 percent of our corn and 70 percent of the grain produced in industrial nations.

More, livestock are forcing other animals out. With species loss accelerating in a virtual “sixth extinction,” livestock currently account for 20 percent of the animal biomass on the planet.

Despite all this, in 2003, the average human ate more than 90 pounds of meat, double the amount 50 years ago. Cultural signals at school, work and church, as well as advertising, continue to reinforce the message that meat is good and necessary for health. Vegetarianism is still depicted as a fringe choice for health faddists.

Sadly, environmental organizations today rarely propose vegetarian diets. Even such an enlightened source as the 2005 Worldwatch report, Happier Meals: Rethinking the Global Meat Industry, is careful not to advocate a vegetarian diet, instead listing it among options that include eating less meat, switching to pasture-raised, “humane” meat, and opting for a few non-meat entrées per week. Vegetarianism has become “the elephant in the living room,” but even in this food-conscious age, it is not easily made the centerpiece of an activist agenda.

Michael Jacobson at the Center for Science in the Public Interest argues that cutting down meat consumption should be a top public health priority. “From an environmental point of view, the less beef people eat the better,” he says, citing the release of methane from livestock and noting the increased risk of colon cancer and heart disease.

When supplied such facts, many meat eaters ask, “Where would I get my protein?” According to the latest medical research, a balanced vegetarian diet provides all the protein needed for glowing health—and avoids meat’s high cholesterol and saturated fat.

Were humans meant to eat meat, just because our ancestors did? “Nonsense,” says Dr. Milton Mills, a physician with Fairfax Hospital in Virginia and a free clinic activist. “The human gastrointestinal tract features the anatomical modifications consistent with an herbivorous diet.”

“The fact that this cornerstone of the American diet aids and abets climate change is an ‘inconvenient truth’ many of us don’t want to face,” remarks Joseph Connelly, publisher of VegNews magazine. He chides Al Gore for not mentioning meat-based diets in his film and giving them only glancing coverage in his book, An Inconvenient Truth.

A 2003 Harris Poll found that between 4 and 10 percent of the American people identify themselves as vegetarians. Five years later, Connelly says that number seems to be holding steady.

“From a sustainability point of view, what’s really needed is for people to understand the connections between factory farming, meat eating and environmental impacts,” he advises. “That’s the first step.”

Jim Motavalli is a freelance environmental writer based in Fairfield, CT. Reach him at www.JimMotavalli.com.