Methane (CH4) is also known as Natural Gas. As a trace gas in the atmosphere, methane has a more warming effect than carbon dioxide - 21 times more on a molecule by molecule basis, and 58 times more on a kilogram for kilogram basis according to the Intergovernmental Panel on Climate Change.

Another piece of bad news is that concentrations of methane in the atmosphere have increased more than twofold since the beginning of the Industrial Revolution, about eight times faster than carbon dioxide, although new data suggests that this process might be slowing down.

Fortunately for us, methane's atmospheric lifetime is much shorter than that of most other greenhouse gases - only 11 years. It is removed from the atmosphere when combined with hydroxyl radical to form water.

Methane is one of the gases formed in the pressure cooker of Earth's interior, and it is vented through volcanoes and other breaks in the Earth's crust. The amount of methane released annually from natural geologic sources has not been measured, forcing scientists to make the assumption that methane sources and sinks closely balance one another in any natural regime, that is one that is undisturbed by the activities of humans.

What is unclear then is how much methane comes from human sources, and how much from natural sources. These totals are only tentative estimates.

Much atmospheric methane is biological in origin. Bacteria produce methane in the absence of oxygen (the so-called "anaerobic" bacteria). It is these bacteria that decompose the plant and animal refuse in wetlands and marshes (other names for methane include "swamp" or "marsh" gas, although the fumes come mostly the hydrogen sulphide mixed with it). Wetlands are currently thought to produce about a fifth of the world's total emissions of methane each year (around 115 million metric tonnes, based on 1991 figures).

Another important source of methane is intestinal gas - especially from cattle, water buffalo, sheep and other ruminant livestock that humans use in farming. Anaerobic micro-organisms in the intestines of these animals make the whole process of digestion possible - but the methane they produce is inevitably vented in to the atmosphere. This so called "enteric fermentation" along with the production of rice paddies and other "agricultural wetlands" contribute a significant additional amount of methane to the atmosphere each year (around 140 million metric tonnes, based on 1991 figures).

Other anthropogenic methane sources include the extraction of fossil fuels (estimated at around 100 million metric tonnes per year) landfills (25 million metric tonnes per year) and biomass burning (40 million metric tonnes per year).

These estimates have changed significantly over the past few years and remain quite uncertain. Whatever the uncertainties, it does seem probable that much of the observed increase in the concentration of atmospheric methane since the beginning of the industrial revolution has been caused by human activity. It follows that, without technological change, further increases in human population, industry and agriculture will bring further increases in atmospheric methane concentrations.

Related Articles:

- Gases Introduction
- Carbon Dioxide
- Halocarbons
- Methane
- Nitrous Oxide
- Ozone
- Water Vapour