CURRENT LAND-COVER IN THE TROPICS AND ITS POTENTIAL FOR SEQUESTERING CARBON

Emissions of carbon dioxide and other greenhouse gases from human acti vity are increasing the concentrations of these gases in the atmospher e. The Earth is expected to warm as a result, with consequences that a re potentially highly disruptive to human societies. Reductions in the use of fossil fuels and in rates of deforestation worldwide will redu ce emissions of CO2, but atmospheric concentrations will continue to i ncrease unless emissions are reduced by more than 60% (about 4.5 billi on tons of carbon annually). Reforestation seems to offer one of the f ew means for reducing the atmospheric concentration of CO2 over period s as short as human generations. We report here an approach for evalua ting the potential for reforestation to help stabilize or even reduce the concentration of CO2 in the atmosphere. Reforestation is defined b roadly to include tree plantations, natural regrowth of secondary fore sts, and the practice of agroforestry. Our premise is that human use o f the land has generally reduced woody biomass and that such lands hav e a potential for reaccumulating carbon if appropriately managed. We u sed published ground studies together with global vegetation index dat a from the NOAA 7 satellite to estimate current land cover in tropical regions. Then, superimposing this map of current land cover over maps depicting the distribution of vegetation cover prior to human disturb ance, we obtained an estimate of about 3200 X 10(6) ha in the tropics (almost 60% of the total land area considered) where woody biomass had been decreased, and where carbon might again be sequestered. We calcu lated the amount of carbon that could be withdrawn from the atmosphere and stored in woody biomass if several management options were implem ented. Biomass accumulations were determined from forestry statistics. Application of the data on biomass to the areas suitable for accumula tion of carbon yielded an estimate of potential accumulation of 160-17 0 Pg carbon, an amount equivalent to the accumulation of carbon in the atmosphere since the start of the industrial revolution, or to about 25 years of fossil fuel emissions at current rates. Estimates of both area and potential accumulation of carbon were crude, probably not bet ter than +/-50%. They are useful for suggesting the role that tropical lands might play in stabilizing atmospheric concentrations of CO2, bu t they should not be used to suggest specific management options in in dividual countries. As maps with higher spatial resolution become avai lable, however, the method should provide more precise estimates overa ll and in specific locations.