Future Trends of Land-Use Emissions of Major Greenhouse Gases

Land-use emissions of greenhouse gases make up over one-third of current total anthropogenic emissions of greenhouse gases and about three-quarters of the total anthropogenic emissions of CH4 and N2O. Considering their contribution to global emissions, it is important to understand their future trends in order to anticipate and mitigate climate change. This paper reviews published scenarios of major categories of these emissions with the aim to provide background information for the development of new scenarios. These categories include CO2 from deforestation, CH4 from rice cultivation, CH4 from enteric fermentation of cattle, and N2O from fertilizer application. Base year estimates of all these categories varied greatly from reference to reference, and hence emissions of all scenarios were normalized relative to their 1990 value before being compared to one another. The range of published scenarios of CO2 emissions from deforestation is widest around the middle of the 21st century and then all scenarios converge to low values towards 2100. By contrast, the different scenarios of CH4 and N2O diverge with time, showing their widest range in 2100. Global emissions of CH4 from rice cultivation vary by a factor of three in 2100 and N2O from fertilized soils by a factor of 2.3. Emissions of CH4 from enteric fermentation of animals have the smallest range (factor of 2.0). The typical long-range trends of land-use emission scenarios vary greatly from region to region - they stabilize in industrialized regions after a few decades, but tend to stabilize later in developing regions or continue to grow throughout the 21st century. To improve the realism of the estimates of future trends of land-use emissions, it is especially important to improve the estimation of the future extent of agricultural land and the rate of deforestation, while taking into account significant driving forces such as the demand for agricultural commodities and crop yields.