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Deforestation contributes 6-17% of global anthropogenic CO 2 emissions to the atmosphere 1 . Large uncertainties in emission estimates arise from inadequate data on the carbon density of forests 2 and the regional rates of deforestation. Consequently there is an urgent need for improved data sets that characterize the global distribution of aboveground biomass, especially in the tropics. Here we use multi-sensor satellite data to estimate aboveground live woody vegetation carbon density for pan-tropical ecosystems with unprecedented accuracy and spatial resolution. Results indicate that the total amount of carbon held in tropical woody vegetation is 228.7 Pg C, which is 21% higher than the amount reported in the Global Forest Resources Assessment 2010 (ref. 3). At the national level, Brazil and Indonesia contain 35% of the total carbon stored in tropical forests and produce the largest emissions from forest loss. Combining estimates of aboveground carbon stocks with regional deforestation rates 4 we estimate the total net emission of carbon from tropical deforestation and land use to be 1.0 Pg C yr −1 over the period 2000-2010-based on the carbon bookkeeping model. These new data sets of aboveground carbon stocks will enable tropical nations to meet their emissions reporting requirements (that is, United Nations Framework Convention on Climate Change Tier 3) with greater accuracy.When forests are cleared, carbon stored above and below ground in leaves, branches, stems and roots is released to the atmosphere. As a consequence, forest clearing, especially in the tropics, is a major source of CO 2 to the atmosphere. Although the proportion of carbon stored in forests comprises 70-80% of total terrestrial carbon 5 , the spatial and temporal variability in carbon storage is substantial 6 . This variability arises from natural and anthropogenic disturbances, as well as differences in stand age, topography, soils and climate. Globally, soils hold two to three times more carbon than that stored above ground in forest vegetation, but with the exception of cultivation, peatland fires and thawing permafrost, much of the carbon in soils is physically and chemically protected and not easily oxidized 7 . In contrast, carbon stored in aboveground biomass is readily mobilized by disturbance processes such as fire, wind throw, pest outbreaks and land conversion 8 .Efforts to quantify the amount of carbon stored in aboveground biomass over large areas of the tropics have been fraught with uncertainty. For example, estimates of aboveground carbon storage in tropical African forests vary by over ref. 9). In turn, the lack of reliable estimates of forest carbon storage introduces large uncertainties into estimates of terrestrial carbon emissions 10-14 . In Amazonia, recent studies have suggested

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