Trees in tropical forests are getting bigger and absorbing almost a fifth of all the carbon dioxide emitted in the world, according to a new study.

In this undated photo, study author Bonaventure Sonke and his field team measure trees in the Dja Faunal Reserve in southeast Cameroon. (Photo courtesy of Simon Lewis)

Researchers from a variety of institutions — including the University of Leeds in Britain, the University of Yaounde in Cameroon and the University of Toronto — studied data collected between 1968 and 2007 from 79 intact forest areas in 10 countries across Africa. They found that on average, trees in intact African forest areas soaked up an additional 0.63 tonnes of carbon per hectare per year — significantly more than expected.

Extrapolating these results, the researchers found that African tropical forests are responsible for absorbing 1.3 billion tonnes of carbon dioxide a year.

The study will be published in Thursday's edition of the journal Nature.

Analyzing these results with numbers taken from 156 intact forest areas from 20 countries around the world, the researchers found tropical forests are responsible for absorbing some 4.8 billion tonnes of carbon a year.

"We are receiving a free subsidy from nature," Simon Lewis, lead author of the paper, said in a statement. "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."

The researchers point out that human activity emits 32 billion tonnes of carbon dioxide, of which only 15 billion tonnes remain in the air. Of the 17 billion tonnes that is not circulating, half is absorbed by the oceans, while the rest is taken up somewhere on land, the researchers say.

"According to our study about half the total carbon 'land sink' is in tropical forest trees," says Lewis, who's with the University of Leeds in the U.K.

'We cannot rely on this sink forever'

But the tropical forests cannot be counted on as a permanent bank for carbon, the researchers say.

As forests grow, they soak up carbon from the atmosphere, but the opposite holds true when trees die. In the absence of dramatic and steep deaths of trees (due to events like wildfires or deforestation), the rate at which trees in forests grow eventually equals the rate at which they die.

As a result, the researchers say that forests should eventually reach a state of equilibrium where there is no net carbon absorption or release.

The researchers suggest two possible causes for why today's tropical forests haven't reached equilibrium and are showing an increased propensity to absorb carbon dioxide.

One theory is that the forests are still growing back from some past trauma — a process that could take hundreds or thousands of years.

Another explanation is that these tropical rainforests have been pushed off equilibrium by global climate change — extra carbon dioxide in the atmosphere is effectively fertilizing the trees, the authors say. But the cause remains cloudy, the researchers say.

"Whatever the cause, we cannot rely on this sink forever. Even if we preserve all remaining tropical forest, these trees will not continue getting bigger indefinitely," Lewis said.

"Whether remaining intact forests will continue to sequester carbon, become neutral, or become a net source of carbon in the future is highly uncertain," the study says. "Improved monitoring and modelling of the tropical environment is required to better understand this trajectory."

The researchers say their study demonstrates why tropical rainforests should be conserved.

"Predominantly rich polluting countries should be transferring substantial resources to countries with tropical forests to reduce deforestation rates and promote alternative development pathways," Lewis said.