EVER since “An Inconvenient Truth,” Al Gore has been the darling of environmentalists, but that movie hardly endeared him to the animal rights folks. According to them, the most inconvenient truth of all is that raising animals for meat contributes more to global warming than all the sport utility vehicles combined.

The biggest animal rights groups do not always overlap in their missions, but now they have coalesced around a message that eating meat is worse for the environment than driving. They and smaller groups have started advertising campaigns that try to equate vegetarianism with curbing greenhouse gases.

Some backlash against this position is inevitable, the groups acknowledge, but they do have scientific ammunition. In late November, the United Nations Food and Agriculture Organization issued a report stating that the livestock business generates more greenhouse gas emissions than all forms of transportation combined.

When that report came out, People for the Ethical Treatment of Animals and other groups expected their environmental counterparts to immediately hop on the “Go Veggie!” bandwagon, but that did not happen. “Environmentalists are still pointing their fingers at Hummers and S.U.V.’s when they should be pointing at the dinner plate,” said Matt A. Prescott, manager of vegan campaigns for PETA.

So the animal rights groups are mobilizing on their own. PETA is outfitting a Hummer with a driver in a chicken suit and a vinyl banner proclaiming meat as the top cause of global warming. It will send the vehicle to the start of the climate forum the White House is sponsoring in Washington on Sept. 27, “and to headquarters of environmental groups, if they don’t start shaping up,” Mr. Prescott warned.

He said that PETA had written to more than 700 environmental groups, asking them to promote vegetarianism, and that it would soon distribute leaflets that highlight the impact of eating meat on global warming.

“You just cannot be a meat-eating environmentalist,” said Mr. Prescott, whose group also plans to send billboard-toting trucks to the Colorado Convention Center in Denver when Mr. Gore lectures there on Oct. 2. The billboards will feature a cartoon image of Mr. Gore eating a drumstick next to the tagline: “Too Chicken to Go Vegetarian? Meat Is the No. 1 Cause of Global Warming.”

The Humane Society of the United States has taken up the issue as well, running ads in environmental magazines that show a car key and a fork. “Which one of these contributes more to global warming?” the ads ask. They answer the question with “It’s not the one that starts a car,” and go on to cite the United Nations report as proof.

On its Web page and in its literature, the Humane Society has also been highlighting other scientific studies — notably, one that recently came out of the University of Chicago — that, in essence, show that “switching to a plant-based diet does more to curb global warming than switching from an S.U.V. to a Camry,” said Paul Shapiro, senior director of the factory farming campaign for the Humane Society.

The society, Mr. Shapiro said, is not only concerned with what happens to domesticated animals, but also with preventing the carnage that global warming could cause to polar bears, seals and other wildlife. “Our mission is to protect animals, and global warming has become an animal welfare issue,” he said.

Even tiny pro-veggie operations are starting to squeeze dollars out of their shoestring budgets to advertise the eating meat/global warming connection. Vegan Outreach, a 14-year-old group in Tucson with just three full-time workers and a $500,000 annual budget, is spending about $800 this month to run ads and links to its Web page on about 10 blogs. And, it will give more prominence to the global warming aspect of vegetarianism in the next batch of leaflets it orders.

“We know that vegetarian organizations have sometimes made exaggerated health and environmental claims, but that U.N. report is an impartial, unimpeachable source of statements we can quote,” said Matt Ball, executive director of Vegan Outreach.

Like Mr. Prescott, Mr. Ball is incensed that high-profile people like Al Gore — or environmental groups with deeper pockets than his — have not stepped up to the plate.

“Al Gore calls global warming an existential risk to humanity, yet it hasn’t prompted him to change his diet or even mention vegetarianism,” he complained. “And I guess the environmentalists recognize that it’s a lot easier to ask people to put in a fluorescent light bulb than to learn to cook with tofu.”

Advertising specialists warn that this new attention to global warming may attract enemies as well as converts.

“Using global warming as a tactic for advancing the cause of vegetarianism feels a bit opportunistic,” said Hank Stewart, senior copywriter at Green Team Advertising, which specializes in environmentally themed ads.

He also questions the logistics. “You want to get the message as close to the meat-purchasing moment as possible,” he said, “but can you imagine a supermarket allowing ‘Attention, Planet-Destroying Carnivores’ on the in-store radio?”

Environmental groups, meanwhile, readily concede that mobilizing against meat eaters is not their highest priority.

“We try to be strategic about doing the things where each unit of effort has the most impact,” said Carl Pope, executive director of the Sierra Club. Mr. Pope notes that his group has stopped short of castigating people for driving S.U.V.’s or building overly large homes, too.

“We’ll encourage companies to make more efficient S.U.V.’s, and we’ll encourage consumers to buy them,” he said, “but we do not find lecturing people about personal consumption choices to be effective.”

Environmental Defense is also “in agreement on the value of eating less meat,” said Melanie Janin, director of marketing communications. But, she added, her group would rather spend its time and money influencing public policy — specifically, getting Congress to regulate greenhouse gases.

Mr. Gore declined to make himself available for comment. Chris Song, his deputy press secretary, simply noted that a suggestion to “modify your diet to include less meat” appears on Page 317 of Mr. Gore’s book version of “An Inconvenient Truth.”

He did not address Mr. Gore’s personal food choices.

Source: The New York Times