With the February chill about to hit, I'm in the market for a cozy new sweater. Ordinarily I'd opt for wool, but I wonder whether all those methane-belching sheep are killing the planet. Cotton seems like a sensible alternative, but I'm sure it's not without its environmental drawbacks. So which fabric is greener, wool or cotton?
There's an apples-to-oranges aspect to your question, since wool and cotton have very different environmental downsides. With the former, as you note, the chief concern is methane; for the latter, it's nitrous oxide released by inorganic fertilizers. On top of that, not all wool or cotton is created equal; much depends on the technology employed in the fields or pastures. But the Lantern will take a stab at your query all the same, if only to highlight the ways in which agricultural minutiae can make a real environmental difference.
Sheep, like their bovine cousins, emit an appreciable amount of methane every day, thanks to the parasites that reside in their guts. Estimates vary widely, but the average sheep probably burps out 20 to 30 liters of methane per day—about a tenth of the amount released by a typical cow. These methane emissions are a particular problem in New Zealand, a major wool exporter with roughly 45 million sheep. More than half of the nation's overall greenhouse gas emissions stem from livestock flatulence. And as the Lantern has previously noted, methane is a particularly worrisome greenhouse gas, with a Global Warming Potential of 21 (versus a GWP of 1 for carbon dioxide).
Producing wool also requires a vast amount of water—not only to raise and care for the sheep, but also to rid the raw wool of numerous impurities. It takes approximately 500,000 liters of water to manufacture a metric ton of wool; this figure is even higher when the sheep in question are fed in confined quarters, where extra water is required to manage the manure.
Cotton is pretty thirsty, too, though the crop can get some of its liquid sustenance directly from the heavens rather than via manmade systems. According to the United Nations, producing one cotton T-shirt requires the input of 2,500 liters of water. (It's unclear how much of that water is expended while turning the raw fabric into a finished product, versus just getting it prepped for harvest.)
But when environmentalists grouse over cotton, they usually ignore the crop's water jones and focus instead on the use of nitrogen fertilizers. When these fertilizers are applied haphazardly, large amounts of nitrous oxide—which has a GWP of 310—can be lost to the atmosphere. In Australia, where nitrous oxide emissions have increased 130 percent since 1990 due to fertilizer usage, it's estimated that a third of the nitrogen applied to cultivated fields is lost before serving any purpose. The furrows of cotton fields are particularly egregious emitters; in 2006, researchers from the Queensland University of Technology found that each acre of furrowed land accounted for 13.8 ounces of released nitrous oxide. (The cotton industry counters that its fields act as carbon sinks and ameliorate the crop's effects on the world's climate.)
The Lantern could go on and on about some secondary issues, such as pesticide use in cotton fields and the nitrogen in sheep dung, but he'll spare you the deepest part of the rabbit hole. The bottom line is that cotton probably wins this face-off by a nose, given the sheer volume of methane issuing forth from the planet's sheep.
But if you plan on taking advantage of the fact that cotton is easier to wash in home machines, the balance could tip in favor of wool. According to the University of Cambridge Institute for Manufacturing, washing and drying accounts for more than half of a cotton T-shirt's lifetime energy use—and makes it more energy hungry than a synthetic tee. Yes, the Lantern realizes that T-shirts and sweaters are different beasts, and that dry cleaning a wool sweater takes a lot of juice, too. Still, garment-care plans are worth considering if you're a true green die-hard.
The other assumption inherent in the Lantern's pick is that the farmers in question aren't deploying the latest, greatest technology. Sheep ranchers, for example, are now experimenting with vaccines that inhibit gut organisms, and can thus reduce the animals' methane emissions by up to 20 percent. And wool producers are using new dying procedures that allow for the fabric to be treated at lower temperatures—a small measure, perhaps, but one that reduces greenhouse gas emissions by 20 percent. Genetically modified cotton plants, meanwhile, are raising yields and reducing the need for pesticides. And better crop-rotation practices may help maximize the amount of nitrogen that is sequestered in a farm's delicate soil.
Yet cotton farmers may actually have a bizarre incentive to welcome global warming, at least in the short term: Elevated carbon dioxide levels appear to be good news for cotton yields. So at least we'll have plenty of comfy cotton garments to wear in our overheated future.