Nanogirl Michelle Dickinson: Is banning plastic bags bad for the environment?

Our supermarkets are making bold moves in the ban on single-use plastic bags. They have significant public support too - a 65,000-strong petition was delivered to parliament in support of the ban.

However, could the alternatives for getting your groceries home actually be worse for the environment?

I stopped using single-use bags a long time ago, and feel smug when I turn up to the supermarket with my neatly folded non-woven reusable bags.

But that smugness subsided when I saw a recent life cycle analysis on plastic bag alternatives.

The analysis is a calculation of the environmental cost of a product at every step of its lifecycle, from the extraction and production of the raw materials through to the manufacturing, packaging, transportation and final disposal.

Two major independent studies on grocery carrier bags - one published by Denmark's Ministry of Environment and Food, one by the UK Environment Agency – agree (shockingly to me!) that overall, single-use plastic bags had the lowest carbon footprint compared to the alternatives, especially when the bag's final use was a bin liner.

"How on earth can that be?" I hear you cry.

Well, the majority of the greenhouse gas production from a plastic bag comes from the extraction and production of its raw materials. Single-use bags require the least amount of material and are the lightest, and therefore the most efficient to transport.

An Australian Capital Territory government study found that after banning single-use plastic bags, the sales of thicker-plastic bin liner bags increased by 31 per cent.

Paper bags, considered by many as a more natural alternative, are about four times heavier than a plastic bag and need to be reused four times to reduce their global warming footprint to that of a single-use plastic bag.

Surprisingly, although trees may seem more natural than crude oil, converting hardwood into paper requires a resource-heavy pulping process that uses large amounts of water, energy and chemicals, which can emit toxic and hazardous chemicals into air and water. This was confirmed in a report by the US Department of Energy.

Based on current calculations, the environmental impact of one single-use plastic bag equals five uses of a thicker LDPE plastic bag, four uses of a paper bag, 14 uses of a polypropylene non-woven bag (the reusable ones you can buy at the supermarket today) and 173 uses for a cotton reusable bag.

The global warming impact is not the only factor that determines how environmentally friendly each bag option is. Photos of turtles eating plastic bags are a strong advertisement for how littered our oceans are.

Government research studies in the US and Australia found that single-use plastic bags only account for around 1 per cent of the litter found. Closer to home, a Sustainable Coastlines Queen St drain survey found cigarette packaging, beverage bottles and food containers to be the biggest challenges when it comes to litter.

Although banning bags is a positive step, realistically only makes a tiny dent when it comes to reducing the amount of plastic waste in our ecosystem.

What we need is an evidence-based study that looks at the behaviours of New Zealanders to see how and what we use our bags for. If we are going to offer an effective strategy for reducing the environmental impact of taking our shopping home we need to gather data for an alternative which provides better environmental solutions.

If we don't we will only end up exchanging one type of plastic for another, without looking at the evidence around what New Zealand actually needs.

In the meantime, I'll continue to re-use my supermarket bags but I won't be feeling as smug at least until after my 14th shopping trip.