Animal-rights groups are attacking Al Gore because “you just can’t be a meat-eating environmentalist.”

Environmental groups have embraced Gore since his documentary “An Inconvenient Truth” won an Oscar. But the People for the Ethical Treatment of Animals and the Humane Society of the United States are pointing to a United Nations Food and Agriculture Organization study published last November that states the livestock business generates more greenhouse-gas emissions than all forms of transportation combined.

The animal-rights groups have launched an advertising campaign to raise the issue further. A PETA spokesperson reports that the group will send a billboard-toting truck to a Denver convention center when Gore lectures there on Oct. 2. It will feature a cartoon image of Gore, with the slogan “Too Chicken to Go Vegetarian? Meat is the No. 1 Cause of Global Warming.”

Perhaps more important is the trend that this signifies. The New York Times reports that “even the tiny pro-veggie operations are starting to squeeze dollars out of their shoestring budgets to advertise the connection between eating meat and global warming.”