Researchers from several universities around the world worked together for seven years growing trees that were exposed to 50% more carbon dioxide (CO2) than is currently in the environment. At first the trees reacted well to the CO2 and grew bigger but, as time went on, they stopped taking in the excess carbon dioxide.

"We found that the trees could not sustain the effect of the excess CO2, " said David Ellsworth of the University of Western Sydney.

"They grew bigger and at a faster rate than normal at first but the rate of carbon storage soon slowed down.

By the end of the study, the trees had only taken in about 10% of the 50% extra carbon dioxide they had been exposed to."

Bands such as Bell X1, Coldplay and the Rolling Stones have all paid out for trees to be planted to offset the environmental damage of their tours, while the Green Party last year committed to offset carbon emissions from flights made by its staff on party business.

Minister for Communications, Energy and Natural Resources Eamon Ryan offset his recent trip to New York and, as of next year, all government ministers will be offsetting official business trips, a spokesman confirmed.

"Practical arrangements will be put in place to enable each department to compute annually the emissions associated with its air travel and to make a contribution to an appropriate fund to securef emission reductions of an equivalent amount, " a Deptartment of the Environment spokesman said.

However, Ellsworth warned buying up trees is unlikely to cancel out the damage done to the environment by air-travel emissions.

"It is certainly not working in a lot of places, " he said.

"Our research found that if trees are fed nutrients then they will take in more CO2 . . .

but generally that is not financially viable.

"Those who are taking part in carbon-offsetting projects should probably be aware of this."