Dr Simon Lewis, a Royal Society research fellow at the University of Leeds and author of the paper, said: "We are receiving a free subsidy from nature.
"Tropical forest trees are absorbing about 18 per cent of the CO2 added to the atmosphere each year from burning fossil fuels, substantially buffering the rate of climate change."
Dr Lewis said the trees could be mopping up even more carbon dioxide than before because CO2 already in the atmosphere is acting like a fertiliser, but man could not rely on them forever.
"Even if we preserve all remaining tropical forest, these trees will not continue getting bigger indefinitely," he added.
The Intergovernmental Panel on Climate Change has estimated that human activity emits 32 billion tonnes of CO2 each year, but only 15 billion tonnes actually stays in the atmosphere adding to climate change. The new research shows exactly where some of the 'missing' 17 billion tonnes per year is going.
It is particularly important in the run up to the UN Climate Change Conference in Copenhagen at the end of this year when world leaders will decide a new Kyoto Protocol, including international agreements to halt deforestation.
David Ritter, forest campaigner at Greenpeace UK, said the new findings would put pressure on the world to act. But he warned slowing deforestation should not be seen as a substitute for cutting man made carbon emissions, which make up the bulk of greenhouse gases.
"This groundbreaking research reveals how these rainforests are providing a massive service to mankind by absorbing carbon dioxide from our factories, power stations and cars, " he said. "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."