Plastic bags: what the science says

Man: one who thinks of what he sees. Credit.

Over one hundred countries have banned or taxed single-use plastic bags due to worries about their environmental impact. Should you feel bad about using plastic bags? Let’s look at the underlying science.

Ocean Pollution

When plastics enter the ocean, they break into small pieces called microplastics, which can persist for hundreds of years. They can then enter the food chain and eventually be eaten by humans. At the moment the consensus is that the risk is low, but there’s worry that if concentrations go up there could be real risks to people and the environment.

The obvious question is, how do plastic bags get to the ocean? Googling, I see suggestions that plastic bags can blow away from landfills and eventually enter the sea. Looking at the research, there’s one paper in Science with almost four thousand citations that seems to be the source for this idea. Here’s the key bit, with my emphasis:

We defined mismanaged waste as material that is either littered or inadequately disposed. Inadequately disposed waste is not formally managed and includes disposal in dumps or open, uncontrolled landfills, where it is not fully contained. Mismanaged waste could eventually enter the ocean via inland waterways, wastewater outflows, and transport by wind or tides.

To summarise:

  1. mismanaged = inadequately-disposed + litter
  2. landfills that let waste leak out count as inadequately-disposed and can lose material to the wind.

How much mismanaged waste is there? Here’s what they estimate:

Spot the waste mismanagement in China — it’ll be relevant at the very end of this article…

That picture suggests that the issue is widespread. But the paper also notes that the whole of the EU only produces about as much waste as Morocco. That made me curious, so I dug into their data. Here’s the part for high-income countries with significant waste:

Huh? Most of those countries, including the UK, have no inadequately managed waste. All of the mismanaged waste is litter. And they assume that in every country:

Mismanaged waste is the sum of inadequately managed waste plus 2% littering.

There’s no justification for that 2% figure; in fact, that’s the only mention of ‘2%’ in the article. This is bad science; you can’t just pull numbers out of thin air without any justification.

Bad science. It makes me cry. Credit.

In fact, according to their data, if you live in any of these countries…

Australia, Belgium, Canada, Denmark, Finland, France, Germany, Greece, Greenland, Iceland, Ireland, Italy, Japan, Netherlands, New Zealand, Norway, Portugal, Qatar, South Korea, Spain, Sweden, UK, US

… there is no waste mismanagement, meaning that if you put a single-use plastic bag in the trash, it will not end up in the ocean. Nor will it end up stuck in trees or eaten by animals, because it can’t get out of the landfill.

  • There’s a case to be made that governments in the listed countries should tax or ban bags to reduce littering, with some weak support from this paper. (The statistical method used there is flawed because it doesn’t correct for multiple hypothesis testing, but the raw graphs are somewhat convincing.)
  • I’m not going to go into details, but if you read about the (relatively small) environmental problems with landfills, notably decomposition gases, you’ll see that plastic bags don’t contribute to these.
  • For the avoidance of doubt, mismanagement of trash is a real problem in many countries. Do check their spreadsheet yourself if your country isn’t in the list above.
  • …it probably doesn’t need saying, but if you put plastic bags down the drain, those can end up in the oceans.

Climate Change

What about the climate change impact of plastic bags? They’re made from oil, and they take energy to make, so there will be some emissions from their manufacture.

My go-to source for emissions of estimates is Mike Berners-Lee’s How Bad are Bananas. It considers all greenhouse gases, rather than just CO₂, and it uses the best available method for counting embedded emissions. It suggests that a single-use plastic bag generates 10 grams of CO₂-equivalent emissions.

It’s never a good idea to rely on one source. This paper from the Environment Agency suggests that 82.14 single-use bags come with 2.082 kg of emissions,* meaning a single bag comes with 25g of emissions. This one suggests 520 single-use bags come with a 6.13kg footprint, meaning a single bag comes with 12g of emissions.

Emissions footprints are never precise, so I’m happy that the 10g figure is reasonable.

*2.082 kg is weeeirdly specific for an emissions measurement. It suggests that someone somewhere doesn’t quite understand significant figures… .

UK government data says that in 2014, before a plastic bag charge was introduced, each person used 140 bags. That means plastic bags resulted in 1,400 grams (1.4kg) of CO₂-equivalent emissions per person per year.

How significant is this? Well, the average person in the UK generates 15 metric tons of emissions a year. 15 metric tons is about 10,000 times as large as 1.4kg grams, so plastic bags accounted for about 1/1,0000th of the average UK carbon footprint. That’s 0.01%. Travelling one mile on a city bus produces 15 times the emissions of one plastic bag.

Credit.

Other kinds of bags have much higher emissions than plastic bags, so people sometimes worry that banning plastic bags might increase emissions. In fact, it’s unlikely to matter unless you’re planning to go through 140 cotton tote bags in a year. 0.01% of your carbon footprint is so small that the emissions numbers for all the alternatives are also going to be negligible.

Man: one who thinks of what he sees

If there’s a moral here, it’s that things that are very visible can take over the general conversation without any underlying scientific reason. The images of plastic bags at sea are ugly enough that they’ve captured everyone’s attention, resulting in a lot of misplaced effort in countries with good waste disposal systems. As an individual, so long as you don’t litter, using plastic bags will not cause any environmental harm.

One last note: while recycling plastic bags may make you feel virtuous, a lot of recycling is shipped to China and other countries that don’t have reliable waste disposal procedures. You’re liable to do more harm than good. If you live in a country with good waste disposal, that gives you a guaranteed way to dispose of a bag with no environmental impact.

tl;dr

If you see your country in the list below, use plastic bags as much as you like as long as you throw them in the trash. It won’t harm the environment.

Australia, Belgium, Canada, Denmark, Finland, France, Germany, Greece, Greenland, Iceland, Ireland, Italy, Japan, Netherlands, New Zealand, Norway, Portugal, Qatar, South Korea, Spain, Sweden, UK, US

Key science, with sources. Minus bad statistics. Minus shaky methodology. Minus politicisation, left or right.