Global Warming and Tropical Land-use Change: Greenhouse Gas Emissions from Biomass Burning, Decomposition and Soils in Forest Conversion, Shifting Cultivation and Secondary Vegetation

Tropical forest conversion, shifting cultivation and clearing of secondary vegetation make significant contributions to global emissions of greenhouse gases today, and have the potential for large additional emissions in future decades. Globally, an estimated 3.1× 109 t of biomass carbon of these types is exposed to burning annually, of which 1.1×109 t is emitted to the atmosphere through combustion and 49×106 t is converted to charcoal (including 26–31×106 t C of black carbon). The amount of biomass exposed to burning includes aboveground remains that failed to burn or decompose from clearing in previous years, and therefore exceeds the 1.9× 109 t of aboveground biomass carbon cleared on average each year. Aboveand belowground carbon emitted annually through decomposition processes totals 2.1 × 109 t C. A total gross emission (including decomposition of unburned aboveground biomass and of belowground biomass) of 3.41× 109 t C year−1 results from clearing primary (nonfallow) and secondary (fallow) vegetation in the tropics. Adjustment for trace gas emissions using IPCC Second Assessment Report 100-year integration global warming potentials makes this equivalent to 3.39 × 109 t of CO2-equivalent carbon under a low trace gas scenario and 3.83×109 t under a high trace gas scenario. Of these totals, 1.06×109 t (31%) is the result of biomass burning under the low trace gas scenario and 1.50 × 109 t (39%) under the high trace gas scenario. The net emissions from all clearing of natural vegetation and of secondary forests (including both biomass and soil fluxes) is 2.0 × 109 t C, equivalent to 2.0–2.4 × 109 t of CO2-equivalent carbon. Adding emissions of 0.4 × 109 t C from land-use category changes other than deforestation brings the total for land-use change (not considering uptake of intact forest, recurrent burning of savannas or fires in intact forests) to 2.4 × 109 t C, equivalent to 2.4–2.9 × 109 t of CO2-equivalent carbon. The total net emission of carbon from the tropical land uses considered here (2.4× 109 t C year−1) calculated for the 1981–1990 period is 50% higher than the 1.6 × 109 t C year−1 value used by the Intergovernmental Panel on Climate Change. The inferred (= ‘missing’) sink in the global carbon budget is larger than previously thought. However, about half of the additional source suggested here may be offset by a possible sink in uptake by Amazonian forests. Both alterations indicate that continued deforestation would produce greater impact on global carbon emissions. The total net emission of carbon calculated here indicates a major global warming impact from tropical land uses, equivalent to approximately 29% of the total anthropogenic emission from fossil fuels and land-use change. Climatic Change 46: 115–158, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 116 PHILIP M. FEARNSIDE