7. Sources of Methane: An Overview M.A.K. Khalil and M.J. Shearer Department of Physics, Portland State University, P.O. Box 751, Portland, Oregon 97207 1. Introduction The sources of methane are the most complex and critical element in understanding the concentrations of atmospheric methane and their trends. For those who want to reduce methane in the atmosphere or prevent it from increasing, controlling the sources is perhaps the only practical approach. Accordingly, a significant portion of this book is devoted to estimating the global and regional emission rates. The purpose of this chapter is to introduce the subsequent chapters on individual sources and to lay the foundation for the common elements of determining global emission rates from the many and varied sources of methane. There are three major sources ( > 50 Tg/yr), all biogenic, namely rice agriculture, ruminants (particularly cattle), and the natural wetlands. There are many more minor sources that each emit between 10-50 Tg/yr but collectively are a significant fraction of the global budget. These sources include landfills, coal mines, biomass burning, urban areas, sewage disposal, natural gas leakages, lakes, oceans, termites, and tundra. Finally there are yet smaller sources including biogas pits, asphalt, several industrial sources, and possibly others that have not yet been identified ( - < 5 Tg/yr) (see Judd, this volume; Lacroix, 1993). The very small sources are thought to be a small fraction of the annual emissions even when taken together. There is still enough uncertainty in the estimates of emission rates from individual sources that some may go from minor to major or vice versa, but it is very unlikely that there are any unknown major sources. Research has concentrated more on the major sources so less is known about the minor sources, particularly the smaller of the minor sources. 2. Estimating Global Emissions: Issues and Procedures The process of evaluating global emissions usually consists of two fundamental pieces of information. The first is data on fluxes or emission factors, often measured directly under field or laboratory conditions. The second is an extrapolating factor or extrapolant that, when associated (or multiplied) with the measured fluxes, results in the global emission rate. Mohammad Aslam Khan Khalil (Ed.) Atmospheric Methane © Springer-Verlag Berlin Heidelberg 2000