If you're worried about global warming or water pollution, you can blame cars and factories, the White House, and the oil companies.
Or you can blame cows and pigs. Yes, cows and pigs.
The Food and Agriculture Organization of the United Nations, in a report called "Livestock's Long Shadow," says, "The livestock sector emerges as one of the top two or three most significant contributors to the most serious environmental problems, at every scale from local to global."
"The findings of this report," it says, "suggest that it should be a major policy focus when dealing with problems of land degradation, climate change and air pollution, water shortage and water pollution, and loss of biodiversity."
Cows do not add to the amounts of carbon dioxide in the atmosphere.
They do not run aground and spill crude oil. But they do ruminate -- which is to say that they give off methane when they chew their cud and belch, and nitrous oxide and ammonia when they leave manure all over the barnyard.
So that pungent odor you smell on a farm? It's bad for the global environment.
Methane, while less prevalent in the air than carbon dioxide, is 23 times more potent as a heat-trapping gas, the FAO report says.
Do some math, the authors say, and you find that livestock is responsible for 18 percent of the world's greenhouse gas problem.
What's more, cows take up a lot of space, grazing on land that could feed many more of the world's people if it were used for crops.
The FAO says grazing takes up 26 percent of the land on Earth that is not covered by ice -- 30 percent if you count the land used to grow feed for the animals.
And livestock does a lousy job of cleaning up after itself.
Farm waste, washed downhill by rain, is a major source of water pollution. The methane and ammonia in cow dung rise into the air with evaporating water -- and fall back down as ingredients of acid rain.
In China, phosphorus and nitrogen contamination are flowing into the South China Sea, killing off marine life.
In Latin America, forests that would soak up carbon dioxide are instead chopped down, so that farmers can use the land for their cattle.
And on and on, enough to make for a 400-page report.
The FAO says the problem "needs to be addressed with urgency." But it says the good news is that "major reductions in impact could be achieved at reasonable cost."
It might be cheaper, some scientists have argued over the years, to control methane emissions from agriculture than carbon dioxide from cars and industry.
Cows' diets could be modified. Manure could be recycled; it's already dried and burned as a fuel in many poorer countries -- and because it comes from animals, it counts as "renewable."
What complicates it, though, is that livestock is used for food. If you want to control greenhouse gases, will people be willing to eat less meat?
Different groups that spar over environmental issues had reactions that fit their existing positions.
"Yup. … Next year the Democrats will have the EPA [Environmental Protection Agency] regulating exhaust emissions of cows," wrote one person in a blog forum. "Heehee."
But at one environmental group that has called for caps on carbon dioxide emissions, a staff member, asking that the group not be named, called the report "bullsh--."