“Imagine a country where carbon becomes a currency. We carry bank cards that store both pounds and carbon points. When we buy electricity, gas and fuel, we use our carbon points, as well as pounds. To help reduce carbon emissions, the government would set limits on the amount of carbon that could be used.” This is not the future seen by a science fiction writer, but an idea put forward by David Miliband, the UK environment minister in July.

Personal carbon trading, or domestic tradable quotas (DTQs), to use the technical term, is an idea gathering support. In the UK, it would extend to every adult the “cap and trade” principle behind the EU Emissions Trading Scheme (ETS). Each person would have a carbon allowance, a slice of the national carbon budget, which would shrink every year.