Which is the most earth-friendly: paper bags, plastic bags or cloth bags?
The answer to the question depends upon whether or not you really believe in science, because as they say in certain environmental activist circles, the "science is settled"! Here's the summary description of the bag found to be the best for the environment, which is defined as being the bag with the least negative impact upon the environment, as found in a very recent and thorough study on the topic:
The conventional HDPE bag had the lowest environmental impacts of the lightweight bags in eight of the nine impact categories. The bag performed well because it was the lightest bag considered. The lifecycle impact of the bag was dictated by raw material extraction and bag production, with the use of Chinese grid electricity significantly affecting the acidification and ecotoxicity of the bag.
Yes, the convential High-Density Polyethylene (HDPE) (aka "plastic") bag had the least impact upon the environment of all the bags considered in the study, which considered a number of bags made from different plastics, as well as both paper and cotton-based materials!
But that's only considering using each type of bag just once. For many eco-oriented people, the whole point is to reuse other kinds of bags to counteract the perceived environmental hazards posed by the conventional plastic bag.
Fortunately, the study revealed how many times the alternatives to the conventional plastic bag would have to be reused to overcome their own negative impacts to the environment:
Here, we find that if a consumer only uses a conventional HDPE plastic bag just once (say to carry their groceries home before throwing the bag away), a paper bag would have to be reused 3 times, a heavy-duty plastic bag made from Low-Density Polyethylene (LDPE) would have to be used 4 times, a plastic "bag-for-life" made of non-woven Polypropylene (PP) would have to be used 11 times, and a cotton (or canvas) bag would need to be re-used 131 times!
The study reports that a canvas bag is expected to last for 52 trips (Table A.3.1). With that as a reference, a cotton/cloth canvas bag user does over twice the damage to the environment that a plastic bag using grocery shopper who throws away every plastic bag they get immediately after each shopping trip, as they will likely have to replace their more environmentally-destructive bag at least once long before they reach 131 uses!
However, if a consumer reuses 100% of their conventional HDPE plastic bags (say as trash bags), the number of uses needed for the other bags to have a lesser environmental impact than the conventional HDPE plastic bag rises by a factor of anywhere from 2.2 to 2.5, which we see in the table above. For example, that re-usable canvas bag would need to be used at least 327 times to be less damaging to the environment!
Which makes the eco-friendly canvas bag user over six times as destructive to the environment as the conventional consumer who simply re-uses all the plastic bags they get from the grocery store just once.
If only those anti-plastic bag advocates cared more about the environment....
Edwards, Chris and Fry, Jonna Meyhoff. Life Cycle Assessment of Supermarket Carrier Bags. Report SC030148. Environment Agency.