An episode of the British show Dispatches aired "The Great Green Smoke Screen," which revealed how academics and environmentalists are questioning the ethics and impact of offsetting -- and suggesting that offsetting schemes have not been effective as claimed.

Dispatches claims that schemes run by Climate Care, and promoted by BA, took no account of the "multiplier" effect which increased the damage done by aviation fumes in the stratosphere by a factor of three. In other words, the schemes funded by BA passengers only mitigate one third of the damage that their flights cause. Climate Care, which uses a multiplier of two on its website, told Dispatches that it was unhappy about the way Britain's biggest airline ran its scheme.

A project funded by BP to siphon methane gas from excrement at pig farms in Mexico will only produce half the savings claimed on the company's website, which is now being amended.

In Bulgaria, Sky funds a hydro-power plant that turns water into low-pollution electricity but the manager, Vladislav Tsvetkov, said that Sky's money, though welcome, was "not required" -- a view backed by the Bulgarian bank which lent the capital. The manager later retracted his comments and the Carbon Neutral Company which administers the offsets insisted the money had been critical in establishing the plant.

At Donkleywood forest in Northumberland, the Carbon Neutral Company said the planting of the trees helped the climate. But it emerged that 70 percent of the funding came from the Forestry Commission.

The programme analysed the behaviour of the electricity suppliers which offer customers the opportunity to match their bills to wind and solar power with "green tariffs" while doing no more than meeting their legal obligations to buy renewable power.