A brief link to this article from Philly.com talking about the popularity of the reusable bag on Earth Day.
The best quote?:
Not every reusable bag is environmentally equal. A nonwoven polypropylene bag, for example, would have to be used just 11 times to make up for the negative effects of a plastic bag used one time, according to a British Environment Agency study that compared bags. A cotton bag, however, would have to be used 131 times.
Why would a cotton bag have to be used so much? Because the energy that goes into its production is significantly higher... check this article from Daily Mail and the quote:
Using a thin plastic bag - made from a plastic called high-density polyethylene (HDPE) - equates to generating 1.57kg of carbon dioxide, the greenhouse gas that scientist believe leads to global warming according to the report. A cotton bag would have to be re-used 171 times to emit the same level of CO2.
Cotton bags typically made in China have a greater environmental impact because of the water and fertiliser required in their production, as well as their transportation and greater weight.
As with most things marketed to us for sale as "Green", its actually a load of crap. Buy less, consume less.
Also, Earth Day shouldn't be allowed to overlap with Good Friday. Jesus totally loved his creation, but that's just putting the cart before the horse.