Trees across the tropics are getting bigger and offering help in the fight against climate change, scientists have discovered.

    A laborious study of the girth of 70,000 trees across Africa has shown that tropical forests are soaking up more carbon dioxide pollution than originally thought. Almost one-fifth of our fossil fuel emissions are absorbed by forests across Africa, Amazonia and Asia, the research suggests.

    Simon Lewis, climate expert at the University of Leeds, who led the study, said: "We are receiving a free subsidy from nature. Tropical forests are absorbing 18% of the CO2 added to the atmosphere each year from burning fossil fuels."

    The study, published tomorrow in Nature, measured trees in 79 areas of intact forest across 10 African countries from Liberia to Tanzania, and compared records going back 40 years. "On average the trees are getting bigger," Lewis said.

    Compared to the 1960s, each hectare of intact African forest has trapped an extra 0.6 tonnes of carbon a year. Over the world's tropical forests, this extra "carbon sink" effect adds up to 4.8bn tonnes of CO2 removed each year - close to the total carbon dioxide emissions from the US.

    Although individual trees are known to soak up carbon as they photosynthesise and grow, large patches of mature forest were once thought to be carbon neutral, with the carbon absorbed by new trees balanced by that released as old trees die.

    The discovery suggests that increased CO2 in the atmosphere could fertilise extra growth in the mature forests.

    Lewis said: "It's good news for now but the effect won't last forever. The trees can't keep on getting bigger and bigger."

    Helene Muller-Landau of the Smithsonian Tropical Research Institute in Panama, said the growing forests could recovering from trauma - droughts, fire and human activity - going back hundreds or even thousands of years.

    The research comes as efforts intensify to include protection for tropical forests in carbon credit schemes, as part of a new climate deal to replace the Kyoto protocol.