Experts say safeguarding forests like Madagascar's will be key to tackling warming, since deforestation in the tropics produces about 20 percent of all carbon dioxide emissions.

A UN report this week warned that Africa is suffering deforestation at twice the world's average rate.

Ray Victurine, finance programme director for the non-profit Wildlife Conservation Society (WCS), told Reuters by telephone that the offsets were expected to sell over 30 years, with prices now at an average of between US$4 and US$10 per ton.

"It is linked to the voluntary carbon market," he said.

Madagascar's forests may be small in comparison with those in Indonesia or Brazil. But they contain rich biodiversity, from chameleons to lemurs and enormous baobab trees.

WCS said proceeds from the sale would be used to protect the 400,000-hectare Makira Forest, which is home to 22 species of lemur, hundreds of varieties of birds and thousands of plants.

About half of Madagascar's unique biodiversity was found in Makira, the group said.

Half the expected revenue from the sales would go directly to communities living in the forest, WCS said, while a quarter would go to forest conservation and 15 percent to the government's conservation and climate change projects.

The remainder would go on monitoring and overheads.

Conservationists say deforestation in Madagascar has slowed after the authorities there decided to set aside some 6 million hectares as nature reserves.

But farming and charcoal use still lead to the loss of 100,000 hectares of forest a year on the huge island, where more than three quarters of the almost 20 million population live on less than US$2 per day. (Editing by Daniel Wallis and Mariam Karouny) (For Reuters latest environment blogs click on: http://blogs.reuters.com/environment/)