The Role of Methane

Introduction

Graph showing the increases in the concentration of methane in the atmosphere - as measured in ppbv (parts per billion by volume) - roughly parallel world population growth. As indicated above, over the last two centuries (i.e., since the industrial revolution), methane's concentration in the atmosphere has more than doubled. Over the past decade (1990-2000), although methane concentrations have continued to increase, the overall rate of methane growth has slowed. In the late 1970’s, the growth rate was approximately 20 ppbv/year. In the 1980’s, growth slowed to 9-13 ppbv/year. The 1990’s have seen growth of between 2 and 8 ppbv/year.
The “natural gas” that we burn to heat our homes and fuel our stoves is called methane. Methane (CH4) is a greenhouse gas. It affects the radiation budget of Earth directly (by intercepting heat radiation) but also indirectly, by reacting with other trace gases in the atmosphere, and by delivering water molecules upon oxidation in the stratosphere. The amount of methane in the atmosphere has more than doubled over the last 200 years, according to ice core data. Adding methane to the atmosphere is about 20 times more effective in increasing global temperature than adding equal amounts of CO2. The rise within the 20th century is very steep and there can be little doubt that this rise is tied to human activities. The rise has been on the order of 1% per year.

Sources of Methane
What is the source for all this methane? Methane has both natural and anthropogenic sources. It is produced wherever organic matter decays under conditions where oxygen is lacking. This is the case, for example, in wet rice fields, in waste deposits, and in the stomachs of cattle and of termites. Also, methane is released while obtaining it from underground reservoirs for industrial use, while pumping oil, and while mining coal. A substantial amount of methane apparently also originates from the burning of forests and other vegetation ("biomass" burning). Methane is relatively short-lived in the atmosphere; a molecule of methane is oxidized to water and carbon dioxide within a decade or so, mainly by reaction with another trace gas, the hydroxyl radical OH-. Thus, unlike the case of carbon dioxide (which stays in the atmosphere longer than methane), a concerted effort to reduce methane emissions would have almost immediate results in terms of reduction of greenhouse effect. As in the case of carbon dioxide, the origin of methane can be partly traced by recording the amount of radiocarbon in the methane (fossil fuels have none) and by measuring the ratio of carbon-13 to carbon-12 (bacteria produce methane high in 12C).

There is a strong suggestion, from studying the history of atmospheric composition in ice cores over the last several hundred thousand years, that the amount of methane in air is tied to the global temperature. When it was warm methane content was roughly twice higher than during glacial periods. From this relationship (on long time scales) it may be hypothesized that methane will rise when the climate warms. Thus, methane would seem to be part of a positive feedback for climate change.


The principal anthropogenic sources of methane emissions in the United States. (From: EPA)
A similar trend may be expected for the future. Upon warming increased flux of methane can be expected from wetlands (especially susceptible are northern wetlands). Also, the lowering of permafrost levels in the Arctic should add to the methane burden, by making organic carbon available to methane-producing bacteria. It is possible that some of the recent increase is due to such feedback. Even without such feedback, the growing number of people on the planet will result in additional demands for rice fields, cattle, forest clearing and fossil fuels, all of which contribute to the release of methane into the atmosphere.