The Wilsons began by getting rid of their son's heated water bed, turning off power to computers and televisions when not in use, changing to energy-efficient light bulbs, hang-drying their laundry and, finally, investing $58,000 (€42,000 £37,000) in a solar panel system - until they reduced their electricity bill to zero.

Then they signed up on the MyEmissionsExchange. com site to have their energy savings calculated. They found that they had already saved one tonne of carbon, which earned them a carbon credit. The exchange sold the credit for $21.50 to Molten Metal Equipment Innovations of Ohio, taking a 20 per cent commission.