Carbon dioxide emitted from lakes means forests are not the World’s carbon sponges

Tropical forest waterways give off 900 million metric tonnes of carbon across the globe, comparable to one-fifth that generated by human activities, suggesting that tropical rain forests are not the carbon sponges of the World as many scientists and politicians believe, according to a new study by US and Brazilian researchers.

The new figure matches the amount that some leading modellers have speculated was missing when tallying up the worldwide movement of carbon, says lead researcher Jeffrey Richey, Professor of Oceanography at the University of Washington.

The researchers used radar imagery in conjunction with 13 National Science Foundation expeditions to the Amazon in the 1980s and ’90s. The study has revealed that, with the new figures for waterways, the amount of carbon emitted by tropical forests is approximately the same as that being absorbed by them.

“The land-water connection appears to be far more important than anyone thought,” said Richey. Around 20% of the carbon being released from streams and rivers was found to be from aquatic processes, with the remaining 80% originating in the forests.

Although carbon dioxide is drawn out of the air during photosynthesis, carbon is subsequently ‘swept’ into rivers and streams by the rain and floods that draw it out of the soil and carry woody debris, leaf litter and other matter downstream. As the material decomposes and is eaten by organisms including insects and fish, and as river conditions change, carbon dioxide is released from surface waters.

“If you want to know where carbon from today’s tropical forest goes, look a thousand kilometres downstream in 20 or 30 years,” said Richey.

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