New research suggests that older Amazon rainforests that have experienced drought or other natural disturbances may give out more carbon than they take up.

The findings, by researchers based in Brazil and in the United States, suggest that earlier studies have overestimated the ability of the Amazon rainforest to absorb carbon dioxide by neglecting to consider the effect of natural disturbances.

Scott Saleska of Harvard University and colleagues report in this week's Science that their studies near Santarém, Brazil, show that drought or other disturbances that kill trees can lead to higher levels of carbon dioxide being released than would normally be expected.

These increases occur during wet seasons, when the dead wood breaks down, whereas more generally studies have found that is the dry season during which there is an increase in carbon loss.

Forests are estimated to absorb about one-quarter of annual emissions of carbon dioxide caused by human activities. The functioning of such 'carbon sinks' has become a key focal point of negotiations to reduce carbon emissions.

Link to research paper in Science by Saleska et al  

Reference: Science 302, 1554 (2003)