Forest carbon stores may be massively overestimated

Rainforests may store much less carbon than we thought. It could be time to dramatically revise our estimates following the discovery that apparently similar forests hold vastly different amounts of the stuff.

The finding is important because there are plans for governments worldwide to compensate tropical countries for protecting their forests as "carbon sinks" to curb global warming. If carbon cannot be counted, then dollars cannot be disbursed.

Greg Asner of the Carnegie Institution for Science in Stanford, California, and colleagues say they used satellite mapping, laser probing of forest undergrowth from aircraft and local ground surveys across a large area of Peruvian rainforest to crack the problem of estimating how much carbon is locked up in forests. But the new technique has revealed a large, previously unknown variability in the density of carbon stored in apparently similar forests.

The Intergovernmental Panel on Climate Change says that there should be about 587 million tonnes of carbon stored in the study area, 43,000 square kilometres of lowland forest in the Madre de Dios region of Peru.

But after dividing the area into 40 million individual grid squares and estimating for each the carbon stored as well as the rate at which it is being fixed and released, Asner says the real figure is just 395 million tones, a third less.

Hidden patterns

"What really surprised us was how carbon storage differed among forest types and the underlying geology," says Asner. For instance, where the underlying rocks are younger, the soils and forests contain more carbon. "There has been no way to uncover these incredible patterns till now."

The study also reveals major carbon loss due to logging, farming, mining and road construction – even in areas still covered by forest. This "forest degradation" made up almost 50 per cent of all carbon lost from the forest. But the study also found substantial carbon accumulated through natural forest regrowth on abandoned land.

Asner is now conducting similar studies elsewhere round the world, which he says can be used to police the United Nations' proposed initiative on Reducing Emissions from Deforestation and Forest Degradation in Developing Countries. This scheme would compensate countries that protect their forests to store carbon. It's important that countries with large rainforests do this, because around 15 per cent of global carbon dioxide emissions come from tropical deforestation.

Markku Kanninen, a forest scientist at the Center for International Forestry Research, a World Bank-backed body based in Bogor, Indonesia, agrees: "This research shows the importance of improving our data on forest carbon. The IPCC global default values are not precise enough for national inventories."

Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1004875107