Roger Hegarty isn't a big fan of fart jokes. But over the course of a decade studying livestock methane emissions for the New South Wales state government in Australia, he's found that few people have been able to resist sharing their favorite wind-breaking witticism. "There isn't a fart joke in the world I haven't heard," he says with good-natured weariness. He'd be thrilled not to hear any of them again, particularly considering that 95% of the gas actually issues from front end of the animal, not the rear.

In any case, the production of methane and nitrous oxide from farming of cows, sheep, and other farmed animals is no laughing matter. In Australia alone, There are now roughly 25 million cattle in pastures and feedlots, and 100 million sheep. In 2004, together they produced some 65 million tons of "carbon dioxide equivalent," a measure that allows the warming effects of...

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