"Shoppers may face 20p bag charge"

Faithfully trotted out by the BBC:

The [Welsh] National Assembly's Sustainability Committee will later publish a report calling for the levy on the use of all disposable plastic shopping bags.

The document says a standard charge of between 10p and 20p per bag would raise between £6.48m and £12.96m which could be used to fund environmental projects.

It is believed that each bag takes between 450 and 1,000 years to degrade and collectively amount to between 0.1% and 1% of visible litter in the UK.

Ahem ...

1. The tax might well "raise between £6.48m and £12.96m" but that's just taking money out of people's hands, so they have less to spend on other stuff.

2. This notion that "each bag takes between 450 and 1,000 years to degrade" is totally insubstantiated. Plastic bags have been in common use for what, half a century? What's the oldest plastic bag that you've ever seen?

3. They "amount to between 0.1% and 1% of visible litter"? That's a handsome margin of error, even by my standards. I am not averse to user charges, so let's pro rate up the 648 million bags used in Wales to 13 billion bags for the whole of the UK every year (four per person per week, looks about right), all of which are used as cheap and cheerful bin liners and go in the normal rubbish. Let's assume that plastic bags' share of normal rubbish is 0.5% and use the government's own assumption that refuse collection costs about £2.4 billion per year (£2 per binload per household per week, seems about right), 0.5% of that is £12 million, divided by 13 billion bags is 0.1p per bag*. That's roughly the correct level of taxation ...

4. ... anything more than that, and we'd have a situation like in Ireland where people buy more, thicker bin-liners, nappy bags etc, so the overall environmental benefits are questionable to say the least.

* My earlier calculations, which were based on the cost of waste incineration per ton, also arrived at 0.1p per bag as a fair charge.