"Imagine a country where carbon becomes a new 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."

The words of UK Environment Secretary David Miliband speaking at the Audit Commission Annual Lecture last year.

The idea has been picked up by UK product innovation outfit Design Stream, which has teamed up with identity specialist Sven Vogel to create the Emissary credit card concept. The idea is to develop a credit card which can display messages about the environmental impact of personal spending decisions and suggest alternative greener options.

The cardholder’s personal carbon credit allowance would be topped up annually and points would be deducted for every purchase made. Users who exceeded their annual carbon quota would be obliged to top up their card with credits bought from energy efficient citizens with spare allowances.

The imposition of such a system nationally would be impossibly complex and wide open to fraud and abuse - not unlike the current system of carbon trading for industrial polluters. 

But the development of a green credit card that informed users of the environmental impact of their spending decisions could well prove popular with the general public. One for the bank innovation teams to ponder.