EDUCATION: GLOBAL METHANE INVENTORY

Assessing the Potential Contribution of Ruminant Animals to the Decreasing Growth Rate of Atmospheric Concentrations of Methane

By David Sarma

Abstract

The growth rate of atmospheric concentrations of methane has been observed to have decreased beginning in the early 1990s. The goal of this research project under the Global Methane Inventory project is to assess the potential contribution of animals to this trend. The set of animals on which data was collected is ruminant animals, which account for almost all methane emissions from all animals. The yearly populations by country of each set of animals for the time period 1980 to 1997 were taken from the Food and Agriculture Organization (FAO) of the United Nations. The populations were used in methodologies provided by the Intergovernmental Panel on Climate Change (IPCC) in order to estimate the amount of methane produced by each set of animals each year. Methane emissions from animals were found to increase from 83 Tg/yr in 1980 to 91 Tg/yr in 1997. However, the record did not exhibit a clear trend in the growth rates. The interannual growth rate is variable and does not appear to be increasing or decreasing. Because the growth rate does not appear to be increasing, we consider animals as a candidate for contributing to the declining growth rate of atmospheric concentrations of methane.

Introduction

The Global Methane Inventory is a project which seeks to compile a complete dataset of terrestrial methane sources and methane emissions. Methane (CH₄) is considered to be one of the most important trace gases in the atmosphere because of its role as "both a greenhouse gas and a player in the chemistry of ozone destruction" (Crutzen, 1991). Methane's role as a greenhouse gas means that it absorbs infrared thermal energy which is reflected upward from the Earth's surface, thus warming the Earth. Though methane is found in the atmosphere in much smaller concentrations than other greenhouse gases such as carbon dioxide, its efficiency as an absorber of infrared radiation, absorbing about 25 times more energy than carbon dioxide, molecule for molecule, over a 100 year period, determines its importance (Houghton et al., 1996). Methane has contributed to the phenomenon of global warming; it has been estimated that "about 12 percent of the added greenhouse warming forcing during the 1980s" (Crutzen, 1991) can be attributed to CH₄. The fact that atmospheric concentrations have more than doubled, from 0.8 parts per million by volume (ppmv) to 1.7 ppmv, in the past 300 years strongly suggests that the increase is related to human activities (Crutzen, 1991).

Atmospheric concentrations reflect the balance between the sources of methane (inputs) and the sinks (removal mechanisms). The major sources of methane, which results from the anaerobic decay of organic matter, are the following: wetlands, rice cultivation, and the enteric fermentation of food in the stomachs of certain animals (Crutzen et al., 1986). These three sources, the latter two of which are anthropogenic, account for half of the world's annual methane emissions (Augenbraun et al., 1997). Other methane sources include landfills, the burning of fossil fuels, biomass burning, sewage treatment, and animal waste. A substantial growth or decline in any of these sources would lead to an observable change in atmospheric concentrations of methane within about ten years, since ten years is the average time methane molecules spend in the atmosphere before reacting with hydroxyl ions, the major methane sink.

A decrease in the growth rate of atmospheric methane has been observed beginning in the early 1990s. Though it has been suggested that both decreasing source emissions and/or increasing sinks are the cause of the decline in the growth rate, the relative contributions of various possible causes are presently unclear (Houghton et al., 1996). The collection of yearly data on all sources of methane will help to ascertain from which source or combination of sources the changing rate of growth stems, if it does in fact stem from the sources.

This paper reports on methane emissions from animals. Previous work on emissions from animals by Augenbraun et al., 1998, reports estimates of emissions at five year intervals from 1965 to 1995, giving a general idea of trends in the amount of methane produced by animals in recent times. The goal of this study was to assess whether or not animals are a potential contributor to the declining growth rate of atmospheric methane. This study was designed to test the hypothesis that methane emissions from animals are directly proportional to animal populations. Thus, a decrease in the growth rate of animal populations could contribute to the decrease in the growth rate of methane concentrations.

In addition to population, management practices also influence the amount of methane produced by animals. Amount and type of food, size of the food, and different types of medication all influence methane production (Johnson et al., 1993). They are reflected in this study by the distinctions made in the methodology between different regions of the world. It is possible that different management practices have reduced methane emissions despite an increase in animal population; methane's status as a waste product of animal digestion leads to a general trend towards decreasing per capita emissions in developed countries, though increased production in developed countries generally increases methane production. Understanding the causes for variations in the growth rate of methane contributes to the body of knowledge concerning greenhouse gases and could contribute to the formulation of policies to decrease greenhouse gas emissions into the atmosphere.

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