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Science
| 10/04/17

Tropical forests emit more carbon than they absorb

TCE

The world’s tropical forests are releasing more carbon dioxide into the atmosphere than they are absorbing, according to a new study that uses some of the most accurate data on forest growth and loss to date.

It’s not all bad news, though. Researchers, whose work was published last week in the journal Science, say that the situation can be turned around fairly straightforwardly, and even cheaply, converting “carbon sources” back into “carbon sinks.”

Accurate assessments of carbon flux from forests are notoriously difficult. Up to now, scientists have evaluated carbon gains or losses using measurements of atmospheric carbon flows and emphasizing the role of forest growth on the one hand and clear cuts and large fires on the other. This skips over the influence from forest degradation or disturbance, which includes such phenomena such as drought and pests—the incidence, severity and duration of which are increasing in a warming world—and more difficult to quantify impacts such as selective logging.

To make up for the degradation and disturbance gap, the researchers considered field measurements, remote sensing and satellite imagery from 2003 to 2014. They found that as feared, there is a net release of carbon from tropical forests, amounting to 437 teragrams (437 billion kilograms), the bulk of this from South America. The western hemisphere represents 60 percent of these losses, with 24 from Africa and 16 percent Asia.

However, variations are sharp within regions. The western hemisphere simultaneously enjoyed the greatest forest gains and suffered the greatest losses. The researchers noted that they can return to being carbon sinks with better conservation efforts and enforcement of existing regulation.

They also are careful to note that not all carbon losses from forests immediately correspond to increases in atmospheric carbon. When the losses are in the form of wood products such as furniture of building structures—which constitute four to 14 percent of the losses in the study—this carbon is in effect locked away from the atmosphere for decades or, in the case of long-lived buildings, even longer.

The accurate measurement work of the researchers and the follow-on efforts of developing policies and best practices are at the heart of PICS’ own Forest Carbon Management project. British Columbia also needed to have more accurate measurements of the carbon flux of its forests, and based on that what sort of forestry practices would enable optimal carbon mitigation.

The BC scientists note that the period investigated by the tropics researchers is however very short. They caution that for Canadian regions, the variability from year to year in carbon losses or gains in a region is very large. This means that for British Columbia, all it would take is one moderate drought year in the second half of the study period, or conversely one really great growth year in the first half, for the trend to appear to swing back to “sink.”

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Tropical forests carbon 'sources' not 'sinks', but situation can be turned around with better management

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