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Whatever agreement emerges from the climate meeting in Copenhagen, many expect that it will include a mechanism allowing rich nations to offset their emissions by paying poorer countries to protect their forests — and the carbon they contain. But just how much carbon is at stake? Researchers at the meeting have given their best answer yet: the first satellite-based estimates of the biomass contained in the world's tropical forests.

Current biomass estimates for the tropics are based on data gathered by the Food and Agriculture Organization of the United Nations (FAO), and their quality varies greatly from country to country. As a result, baseline figures for biomass are some of the biggest uncertainties in calculating emissions from deforestation and forest degradation, recently estimated to be around 15% of global carbon emissions (G. R. van der Werf et al. Nature Geosci. 2, 737–738; 2009).

The latest assessments, presented at Copenhagen, harness data from multiple satellites as well as thousands of ground plots, and should help governments and other scientists to estimate how much carbon is locked within trees, vegetation and soils on a given patch of land — rather than relying on rough averages that are calculated across a forest.

Sassan Saatchi, an environmental scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, worked on one study with researchers at the carbon consulting firm Winrock International in Arlington, Virginia. He says that their preliminary calculations (see map) accord well with previous estimates. South America comes in with about 145 gigatonnes of carbon in vegetation and soils, about 26% higher than what has been reported by the Intergovernmental Panel on Climate Change (IPCC). The figures for Africa (51 gigatonnes) and south Asia (46 gigatonnes) are about the same as the IPCC figures.

A question of scale

But Saatchi says that the study provides additional information about biomass levels at regional and national levels. "You cannot really nail down this problem unless you have the distribution," he says. "You need to know how biomass is distributed and how it's changing over time, almost everywhere, with some resolution and accuracy."

Funded in part by the World Bank, the work provides a snapshot at 1-kilometre resolution of tropical forests as they were in 2000, when most of the satellite data were collected, as well as more recent deforestation trends.

Also at Copenhagen, researchers at the Woods Hole Research Center in Massachusetts presented another pan-tropical biomass assessment, which had a resolution of 500 metres. Like the Winrock study, it includes spectral data from NASA satellites as well as laser measurements of forest canopy height from an instrument on NASA's Ice, Cloud, and land Elevation Satellite (ICESat) that was designed to study polar ice caps. The two teams have yet to compare results.

Richard Houghton, a biomass expert at Woods Hole, says that it is good news that multiple teams are tackling the big-picture question of tropical forest biomass. "We need a couple of independent estimates just to see how well they match," he says. "Anybody can make a map. If they differ, at least it identifies the areas that need further analysis."

The next step, says Alexander Lotsch, a geographer at the World Bank in Washington DC, is to produce better estimates for carbon emissions from deforestation. He adds that Saatchi's research is still a "work in progress".

Satellites can reliably track deforestation and, increasingly, small-scale logging. In Copenhagen, Greg Asner of the Carnegie Institution of Science in Stanford, California, and unveiled an online tool that allows tropical countries, beginning in South America, to map deforestation using an automated system to analyse satellite imagery. Asner has also developed a system for assessing biomass at finer resolution, which will be necessary if forests are going to be linked to international carbon markets.

The new pan-tropical biomass maps from Saatchi and Woods Hole won't accomplish that goal, but they can provide scientists and policy-makers with a better understanding of carbon trends. For example, using a similar technique to Saatchi, Asner has found that deforestation in Brazil is moving into higher biomass areas in the interior of the forest. That suggests that emissions will probably rise over time on a per-hectare basis, offsetting some of the reductions in deforestation that Brazil aims to achieve in the coming decade (see Nature doi:10.1038/news.2009.752; 2009).