A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Do cow and termite flatulence threaten the earth's atmosphere?

March 24, 1989

Dear Cecil:

Is it true that cow, sheep, and termite flatulence does more damage to the ozone layer than fluorocarbons? How much damage do human farts do?

Cecil replies:

Couple issues we need to deal with here, Moe.  The first is your imperfect grasp of the threats to the earth's atmosphere.  While it's true that gas of biological origin is a problem, the concern isn't the ozone layer (which is being damaged by chlorofluorocarbons, or CFCs) but rather the greenhouse effect, which causes global warming.

Some newspaper reporters aren't quite getting it either. You were no doubt moved to write by a story in the Washington Post headlined "Feed, Animal Flatulence and Atmosphere." It described the work of Donald Johnson, an animal-nutrition specialist at Colorado State University who supposedly studies cow flatulence. According to the story, animal flatulence "contributes in a large way to the potentially catastrophic warming of the globe, the 'greenhouse effect.'" Each cow emits 200 to 400 quarts of methane gas per day, or 50 million metric tons per year.

Just one little problem. Cows don't emit 400 quarts of daily flatulence, as the term is usually understood. According to Professor Johnson, they emit 400 quarts' worth of burps, known in polite circles as eructation. The Post, in other words, doesn't know one end of a cow from the other! And this is the paper that broke Watergate — although, to be fair, I don't suppose they assign their top reportorial resources to the cow burp beat.

Details aside, animal methane presents a definite threat to the biota. It's believed 18 percent of the greenhouse effect is caused by methane, putting it second on the list of offending gases behind carbon dioxide. Methane breaks down in the atmosphere to form carbon dioxide, ozone, and water, all of which absorb heat. The temperature of the atmosphere rises, the ice caps melt, and next thing you know you're pumping the Atlantic Ocean out of your basement.

There are several major sources of methane: rice paddies (methane-producing bacteria thrive in the underwater environment), swamps and wetlands (ditto), mining and oil drilling, landfills, termites (although there's still some controversy on this one), "biomass burning" (notably in the Amazon rain forest), and animals. Ninety percent of animal methane is produced by ruminants (i.e., cud-chewers). These include sheep, goats, camels, water buffalo, and so on, but most of all cattle, of which the world has an estimated 1.2 billion.

Ruminants eat hay, grass and other plant material containing cellulose, which can be digested only by special microbes that, to minimize commuting problems, live in the ruminants' guts. Unfortunately, the microbes aren't as thorough as one might like, and about 6 or 7 percent of what they eat winds up as methane. Thus the problem.

Now, you're probably saying, cows have been around forever, how come all of a sudden they're a threat? All we know is this: atmospheric methane has been increasing at the rate of 1 percent a year, and something's got to be causing it. The world cattle population is thought to have increased in the last decade, and Lord knows the Brazilians don't feel like taking any more heat for torching the Amazon. So let's blame the cows.

Is there hope? Professor Johnson thinks a timely application of antibiotics in cattle feed could retard the microbes' methane production. But by and large antibiotics are already in use in the U.S., while in many third-world countries cattle forage in the fields, making antibiotics difficult to administer.

In other words, we've got yet another largely insoluble problem that threatens life as we know it. Sometimes I wish one of these looming disasters would go ahead and happen, just to end the suspense.

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