What would our Paleolithic ancestors think if they could witness the collective hissy fit we’ve gotten ourselves into over the banning of plastic bags in Toronto?  Surely, they would spear us to prevent such feeble specimens of humanity from polluting the gene pool with our dependencies, lack of imagination, and unwillingness to change! 

Because, to hear some people tell it, banning plastic bags is the first stumble on a slippery slope that leads to the collapse of civilization.  Mayor Rob Ford, chief apologist for all things retrogressive, spluttered “You can’t tell people they can’t give out plastic bags. To me it’s ludicrous.”

Look, let me tell you a story.  Imagine, if you will a simpler time – the late seventies.  The grumpy professor – then just a grumpy art student – leaves his student digs in Tooting Bec, south London, to do some shopping.  He arrives at the local street market, one of hundreds scattered around the metropolis, and asks the avuncular vendor for two pounds of potatoes.

“Two pahnds o’ taters” yells the vendor as though advertising his wares to the whole borough.  He then weighs the spuds; I hold out my open back pack, and he pours the spuds into it.

I ask for a pound of carrots.  “Pahnd o’ carrits” – and in they go.  And so we go on, with beets, onions, and finally tomatoes and lettuce.  Backpack bulging with produce, I make my way home.  Maybe a couple of tomatoes got bruised, but otherwise the different veggies suffered no ill effects from their enforced cohabitation.

And most importantly, no bags of any kind, paper or plastic, were needed in this ancient shopping ritual. 

My point, which you may have already guessed, is that the ubiquitous use of plastic shopping bags is a manufactured need, not some absolute necessity.  “Back in the day”, people lived without them, just as they lived without all the electronic crutches that are enfeebling our mental capacities in the here and now.

But of course, people are used to having plastic bags, and Toronto City Council’s bag ban may have been rather precipitous.  Meanwhile, paper bag manufacturers are making plans to ramp up production to fill the gap.  Meanwhile, Globe and Mail Columnist Margaret Wente, who regards any green by-law as a sort of enforced medieval hair shirt penance, threw us the following piece of “research”:

“….banning plastic bags will do exactly nothing to save the planet…..Because they take very little energy to produce and are reused a lot, they are more environmentally friendly than either cloth or paper. (There’s a ton of research on this. Honest.).”

This got my attention, because even Margaret Wente usually has a factor two from somewhere to boulster her arguments. And as a scientist, I am obligated to consider all sides of an issue.  So I asked myself “Could she be right?”, and did a bit of research of my own.

This is what I found:

• Contrary to Wente, there is not “tons” of research on this subject.  I was able to track down about six studies that attempt Life Cycle Analyses (LCAs) of different types of bag, but only two that include reusable cloth bags as part of the mix (see “Citations”).  A LCA is an attempt to describe the total materials and energy use that goes into manufacturing a product, right from harvesting raw materials with primary processing through to the use, re-use and disposal of the final product (Dilli 2007, Fig. 3.1, Edwards and Fry, 2011 (Appendices)).
• The studies that are available are written for different jurisdictions (UK and Australia) using a variety of different measurements and norms.  This means that comparative analyses are difficult, and susceptible to cherry picking.  For example, the UK study uses a baseline of the number of items carried in a month’s worth of shopping as their standard of comparison among different bag types.  The Australian study uses “quantity of shopping bags used to carry 70 grocery items home from the supermarket each week for 52 weeks in relation to relative capacity and adjusted in relation to expected life span” (Table 3.1).
• The numbers of tables and graphs contained in these reports is enough to make the most dedicated researcher’s eyes glaze over.  So some cherry picking is unavoidable. That said, the grumpy professor is duty bound to be objective (that is why he’s grumpy sometimes…). 

So here are the most important results I have gleaned (some of them may surprise or frustrate you):

1. “Calico” (cotton) and re-usable non-woven plastic “green” bags used less materials and energy to produce than any form of paper or plastic bag, according to the Australian study (Dilli 2007, Table 4.4).  In fact, the green plastic bags came out ahead of cotton because cotton bags need a hell of a lot of water to produce them.
2. Also according to the Australian study, paper bags made of recycled material are not particularly virtuous.  They require more primary materials and energy, and have a far higher global warming potential (GWP – measured as kilograms per CO₂ equivalent) than cotton or green bags. 
3. By contrast, the UK study, which quantifies all impacts based on GWP equivalents, suggests that plastic and paper have much lower GWP than green bags or cotton provided you just account for the manufacturing and transport process, and ignore the numbers of times bags are re-used (see Figure 1).
4. But things change dramatically if you include the fact that almost all bags (including plastic ones) get re-used.  According to the UK study, you would have to use a cotton bag 131 times to produce less global warming than a single use HDPE bag, or 173 times if the HDPE is used as a bin liner (see Table 1).  The PP (Polypropylene “green bags”) fare better, and only have to be used 11 times to cancel out a single use HDPE bag.

Figure 1.  Global Warming Potential (GWP) of different bags without re-use.  This is Figure 5.1 of Edwards and Fry 2011. HDPE is high density polyethylene (your typical shopping bag you get at the checkout), LDPE is low density polyester (“bag for life”, a type of green bag) and PP is polypropylene, another type of green bag.  Cotton is not shown because it apparently has ten times the GWP of other bags.

Figure 2.  GWP, assuming bags are re-used enough times to outperform a HDPE bag that is used only once. This is Figure 3.2 of Edwards and Fry (2011).

 Table 1.  Number of times different bags have to be re-used to have lower GWP than standard HDPE shopping bags (from Edwards and Fry 2011, Executive Summary).

“Green” plastic bags (the “non-woven PP above) and cotton bags beat out ordinary plastic shopping bags, provided you re-use them sufficiently often.  This entails having them to begin with, and then remembering to take them shopping – which doesn’t seem like so much to ask, does it?  Paper bags, recycled bags and compostable (which I assume means “biodegradable”) bags fare worse than cotton bags, green bags, and even plastic bags made of primary materials on several indices, including primary materials used, energy consumption, and global warming potential (Dilli 2007).  So the folks who say that biodegradable bags are the answer may be asking the wrong questions.

So, what is the bottom line?  If you do not shop too often, and do not mind using scarce fossil fuels to make your bags, then the “green” shopping bag is the way to go.  If you shop rather a lot, and do not mind using a lot of water (not to mention agricultural land) to make your bags, then go for cotton.  Either way, making a habit of re-using your bag is the key.

Which brings me to self-justifying apologists for plastic bags.  These folks have come up with some pretty lame and desperate arguments in defence of the plastic grocery bag.  We hear that plastic is re-used – well, yes, but only to a point.  According tot the UK study, 53% of people re-used plastic bags, and 43% used them again for dog poo.  These are one-time re-uses for plastic.  Only 8% re-used them for shopping.  So, levels of re-use are low. 

Other lines of defence include “convenience”, forgetfulness, cloth bags accumulate “harmful bacteria”, and cloth bags are manufactured in China.  These arguments are so pathetic that they scarcely deserve to be included in serious discourse.  Forgetfulness?  Write a note to yourself.  Bacteria?  I know this is stunningly obvious, but wash the damned bag!  You’ve added a little to the GWP by washing it, but not much really.  As for Chinese manufacture?  Sadly, almost everything we use these days is made in China, so if you are trying to say domestic plastic is more “ethical”, then you will have to also repatriate our entire industrial base.

And then, there is the coalition to save the plastic bag (www.savetheplasticbag.com).  Now there is a name designed to inspire trust.  Not surprisingly, these folks cherry pick data from the UK report to favour plastic.  For example, they make a big deal of Figure 1 and Table 1, which show that you have to re-use your cotton bag quite a bit to beat the GWP of plastic.  Oh, the suffering!  They neglect to show Figure 2, which portrays materials use and disposal costs in a rather different light!  In any case, my wife and I have been using our cloth bags for at least five years each, and they show no sign of wearing out – so clearly, it is easy to beat the required 131 uses (Table 1)

Getting back to our Paleolithic ancestors.  It is an axiom of anthropology that Humanity succeeded through imagination and cooperation.  So, why is it that are so reluctant to change even minor aspects of our lives in modern times.  Back in the seventies, I did not need plastic bags when I went to the market.  OK, my back pack was nylon; but mea culpa, I never claimed to be perfect – and I kept that back pack for at least seven years. 

I have to admit, the dog poo thing is difficult.  But what about all the bags that contained breakfast cereal boxes, frozen peas, coffee and the like.  These do not leak and could be easily repurposed for dog poo.  And if you don’t like that, well, the market will eventually provide you with purpose-built one-use dog poo bags at a price.

Look on it as a cost of doing your business!

Dilli, R. 2007. Comparison of existing life cycle analysis of shopping bag alternatives: Final Report. Sustainability Victoria, Melbourne, Australia. http://www.sustainability.vic.gov.au/resources/documents/LCA_shopping_bags_full_report%5B2%5D.pdf> Accessed on June 9th, 2012.

Edwards, C., and J. M. Fry. 2011. Life cycle assessment of supermarket carrier bags: a review of the bags available in 2006. Report: SC030148, UK Environment Agency, Bristol, UK. http://publications.environment-agency.gov.uk/PDF/SCHO0711BUAN-E-E.pdf> Accessed on June 9th, 2012