Tropical rainforests seem to release about three times the amount of carbon dioxide previously expected, according to a new study by U.S. and Brazilian researchers.

The new-found source of carbon dioxide suggests tropical forests are neither a source nor a sink for the greenhouse gas.

It was thought that forests may act as carbon "sinks" that absorb excess carbon and moderate global warming. But it's been difficult for scientists to get a handle on what role tropical forests play in climate change.

Researchers use floating chambers to measure changes in carbon dioxide levels in the Amazon basinCourtesy: University of Washington

Estimates of carbon dioxide emissions taken on the ground of Amazon forests suggested they were acting as a carbon sink. On the other hand, global measurements in the atmosphere suggested the amount of carbon dioxide released by the Amazon almost matched the amount it absorbs.

In this week's issue of the journal Nature, researchers found a new source of carbon in the Amazon that could help balance its books.

'River breath' amounts to fifth of human-made carbon dioxide

Floating chambersCourtesy: University of Washington

Researchers used satellite radar imagery and streamflow measurements to figure out how much carbon dioxide is naturally produced by rivers, streams and flooded areas in the Amazon basin.

The researchers calculated that tropical forest waterways worldwide emit 900 million tonnes – that's equivalent to about a fifth of the carbon dioxide generated by humans each year through deforestation and burning fossil fuels.

Their total matches the amount thought to be missing from some leading models of worldwide carbon movement, said University of Washington oceanography Prof. Jeffrey Richer.

Trees and other vegetation draw carbon dioxide out of the air through photosynthesis. Trees use carbon to produce wood cells.

The carbon, which Richer calls "river breath", eventually ends up in rivers and streams when woody debris and leaf litter are carried downstream by rain and floods.

"If you want to know where carbon from today's tropical forest goes, look 1,000 kilometres downstream in 20 or 30 years," said Richer, who led the research.

In an accompanying article in Nature, ecology researchers John Grace and Yadvinder Malhi of the University of Edinburgh wrote that Richer's study needs more work because "their figures are highly uncertain." They suggest chemical tracers could help.