982 Communications Ecological Applications, 14(4), 2004, pp. 982–986  2004 by the Ecological Society of America TROPICAL DEFORESTATION AND GREENHOUSE-GAS EMISSIONS PHILIP M. FEARNSIDE 1,3 AND WILLIAM F. LAURANCE 2 1 Department of Ecology, National Institute for Research in the Amazon (INPA), C.P. 478, Manaus, Amazonas 69011–970, Brazil 2 Smithsonian Tropical Research Institute, Apartado 2072, Balboa, Republic of Panama ´ Abstract. A recent (2002) analysis concluded that rates of tropical deforestation and atmospheric carbon emissions during the 1990–1997 interval were lower than previously suggested. We challenged this assertion with respect to tropical carbon emissions, but our conclusions were disputed by the authors of the original study. Here we provide further evidence to support our conclusion that the effect of tropical deforestation on greenhouse- gas emissions and global warming is substantial. At least for Brazilian Amazonia, the net impact of tropical deforestation on global warming may be more than double that estimated in the recent study. Key words: Amazon; carbon emission; deforestation; global warming; tropical forest. INTRODUCTION The rapid destruction and degradation of tropical for- ests is considered a major source of greenhouse gases such as carbon dioxide, methane, and nitrous oxide, and could play an important role in exacerbating global warming (Fearnside 2000a, Houghton et al. 2000). However, the magnitude of tropical emissions is the subject of considerable uncertainty and debate, with estimates of annual carbon emissions varying from 0.8 to 2.4 gigatons (1 Gt = 1  10 9 metric tonnes = 1  10 15 g; Houghton et al. 2000, Schimel et al. 2001, Achard et al. 2002). Hence, tropical forest conversion could account for as much as one-third, or as little as one-tenth, of all anthropogenic emissions (roughly 7– 8 Gt/yr at present). Correctly quantifying such emis- sions is essential for understanding the earth’s carbon balance, for assessing the impacts of tropical defor- estation on the global climate, and for developing vi- able mechanisms to conserve forests via carbon-offset funds and related international agreements (Fearnside 1997, 2000a, b). In a recent paper, Achard et al. (2002) assessed de- forestation of humid tropical forests worldwide for the 1990–1997 period, using chronosequences of remote- sensing data and a stratified sampling strategy that fo- cused on ‘‘hotspots’’ of rapid forest conversion that comprised a relatively small fraction (6.5%) of total forest cover. A key conclusion of their study was that both annual deforestation rates and atmospheric carbon emissions were substantially lower than was previously estimated for this same interval by earlier investigators. Achard et al. (2002) estimated emissions of 0.64  0.21 Gt (mean  2 SE) for humid tropical forests and 0.96 Gt for all tropical forests. They emphasized that Manuscript received 21 July 2003; revised 14 November 2003; accepted 29 November 2003; final version received 24 De- cember 2003. Corresponding Editor: A. R. Townsend. 3 E-mail: pmfearn@inpa.gov.br this is much lower than the value of 1.6 Gt C for annual emissions from land use, land-use change and forestry in the tropics used by the Intergovernmental Panel on Climate Change (IPCC) (Bolin et al. 2000). We challenged key tenets of the Achard et al. (2002) study, citing seven specific ways by which their meth- odology and assumptions should yield underestimates of greenhouse gas emissions (Fearnside and Laurance 2003). In their response, these same authors argued that their methods were sound, and they attempted to dis- count or dispute most of our criticisms (Eva et al. 2003). Because we disagree with key elements of their response, we provide here a more detailed explanation for our continued belief that Achard et al. (2002) un- derestimate the impact of tropical deforestation on global warming. FOREST BIOMASS AND GREENHOUSE GAS EMISSIONS At the outset, Eva et al. (2003) suggested that we produced ‘‘no evidence at all contesting our [Achard et al.’s] global biomass estimates or global deforesta- tion rates.’’ The key word here is ‘‘global.’’ Because of our long-term experience in Amazonia—which con- tains about half of the world’s remaining tropical for- ests and nearly 60% of all humid tropical forests—we focused on errors and questionable assumptions relat- ing to this critical region. The fact that so many points of concern were raised about Amazonia poses broader questions regarding the general methods and assump- tions of the Achard et al. study. To be fair, Achard et al. (2002) make some valuable contributions to improving remote-sensing estimates of deforestation rates in humid forests, and Eva et al. (2003) emphasize that we did not dispute their global deforestation estimate. We purposely restricted our comments to their estimates of greenhouse-gas emis- sions, for which we believe we have both better data and a better interpretation than that provided by Achard et al.