Trees across the tropics are getting bigger and offering unexpected 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 that anybody realised. Almost one-fifth of our fossil fuel emissions are absorbed by forests across Africa, Amazonia and Asia, the research suggests.

Simon Lewis, a climate expert at the University of Leeds, who led the study, said: "We are receiving a free subsidy from nature. Tropical forest trees are absorbing about 18% of the carbon dioxide added to the atmosphere each year from burning fossil fuels, substantially buffering the rate of change."

The study 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.

A similar project in South America challenged that assumption when it recorded surprise levels of tree growth a decade ago, Lewis said. His study, published tomorrow in Nature, was to check whether the effect was global.

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 Ancon, Panama, said the forests could be growing as they recover from past trauma.

"Tropical forests that we think of as intact [could have] suffered major disturbances in the not-too-distant past and are still in the process of growing back." Droughts, fire and past human activity could be to blame, she said. "This recovery process is known as succession and takes hundreds or even thousands of years."

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

Lee White, Gabon's chief climate change scientist, who worked on the new study, said: "To get an idea of the value of the sink, the removal of nearly 5bn tonnes of carbon dioxide from the atmosphere by intact tropical forests should be valued at about £13bn per year."

David Ritter, senior forest campaigner at Greenpeace UK, said: "This research reveals how these rainforests are providing a huge service to mankind by absorbing carbon dioxide from our factories, power stations and cars.

"The case for forest protection has never been stronger, but we must not allow our politicians to use this as an excuse to avoid sweeping emissions cuts here in the UK."