Tropical forests are adding more carbon dioxide to the atmosphere than they're removing, according to a new study that estimates the world's lush canopies emit more CO2 than all of America's cars and trucks.

The silver lining, the researchers say, is that tropical forests have untapped potential to act as carbon sinks through better conservation and land management.

Historically, scientists have underestimated forest emissions by focusing mostly on clearcutting, big fires and other overt deforestation, said Wayne Walker, one of the report's authors and an associate scientist at the Woods Hole Research Center, a climate change think tank.

But those analyses have missed degradation from humans, like selective logging, as well as natural disturbances like drought and disease, which are becoming worse as the world warms, Walker said. "While [those losses] might be small in any one place, when you add up all those losses of individual trees across an area as large as the whole pan-tropical belt, it's significant," he said.

Walker and other researchers from Woods Hole and Boston University used 12 years of satellite imagery, laser remote sensing and field measurements to compile what they say are the most accurate data on forest loss and growth ever assembled.

Their findings, published yesterday in the journal Science, show that every continent's tropical forests emit more carbon than they absorb—releasing about 862 teragrams of carbon while absorbing 437 teragrams. (A teragram is a billion kilograms.)

Of the 425 teragrams of net emissions, about 60 percent comes from the Western Hemisphere, while about 24 percent comes from Africa and about 16 percent comes from Asia.

Walker warned against reading too much into where the net emissions are coming from, since there's wide variation even within the same region or country.

For instance, the Americas saw the most forest loss as well as the most forest gain. And forest in the Congo only became a net source of carbon in 2007, said Alessandro Baccini, an associate scientist at Woods Hole and an author of the study.

Even though forest degradation is more widespread than previously thought, the researchers said the ecosystems remain relatively intact and can bounce back if governments strengthen conservation policies and enforcement.

If they do bounce back, they could be a cheap way to reduce atmospheric carbon.

"If tropical forests were a sink, then tropical forests wouldn't be able to play much of a role [in reducing CO2 levels]," Walker said. "But they are a source—that's just a fact, based on our paper—and they do represent a tool in the toolbox. And as tools go, as technologies go, the restoration of forests continue to be one of the easiest, least expensive sorts of ways forward."

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at