The picture of Southeast Asia's deforestation is coming into greater focus.

Scientists have developed a new satellite-imaging technique that allows them to have a better bird's eye view of when carbon-rich peatlands were cleared and to what extent they have been replaced by palm oil trees.

The work, which depends on a blend of satellite images, marks the first attempt to systematically quantify carbon loss from peatland destruction across the region and directly tie it to oil-palm expansion.

Published yesterday in the Proceedings of the National Academy of Sciences, the study found that about 6 percent of carbon-rich peatlands in Peninsular Malaysia and the islands of Borneo and Sumatra were cleared to make way for oil-palm plantations by the early 2000s. In the process, about 3 percent of forest-dwelling birds across the region were lost and massive quantities of carbon were released from clearing peatlands, according to the authors' estimates.

By last year, the study concluded, an area in Southeast Asia roughly the size of New Jersey had been felled of its forests.

Whether or not those lands will become palm oil plantations remains unknown. Historically, such lands have been cleared to expand lucrative palm oil plantations or expand pulp and paper businesses.

The satellite images that were culled to form composite images of the area can only pick up well-established palm oil plantations that are at least eight years old due to the 250-meter resolution. More complete information could be drawn from satellite images with greater resolution, but those high-resolution images would not allow for such a large-scale image of the landscape, the study explains.

Maps that reveal new palm oil plantations
"We envisage that our mapping method provides a cost-effective means to monitor future oil-palm expansion across the tropics by remapping any landscape on a regular basis," said study author Lian Pin Koh, a tropical ecologist with the Swiss Federal Institute of Technology.

The work will provide more nuanced mapping of deforestation in the region, helping researchers and policymakers move away from being as dependent on costly in-person surveys to monitor forest clearing.

"I think it will give us considerably better maps so that policymakers will be able to say, when there is any proposed development in different areas, 'OK, are these areas likely to cause greater emissions or loss of biodiversity, or are these truly degraded lands?'" said Doug Boucher, the Union of Concerned Scientists' tropical forest and climate director.

The work also sheds light on the fact that almost 90 percent of oil-palm development before the early 2000s was on non-peat lands -- suggesting something else may have been driving deforestation on those lands.

Koh declined to comment on what that trigger might be, but Boucher suggested that loggers may seek permits for oil palm work and then just clear the land for timber -- never converting the land into palm oil plantations.

Still, while oil palm was not the main driver for peatland destruction across all of Southeast Asia, in some subregions, such as North Sumatra and Bengkulu, more than a third of peat-swamp forests were lost to oil-palm development, said Koh.

Big biodiversity losses
Whatever the reason for the lack of palm oil development on the land, Koh hopes that his findings will lead to more discussions about reforesting such lands and conserving them -- rather than categorizing them as "degraded" and allowing them to be further developed.

"We found that 2.3 million hectares of peatland have been cleared but not yet planted with palm oil," he said in an e-mail, pointing to the New Jersey-sized tract felled as of last year. "These cleared lands should be dedicated to conservation because of the potential for substantial carbon emissions through peat oxidation if these lands were to be drained for oil-palm development," he said.

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