Livestock generate significant greenhouse gases, but how do the emissions of different animals vary? In particular, as an aficionado of goat cheese, I'd like to know if goats generate lower levels than cows.

—Mimi in Brooklyn, New York

A: It's tough to get a reliable estimate on emissions from goat-milk production. It's such a small part of U.S. milk output that goats have not been as thoroughly studied as other livestock. But according to the UN Food and Agriculture Organization, goat milk is responsible (worldwide) for twice the amount of greenhouse gas emissions as cow milk. Goats in the United States may be more efficient, though, and a study from New Zealand finds that its goats are "on a par" with cows in the greenhouse gas department.

Livestock production accounts for 4.2 percent of total U.S. greenhouse gas emissions: 2.2 percent from beef cattle, 1.37 percent from dairy cattle, 0.47 percent from pigs, 0.08 percent from poultry, 0.04 percent from sheep, and 0.01 percent from goats. I'm not condoning a meat-and-dairy binge, but when you consider that transportation accounts for a whopping 26 percent of our emissions, we should maybe worry as much about how we bring home the bacon as we do about the emissions from raising it. 

Official disclaimer: I've been partial to dairy ever since an old girlfriend's family cow set the world record for milk production. "Granny" was a truly exceptional bovine, from whose example we all can learn. "She gets along well with the other cows and doesn't push her way around," her chief milker noted at the celebration honoring Granny, then age 17, for her U.S. record. "She doesn't crowd into the feed bunk but goes to eat when the others lie down."