In February 2018, Denmark's Ministry of Environment and Food published its Life Cycle Assessment of grocery carrier bags, which looked at the overall embodied energy, materials and labor in different grocery bags, and also evaluated the environmental impacts of different kinds of plastic bags.
Their conclusion was that single-use, flimsy disposable plastic bags are the best option in terms of environmental and climate costs (however, the Ministry's analysis did not factor in marine life impacts of these bags, which are severe).
By far the worst performer was organic cotton, which has about 3x the environmental impact of conventional cotton (organic growing requires more land, water, and pesticides than are used when growing GMOs with synthetic pesticides).
Update: Arthur Yip's Twitter thread on the study points out some methodological problems with the study: "The required reuse number actually varies from 52x – 7100x! 52x is very doable! This is a ridiculously different result than 7100x. Use a cotton bag once/week for a year & it will have "paid off". We're talking abt nice cotton cloth bags, not the thicker plastic ones…Notably, 52x on climate. After using a cotton bag 52x, cotton has lower climate impact than plastic! Again, this is very different from "7000x" or even the average number, 840x, which equally weights all the impact categories discussed… It also did not go through academic peer review. I can only hope that further reviews would have identified the problems I noted and forced the authors to provide some more clarity, footnotes, &re-consideration into how the information was presented and would be interpreted."
With plastic bag bans soaring in popularity globally (127 countries have adopted plastic bag restrictions, and New York just passed one this week), the question of what will replace plastic bags has become more pressing. We know that single-use anything is a terrible idea, whether it is plastic or not, so replacing plastic bags with paper ones will surely have deleterious side-effects like increasing deforestation. Making a paper bag also requires more energy and water than making a plastic bag, so for other environmental considerations besides litter, paper products may be worse than plastic ones.
As the Verge pointed out last year, regardless of the bag you choose, what is likely of vastly greater importance is what you choose to put in it and how you carry it around: Eating less meat, cycling or walking to the store, and buying locally-made grocery products are all likely to make a bigger difference in lowering your personal contribution to environmental problems.
The simplest advice for individuals seems to be this: Whatever you have in your house now—be it a pile of cotton totes, or a jumble of plastic bags—don't throw them out. Keep using them until they fall apart. Whatever the material, use it as a garbage bag once you can't use it for other purposes any more. And whatever you do, try not to buy new ones.
Life Cycle Assessment of grocery carrier bags [Ministry of Environment and Food Denmark]
Your cotton tote is pretty much the worst replacement for a plastic bag [Zoë Schlanger/Quartz]
(Thanks, Fipi Lele!)