As a Danish government study citing a "7,100 times" greater environmental impact of cotton over single-use plastic bags is being cited or referenced (often indirectly) here, I thought I'd take a glance at its assumptions. They prove ... interesting.[1]

TL:DR; Cotton, particularly organic, is most heavily penalised for water consumption, though net carbon emissions consider. The study entirely ignores litter and acquatic impacts, and limits consideration to 100 year impacts on Denmark specifically. It is a fascinating case of very carefully delineated boundaries of consideration at odds with an inclusive long-term environmental assessment, and is absolutely not reflected in popular articles linked. You can paint a very good picture of a thing if you hide its worst features.

I'm commenting as I read through the report, conclusions emerge as I do so (most are in a reply to this comment).

"The effects of littering were considered negligible for Denmark and not considered."

That is, one of the biggest concerns of plastics. and the environment -- long-term residency if tossed, especially in waterways or the ocean, simply isn't addressed.

"The environmental assessment of each carrier bag was carried out taking into consideration different end-of-life options: incineration (EOL1), recycling (EOL2), and reuse as waste bin bag (EOL3) before being incinerated."

That is, landfill disposal or composting are not considered. Cotton bags can be composted, if slowly.

"The environmental assessment was carried out for a range of recommended environmental impacts (European Commission, 2010): climate change, ozone depletion, human toxicity cancer and non-cancer effects, photochemical ozone formation, ionizing radiation, particulate matter, terrestrial acidification, terrestrial eutrophication, marine eutrophication, freshwater eutrophication, ecosystem toxicity, resource depletion, fossil and abiotic, and depletion of water resource."

A long list, but again notably lacking long-term environmental residence and impacts of items bypassing disposal processing.

Looking at specific measures and bag types:

Organic cotton bags:

"Reuse for grocery shopping at least 149 times for climate change"

This is well below the 7,000 headline number, and corresponds to three years of weekly shopping trips, fewer if used multiple times weekly.

"at least 20,000 times considering all indicators; reuse as waste bin bag if possible, otherwise incinerate."

Interesting, though I'm dying to find out what the least favourable indicators are, and how those might possibly be mitigated.

Conventional cotton bags:

Interesting to see how the less "eco-friendly" option fares...

"Reuse for grocery shopping at least 52 times for climate
change"

That's one year of weekly use, a few months if more frequent.

"at least 7100 times considering all indicators"

And that's our headline number. Where does it come from?

A footnote on the next page provides a clue:

The highest number is due to the use of water resource, but also to freshwater and terrestrial eutrophication.

I'm aware from previous research that cotton is famously water-hungry. A chief reason it's grown in river deltas (Nile, Mississippi).

Choice of methodology is of interest:

"The LCA carried out for this study was conducted according to the requirements outlined in the International Standards 14040 and 14044 (ISO, 2006a, 2006b). The present Section provides a detailed description of the LCA methodology utilized for the study: the goal of the LCA, functional unit and reference flow, the system boundaries, the choices for the modelling approach for addressing multi-functionality, the modelling tools, data requirements, impact assessment method, assumptions and limitations."

I'm not familiar with these or alternatives, but at the very least the methodology isn't fully home-grown. Though it may have been carefully selected.

"we defined a functional unit that allowed a fair basis for comparison for the grocery carrier bags"

That is, comparison is standardised to weight and volume capacity.

"Carrying ... an average volume of 22 litres and with an average weight of 12 kilograms.... After use, the carrier bag is collected by the Danish waste management system."

Knowing that several multi-use alternatives are capable of both greater volume and mass, by factors of 2-4, I'd be interested to see if this is addressed and how. I'll note that toting 50kg of groceries may not be for the faint (or weak) of heart, though there is assistive technology.[2]

From subsequent discussion, the reference unit is a minimum capacity, not a scaled reference. That is, some options (e.g., LDPE simple) are scaled up to meet this. capacity, others (Jute, cotton, composite) with greater capacity are not scaled down.[3]

"The time horizon of the impacts in this LCA was 100 years."

This raises all kinds of issues, from environmental residency (already excluded by presuming perfect wastestream recovery) to future value. discounting / present value analysis. Suffice to say there are a vast number of bodies buried, literally, in that innocuous sentence.

"Secondary reuse ... was assumed as substituting a waste bin bag."

So all y'all reuse-it-as-a-trash-bag peeps, you're covered. And yes, plastic beats organic here.

Impact categories are listed in table 5, p. 39. As previusly, omissions are highly notable.

The assumptions section,3.10, pp. 41ff, deserves close reading. Highlights only:

"we did not take into consideration customers' behavioural patterns"

That is, this is a highly idealised study. Useful as an engineering reference, perhaps, but not a full description of the real world. All models are false, yes. I've serious doubts this one is even useful, particularly as cited in practice.[4]

"For biopolymer and textile bags, recycling was not considered."

"Recycling of textiles was not taken into account since it mainly occurs outside the Danish waste management system."

I'll simply note that the environment does not care about this distiction.

On production impacts: "it was not possible to retrieve 'market' processes from Ecoinvert for all the carrier bags materials assessed"

That is: while cotton production is extremely well understood and ducumented, chemical synthesis is not. Ignorance is not innocence.

"For organic cotton, we modified the Ecoinvent dataset for conventional cotton production by subtracting environmental impacts connected to fertilizers and by lowering the production yield by 30%."

Any intrinsic high-volume inputs to cotton will have a far greater impact on organic production. Stay tuned.

"We could not find literature data on the production and manufacturing of the waste bin bag."

Though addressed, an interesting and notable comment re: waste-bagging substitution.

Tables 10-12 specifically list impact weights, and yes, water consumption is the most variable factor, from negative water consumption for LDPE (presumably H2O emissions from incineration) to 70 l/unit consumption for organic cotton. Net carbon emissions also penalise cotton.

"The environmental impacts connected to the production of the organic cotton bag (COTorg) were considerably higher than those of the conventional cotton bag (COT). This is due to the fact that organic cotton production does not involve the use of synthetic chemicals such as and pesticides, which lowers the yield of the cultivation."

That is: yield impacts are considered, but not environmental and soil impacts of fertiliser and pesticide use.

Carbon emissions don't seem to be clearly attributed. I'm not sure what to make of this.

Table 23 provides a colour-coded overview of impacts, though it omits the handy acronyms and abbreviations translator you'll need -- that's on p. 21.

Further discussion in follow-up reply.

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Notes:

1. Source: Ministry of Environment and Food for Denmark, "Life Cycle Assessment of grocery carrier bags" (2018)
https://www2.mst.dk/udgiv/publications/2018/02/978-87-93614-... (PDF). Mind the TLS cert errors.

2. The venerable wheeled folding shopping trolley, e.g., https://www.bedbathandbeyond.com/1/1/56758-super-deluxe-swiv...

3. "Life Cycle", Table 3. Required reference flow for each carrier bag

4. See, e.g., Will Dunn, "How Britain fell in love with the tote bag – and why it’s a dangerous fiction", cited elsewhere in this thread, by which I found the Danish report. https://www.newstatesman.com/politics/environment/2019/07/ho...