I spent an hour or so researching the issue a long time ago and I seem to recall another study by a French supermarket chain. You need to reuse your cotton bags a lot -- hundreds of times is right -- for it to become worthwhile. Which is doable, of course.
And of course, the carbon footprint of both plastic bags as well as cotton bags varies wildly depending on the weight etc.
“Nonwoven PP, on the other hand, is less costly than cotton. These bags need to be reused only 11 times to break even with the conventional plastic.”
Of course these are less durable that heavy cotton canvas. I doubt I could use one of these 200 times. I could definitely use a good canvas bag that many times.
Its just a losing gambit. You know how many shopping bags we have around the house? Supposed to save energy, but once you have 4 or 5 of them, you're up to a lifetime supply of plastic bags. And I'm sure we're not the only once.
This 'issue' is eco-theatre, in my view.
It’s not “eco-theater” to want to reduce the ecological cost of bags, though. Not is it “eco-theater” to want to keep millions of plastic bags out of the oceans and rivers, or out of storm drains where they cause clogs, or out of bushes and trees where they are just ugly.
And as I alluded to above, not all bags of the same material are made equal. The original British source has a range of 80-250g for cotton bags. The ones I use weigh in at less than 50g.