EDUCATION: GLOBAL METHANE INVENTORY

Assessing the Potential Contribution to Downward Trend of Atmospheric Methane Concentrations

By Harvey Augenbraun and T. Smith

1. Introduction

Methane (CH₄) is a potent greenhouse gas, which warms the Earth by absorbing infrared (heat) energy and radiating this energy back to the surface. This process, called "The Greenhouse Effect", contributes to Earth's ability to sustain life. Although methane exists naturally in the atmosphere, it has more than doubled in the past 200 years as a result of human activities such as processing natural gas and coal. While atmospheric concentrations of methane continue to increase, the growth rate has declined since the middle 1980s. The causes behind the observed decline are unknown, and could stem from the sources of emissions, the sinks (processes that remove methane from the environment), or a combination of both.

1.1 Background

Sources of methane are categorized as natural or human-related (anthropogenic). Natural sources include the wetlands, enteric (intestinal) fermentation in termites, CH₄ hydrates, oceans and naturally occurring forest fires. Anthropogenic sources account for 70% of annual emissions and comprise the production of fossil fuels, enteric fermentation in livestock, rice cultivation, landfills, biomass burning, sewage treatment, and animal wastes. The sinks, include tropospheric degradation of CH₄ by hydroxyl (OH) radicals, along with a small effect from microbial oxidation of methane in soils. Atmospheric concentrations of methane reflect the net effect of sources and sinks. Because methane's atmospheric lifetime is about 10 years, the amount of methane in the atmosphere at any given moment reflects the source and sink processes over the preceding decade.

Currently, the global mean atmospheric concentration of methane is approximately 1.7 parts per million by volume (ppmv). This figure also represents an atmospheric burden of approximately 4700 Tg. All sources generate approximately 500 Teragrams CH₄ annually. Although methane sinks effectively remove approximately 497 Tg from both the atmosphere and soils, studies suggest that the source emissions have either slowed down or leveled off, wielding a greater influence on the decline than the major sink (troposheric OH).

In an effort to assess this possibility, researchers from the Institute on Climate and Planets at the Goddard Institute for Space Studies have investigated historical trends in three major anthropogenic sources: ruminant livestock production (16% of total CH₄), rice cultivation (12% of total CH₄), and landfills (6% of total CH₄).

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