THE SIGNIFICANCE OF AGRICULTURAL SOURCES OF GREENHOUSE GASES

The impact of development of land for agriculture and agricultural production practices on emissions of greenhouse gases is reviewed and evaluated within the context of anthropogenic radiative forcing of climate. Combined, these activities are estimated to contribute about 25%, 65%, and 90% of total anthropogenic emissions of CO2, CH4, and N2O, respectively. Agriculture is also a significant contributor to global emissions of NH3, CO, and NO. Over the last 150 y, cumulative emissions of CO2 associated with land clearing for agriculture are comparable to those from combustion of fossil fuel, but the latter is the major source of CO2 at present and is projected to become more dominant in the future. Ruminant animals, rice paddies, and biomass burning are principal agricultural sources of CH4, and oxidation of CH4 by aerobic soils has been reduced by perturbations to natural N cycles. Agricultural sources of N2O have probably been substantially underestimated due to incomplete analysis of increased N flows in the environment, especially via NH3 volatilization from animal manures, leaching of NO3-, and increased use of biological N fixation. The contribution of agriculture to radiative forcing of climate is analyzed using data from the Intergovernmental Panel on Climate Change (IPCC)(base case) and cases where the global warming potential of CH4, and agricultural emissions of N2O are doubled. With these scenarios, agriculture, including land clearing, is estimated to contribute between 28-33% of the radiative forcing created over the next 100yr by 1990 anthropogenic emissions of CO2, CH4, and N2O. Analyses of the sources of agriculturally generated radiative climate forcing show that 80% is associated with tropical agriculture and that two-thirds comes from non-soil sources of greenhouse gases. The importance of agriculture to radiative forcing created by different countries varies widely and is illustrated by comparisons between the USA, India, and Brazil. Some caveats to these analyses include inadequate evaluations of the net greenhouse effects of agroecosystems, uncertainties in global fluxes of greenhouse gases, and incomplete understanding of tropospheric chemical processes. Extension of the analytical approach to projected future emissions of greenhouse gases (IPCC moderate growth scenario) indicates that agriculture will become a less important source of radiative forcing in the future. Technological approaches to mitigation of agricultural sources of greenhouse gases will probably focus on CH4 and N2O because emissions of CO2 are essentially associated with the socio-political issue of tropical deforestation. Available technologies include dietary supplements to reduce CH4 production by ruminant animals and various means of improving fertilizer N management to reduce N2O emissions. Increased storage of C in soil organic matter is not considered to be viable because of slow accretion rates and misconceptions about losses of soil organic matter from agricultural soils.