Future Forests cannot sell the offsets on to other companies as the voluntary deal was not done under a recognised international emissions trading system. However, it has corporate clients that pay it for taking positive environmental action on their behalf.

"If we're going to address climate change, you have to engage people and provide solutions," George Fowkes of Future Forests told Reuters.

The transatlantic deal came despite U.S. President George W. Bush's withdrawal from the United Nations Kyoto Protocol on climate change, saying it would hurt economic growth.

Carbon emissions trading was one of the methods envisaged by the protocol to reduce global warming.

"Private entities are clearly forging ahead with global warming solutions despite the continued debate over a global warming strategy," said Laurie Wayburn, president of the PFT, a California-based NGO.

The UK started a voluntary emissions trading system in April. Global trading could begin in 2008. Countries or companies that reduce greenhouse gas emissions or maintain large forests to soak up carbon dioxide will get credits that larger polluters will have to buy to offset their emissions.

Fowkes said the PFT had earned the credits by improving forestry management practices, enabling the woodlands it looked after to absorb more carbon. Trees and plants take in carbon dioxide and give out oxygen.

Forest Futures has 120 corporate clients, including car rental company AVIS Europe and insurances Swiss Re , and around 12,000 individualscustomers, that pay it to make their businesses or lives carbon emission free.

Its clients wanted to reduce their environmental impact or to be seen as environmentally friendly, while the insurance industry in particular was worried about the effects of unpredictable climate change on its business, Fowkes said.

Previously Future Forests has reduced greenhouse gas emissions mostly through tree planting, he said.