Economy 8 May 2012 Global warming and externalities How a carbon tax can "solve" global warming Sign UpGet the New Statesman's Morning Call email. Sign-up Tim Worstall (yes, when he's not trolling he's quite good) has a piece up at the Telegraph explaining how carbon taxes work, and why they could "solve" global warming: In economic theory, the problem here is that my actions that create emissions also damage someone else. But I don't have to pay for the damage I've caused. This is called an externality and the economists' solution is something called a Pigou Tax. That is, we add a tax equal to the damage I'm doing, so that I do pay for that damage. Worstall cites the Stern Review's figure of $80 per tonne of CO2 as a good starting ground for where to set a carbon tax, and explains why it's the most efficient way to deal with climate change: As a made-up example: my car emits one tonne CO2 when I drive it to buy fresh bread for lunch. That's $80 of damage I cause in the future by doing so. But the benefit to me is trivial: if you paid me 50p (alright, £5 in the rain) I'd cycle instead and not emit the CO2. The value to me of driving is that 50p; the costs to someone else are the $80. Clearly, this is a bad deal for everyone else: they're bearing costs much greater than the benefit to anyone at all. An $80 a tonne tax would get me cycling and that would be a good thing: I've stopped doing something where the benefit is lower than the cost. However, we've a pregnant woman in pre-eclampsia. She needs to go to hospital in an ambulance which is going to emit that tonne, that $80 worth of CO2. Without it she and the child will be dead; with it they'll be fine. We usually value a statistical life in the £2 – 3 million range. That's what the railways will spend on safety to save a life on average. Or we could use the £50,000 that NICE applies to one year of good-quality life. If your drug treatment costs more than this, then you won't get it on the NHS; less and you might. Different numbers but much the same outcome: burn that fuel and damn the $80 of future damages, because they're much lower than the benefits that are achieved right now from burning that fuel. This efficiency is why a carbon tax – or the harder to impose, but fairer and economically identical "cap-and-trade" system – really is the best way to deal with global warming. By definition, it deals with "bad" emissions while allowing "good" ones, and it does so far better than a legislature could ever hope to with a sprawling network of tariffs and subsidies. But Worstall does somewhat overstate the case in one area, when he writes: The other part [of a reader's question] – what's the point if we're not going to spend the money on green projects? – misunderstands the purpose of the tax. We're not trying to raise money: we're trying to change prices. Changing prices is only half the effect of a carbon tax – or any Pigou tax. The other half is compensating the "victim" for their loss. Suppose we live in a little two person economy where every tonne of CO2 you produce causes $80 worth of flooding damage to me. Imposing a carbon tax solves half the problem, in that it stops you polluting if you only get $10 benefit from it. But it doesn't solve what happens if you can make $100 from polluting. In that case, you pay $80, and make $20 profit. I'm still left with $80 of flooding damage. The proper use of the money raised is to compensate the me for that loss. Otherwise, a tax which merely sorts out externalities becomes a revenue-raising tool of Government. In practice, this means that money raised from a carbon tax should be used on "green projects". Which would annoy Worstall's fellow Telegraph blogger James Delingpole. › Remembering Maurice Sendak Anti-carbon tax protestors in Australia. Photograph: Getty Images Alex Hern is a technology reporter for the Guardian. He was formerly staff writer at the New Statesman. You should follow Alex on Twitter. Subscribe For daily analysis & more political coverage from Westminster and beyond subscribe for just £1 per month!