Due to a global decline in tropical forests, these areas now emit more carbon than they absorb, according to a new study.

These forests now emit 425 teragrams of carbon each year, more than the total of carbon emissions from all of the automobiles in the US. Until recent years, they had absorbed more carbon than they had emitted. The study was published Friday in the journal Science, and represents some of the most thorough research on the subject yet published. It is a much larger change than scientists had thought, and the researchers say the data should lead policymakers to take more aggressive action to address climate change.

According to Alessandro Baccini, one the study’s lead authors:

“This shows that we can’t just sit back. The forest is not doing what we thought it was doing. As always, trees are removing carbon from the atmosphere, but the volume of the forest is no longer enough to compensate for the losses. The region is not a sink any more.”

The study goes further than earlier work on the topic in its examination of disturbance, gaging the impact of the decline in tree density and biodiversity due to fire, drought, selective logging, and hunting. This reduction in the density of foliage can reduce biomass by as much as 75 percent in a given area, but can be more difficult to monitor from satellites than total deforestation, with most of the damage hidden below an intact canopy.

The researchers examined 12 years of satellite data alongside field studies, finding a net carbon loss on every continent. Sixty percent of emissions came from Latin America, 24 percent from Africa and 16 percent from Asia. More was lost as a result of disturbance and degradation than from deforestation. The authors of the study point out that this data provides a chance to see in detail which areas are being affected, and to protect these forests before they are gone completely.

Another lead author of the study, Wayne Walker, explained: “Prior to this we knew degradation was a problem but we didn’t know where or how much. It’s easier to address the problem when there is still some of the forest left standing.”

Walker argues that the most important priority should be the protection of pristine, carbon-dense forests, and that the best way to accomplish this is to support land rights for indigenous people in these areas.

“Those living in the forest can make a difference,” explains Walker.

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