Refining estimates of carbon released from tropical land-use change

We report the results of incorporating two changes in the data base that we used for our previously published summary estimates of the quantity of carbon released to the atmosphere from tropical land-use change (R.P. Detwiler and C.A.S. Hall. 1988. Science (Washington, D.C.), 239: 42–47). First, and most important, we used new statistical approaches to estimate biomass. Second, we incorporated recent minor modifications in the estimates of land-use change provided by the Food and Agriculture Organization of the United Nations. Our new, best estimate of carbon release for 1980 is 0.58 ± 0.06 Gt/year. The range of 0.06 Gt/year is due only to the statistieal uncertainty associated with the biomass data base and not to the uncertainty associated with other factors. (Our previous high estimate, based on the use of destructively sampled biomass, remains at 1.6 Gt/year). This new estimate for the tropics as a whole is 26% higher than the analogous number by Detwiler and Hall (R.P. Detwiler and C.A.S. Hall. 1988. Science (Washington, D.C.), 239: 42–47). About 7% of this increase in our estimates of carbon release from the tropics is a result of the new estimates of land-use change for open forests; the rest is due to changing estimates of biomass. In addition, we explored further uncertainties in our data base. When we reduced our estimate of the proportion of cut biomass that goes to long-term storage from 16 to 3%, carbon release increased by about 30%. We also examined the hypothesis that shifting cultivation could be ignored in our analyses. When shifting cultivation was not considered at all in our analyses, an underestimate of 20% resulted for the carbon release in 1980 for the entire tropics. The results of using different data sources for particular countries with large areas in shifting cultivation showed larger differences in carbon flux.