You may have seen an IMF report in the news last week claiming that fossil fuels are subsidised to the tune of over five trillion dollars every year. This made good headlines, but only because the IMF chose to describe untaxed externalities as 'post-tax subsidies'. This is unusual and misleading. I wrote about why in The Daily Telegraph:

The IMF’s idea of “subsidies” to fossil fuels refers to something completely different. They have taken the indirect costs to society of using energy – air pollution, traffic congestion, climate change – and, if governments haven’t imposed special taxes on one, called it a “subsidy”. The problem is, we already have a word for these things: externalities. And there is something rather Orwellian about describing a failure to tax something as a subsidy. Here’s an example of what we’re talking about: when my neighbours play loud music at night, it makes me worse off. I’d pay, maybe, £20 for them to shut up, if it wasn’t so awkward to go to the flat downstairs, knock on their door and start negotiating prices. Economists would say that they are imposing a £20 externality on me, and that in a perfectly efficient world, my building would charge residents around that much to play music, and give it to sleep-deprived neighbours like me. But, in the absence of that charge, nobody would say that those neighbours are being subsidised by me. It’s just not what the word means. Except, apparently, to the IMF.

That isn't to say that externalities should never be taxed, if a private solution can't be found. But we already have high fuel taxes in most of the developed world, and in the developing world these taxes will hold back growth. Since economic development has positive externalities, it's not obvious that the negative externalities of fossil fuels outweigh the positives. You can read the whole piece here.