This is a column about how good intentions can run amok. It tells the story of how an honourable, intelligent man set out to avert environmental disaster and ended up accidentally promoting the economics of the slave trade. It shows how human lives can be priced and exchanged for goods and services.

The story begins in a village a few miles to the west of London. The British government proposes to flatten Sipson in order to build a third runway for Heathrow airport. The public consultation is about to end, but no one doubts that the government has made up its mind.

Its central case is that the economic benefits of building a third runway outweigh the economic costs. The extra capacity, the government says, will deliver a net benefit to the UK economy of £5bn(. The climate change the runway will cause costs £4.8bn, but this is dwarfed by the profits to be made.