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.