Paper bags vs plastic: What's really best for the environment?

The Warehouse, Noel Leeming and Fresh Choice Nelson are the latest retailers to strike a blow for the environment and ditch plastic bags - or jump on the bandwagon, depending on your point of view.

The Warehouse Group, which includes the 'red sheds', Noel Leeming, Warehouse Stationery and Torpedo 7, is opting for 'compostable' bags, while Fresh Choice Nelson is opting for paper.

"We care for the environment and the community, and we know how negative the impact of plastic is," said Warehouse chief experience officer David Benattar.

The Warehouse Group currently goes through about 21 million plastic bags every year - even though the red sheds charge 10c for each one.

"The only way to solve the problem is to ban the bag and that's exactly what we're going to do," Fresh Choice Nelson co-owner Mark A'Court told Stuff.

The problem he's talking about is pollution. Plastic bags end up in the oceans, can take hundreds or even thousands of years to decompose, and are a threat to wildlife, which can mistake them for food.

But are paper and reusable bags any better? The answer, as it often is when it comes to environmental impacts, is that it depends.

The problems with paper

A UK study published in 2011 found the biggest environmental impacts of all types of shopping bags came during the production process - not their use or how they're discarded.

And reusable and paper bags require more resources to produce than the lightweight plastic bags commonly found in supermarkets.

Paper bag
Paper bags - not a silver bullet. Photo credit: Getty

"A lot of people jump straight to paper as a solution, but paper takes trees, water, fossil fuel and chemicals to produce," says James Blackwood of the anti single-use plastic bag campaign Bags Not.

A Scottish study in 2006 found paper bags take four times as much water to make. The researchers also said they result in the release of three times as much greenhouse gas, increase atmospheric acidification, result in more solid waste (because they're heavier and thicker) and release more air pollutants.

Paper bag production also causes eutrophication (nutrient overload) of waterways 14 times more than plastic bag production.

Plastic bags are also much more likely to be re-used than paper, particularly as bin-liners. Almost 90 percent of people who took part in an unscientific poll Newshub ran in February said they do this.

The UK study, using a more conservative estimate of 40 percent, found paper bags would have to be reused at least four times to compete with that kind of efficiency, and seven times if the Newshub poll was accurate. The same study found re-use of paper bags was insignificant.

The only real advantage paper bags held over plastic is that they're far less likely to end up as litter, because they degrade much more easily and there are more options for recycling.

"Paper carrier bags have a bigger environmental impact than lightweight plastic bags in all categories apart from risk of litter," an update to the research issued in 2014 noted.

Also, paper bags rip easily, especially when wet. The UK study notes this as one of the primary reasons consumers flocked to plastic bags when they hit the market in the 1970s.

The problems with reusables

Because much more goes into the production of a bag that's meant to last, if they're not reused the environmental impacts are comparatively devastating. The UK study found a cotton bag would have to be used 173 times to compete with a plastic bag - rising to 327 times if every plastic bag found a second life as a bin-liner.

A 2015 study also found shoppers with reusable bags tended to purchase more junk food - perhaps rewarding themselves for looking after the environment, but at the risk of their own health.

The problems with biodegradable bags

"A growing number of people do indeed compost their products and their food waste - these bags have way less impact on the environment," said Mr Benattar.

But not only are biodegradable bags - usually made from a mix of plant and synthetic matter - much more resource-intensive to make than lightweight plastic bags, they can release methane when sent to landfill, research has found. Methane is about 21 times more effective at trapping heat in the atmosphere than carbon dioxide.

They also require particular environments to break down properly - just putting them in a landfill with everything else won't guarantee they'll actually disappear over time.

"Compostable sounds great, but you need commercial compost facilities to deliver on the promise," says Mr Blackwood. "Home compostable is best."

So what should I use?

Whichever bag you opt for, the key is reuse. Because most of the environmental impact comes from the production, the more a bag can be used, the better - whatever it's made of.

The UK researchers looked at a number of past studies, one of which was conducted in Australia. It found that re-using thicker, heavy-duty plastic bags - the kind you see used at bookshops and clothing stores - was the arguably best compromise. They use fewer resources to make than cloth reusables, and last longer than a typical supermarket bag.

Reusable plastic bag
Reusable plastic bags - the best compromise? Photo credit: Getty

The researchers also said a ban on plastic supermarket bags doesn't work, because wherever it's been tried, consumers end up making the up difference by purchasing plastic trash bags.

"For me it's impossible to think that a substance alien in the natural world and that never decomposes can ever be better than anything," says Mr Blackwood.

"But there is no silver bullet - yet. Just better alternatives, and everything has some kind of impact.

"The ultimate is waste minimisation in the first place and re-using what already exists. Realising you can make a difference, even if it's only one bag at a time, is very empowering."