Does cutting down tropical forest emit less carbon?

To date, most estimates of the carbon emissions resulting from tropical deforestation have used data on changes in forest area and timber harvesting submitted to the UN Food and Agriculture Organization (FAO). However, figures submitted to the FAO can be unreliable and the carbon cycle "bookkeeping" models employed make many assumptions about carbon stocks. Now researchers from the US have combined satellite data on both deforestation and the amount of carbon stored at each site to come up with a figure of 0.81 Petagrammes of carbon per year for emissions caused by deforestation in tropical regions between 2000 and 2005. The value is just one-quarter to one-half of earlier estimates.

"Our results serve as a better benchmark for monitoring global progress on reducing emissions," Nancy Harris of Winrock International told environmentalresearchweb. "Satellite data has allowed us to sharpen the overall picture of tropical deforestation and pinpoint locations where carbon emissions are highest. For the first time using consistent methods, we were able to give each country an emissions number, with a statistical uncertainty range, that can be refined through time."

Harris and colleagues from Applied GeoSolutions, California Institute of Technology, University of California, Los Angeles, University of Maryland and the World Bank, employed forest-carbon stock data with a resolution of 1 km together with forest-loss estimates in 18.5 × 18.5 km blocks. To marry together these different data scales, the team employed a statistical-sampling technique.

The researchers found that from 2000 to 2005, the gross loss in tropical forest cover resulted in carbon emissions of 0.81 Petagrammes per year, with a 90% prediction interval from 0.57 to 1.22. This equates to 7–14% of total global man-made carbon-dioxide emissions over the same time period.

"Our estimate may be lower than previous estimates, but 0.8 billion metric tons emitted into the atmosphere is still a very large and troubling number," said Harris. "The good news is that it is a number that we can do something about now – we are not waiting on undeveloped technology to start taking action."

Between them Brazil and Indonesia accounted for 55% of the total. Deforestation in Latin America was responsible for 54% of the emissions, followed by 32% from South and South-east Asia, and 14% due to forest destruction in sub-Saharan Africa. Emissions were high in the Brazilian Amazon, Peninsular Malaysia, Laos, Sarawak, Sumatra and Kalimantan and, to a lesser extent, the Congo Basin. Although nearly 40% of the forest loss took place in the dry tropics, these areas made up only 17% of the carbon emissions because their forests have a much lower carbon density than tropical moist forests.

"Our work highlights the fact that technology for monitoring emissions from deforestation has improved," said Harris. "We were able to conduct a much more refined analysis using satellite data than previous work."

The researchers decided to look at gross emissions because estimating net emissions currently requires assumptions about the fate of converted lands. They reckoned that including emissions from peat fires would add 0.099 Petagrammes of carbon per year to their total, while emissions from peat drainage would add 0.173 Pg, together increasing the total value by about 25%.

Now the team, who reported the results in Science, plans to analyse the period 2006–2010 in order to assess whether emissions have gone up or down.