Slate: Should I buy wool or cotton

If I want to help the environment, should I buy wool or cotton? - By Brendan I. Koerner - Slate Maga
http://www.slate.com/id/218295...
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 problems...

"These days, green fashion is all the rage. Major fashion houses have jumped into the fray. Want to spend $2,000 on a handbag and save the environment at the same time? No problem say the denizens of the runway (as long as you do not factor in the incredible amount of resources used by luxury chains to market that little wonder of environmental ingenuity). However, for those of us who live on a more meat and potatoes budget, we occupy ourselves with more mundane musings like: what is the more climate friendly fabric? Cotton or wool? You decide. (The correct answer is organic cotton, which is barely discussed in the article. Hemp is even better)".