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Methane (Rev #19, changes)

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Idea

This page is about methane, a fuel and a greenhouse gas. See also Methane clathrate, Arctic methane deposits and Permafrost.

Basic chemistry and use as a fuel

From Wikipedia:

Methane is a chemical compound with the chemical formula CH 4CH_4. It is the simplest alkane, and the principal component of natural gas. Methane’s bond angles are 109.5 degrees. Burning methane in the presence of oxygen produces carbon dioxide and water.

The relative abundance of methane makes it an attractive fuel. However, because it is a gas at normal temperature and pressure, methane is difficult to transport from its source. In its natural gas form, it is generally transported in bulk by pipeline or LNG carriers; few countries transport it by truck.

Lifetime of methane in the atmosphere

Chemists are sometimes surprised to hear that in the atmosphere the average lifetime of a molecule of methane is about a decade. After all, methane is the most reduced form of carbon. Outside the atmosphere it can be stable and accumulate over millions of years e.g. in methane hydrate ocean sediments. In the atmosphere

Methane is primarily oxidized by reaction with hydroxyl radical, OH•, in a stew of chemical reactions in the troposphere, similar to the chemistry in a candle flame. Additional sinks, accounting for about 10% of the methane, include destruction by ultraviolet (UV) light in the stratosphere and oxidation by bacteria in soils (see table 1).

OH• is a water molecule missing a hydrogen atom and hungry to get that hydrogen back. (…) The lifetime of methane in the atmosphere depends on the concentration of OH•. At night, or trapped in a dark bubble in an ice core, methane can sit in oxygen forever without reacting much, (…) The concentration of OH determines the rate at which the atmosphere can cleanse itself of reduced gases such as methane, CO, and hydrocarbons

  • David Archer, The Global Carbon Cycle, Chapter 5

Methane as a greenhouse gas

From Wikipedia:

Methane is a relatively potent greenhouse gas. Compared with carbon dioxide, it has a high global warming potential of 72 (calculated over a period of 20 years) or 25 (for a time period of 100 years). It has a net lifetime of about 10 years and is primarily removed by reaction with hydroxyl radicals in the atmosphere, producing carbon dioxide and water.

(Note that carbon dioxide is also a greenhouse gas. It has a lower global warming potential than methane, but it effectively stays in the atmosphere much longer.)

In addition, there is a large volume of methane deposited in the Arctic whose stability is poorly understood.

The effect of rising CO2 on plant life is a controversial and important topic. A new meta-analysis in Nature claims that more CO2 boosts soil emissions of nitrous oxide in all ecosystems, and more methane in in rice paddies and wetlands:

The claimed explanation is that higher CO2 concentrations reduce plant water use, making soils wetter, in turn reducing the availability of oxygen in soil, favoring microorganisms that make methane and N2O. Also: increasing CO2 makes plants grow faster, which supplies these microorganisms with extra energy.

Methane cycle

This image is from Wikipedia showing the global Methane cycle:

methane cycle
  • A. Permafrost, glaciers, and ice cores – A source that slowly releases methane trapped in frozen environments as global temperatures rise.
  • B. Wetlands – Warm temperatures and moist environments are ideal for methane production. Most of the methane makes it past methane-consuming microorganisms.
  • C. Forest fire – Mass burning of organic matter releases huge amounts of methane into the atmosphere.[citation needed]
  • D. Rice paddies – The warmer and moister the rice field, the more methane is produced.
  • E. Animals – Microorganisms breaking down difficult to digest material in the guts of ruminant livestock and termites produce methane that is then released during defecation.
  • F. Plants – While methane can be consumed in soil before being released into the atmosphere, plants allow for direct travel of methane up through the roots and leaves and into the atmosphere. Plants may also be direct producers of methane.
  • G. Landfills – Decaying organic matter and anaerobic conditions cause landfills to be a significant source of methane.
  • H. Waste water treatment facilities – Anaerobic treatment of organic compounds in the water results in the production of methane.
  • I. Hydroxyl radical – OH in the atmosphere is the largest sink for atmospheric methane as well as one of the most significant sources of water vapor in the upper atmosphere.
  • J. Chlorine radical – Free chlorine in the atmosphere also reacts with methane.

For more, see:

Atmospheric methane concentration

It seems that the concentration of methane in the air depends a lot on the latitude:

(Click for details.)

Ed Dlugokencky’s data for monthly averages of methane concentrations from 1983 to 2011 can be found here:

In particular, his monthly in situ measurements made at Mauna Loa are here while his monthly measurements using flasks of air are here. Dr. Dlugokencky also has an ftp server where the Mauna Loa files naming conventions seem to be explained, here. The molar fraction for CH4 is expressed in terms of nanomoles of methane per mole of dry air.

References

As always, it’s good to start with Wikipedia:

Additional articles:

Abstract. We have reviewed the available scientific literature on how natural sources and the atmospheric fate of methane may be affected by future climate change.We discuss how processes governing methane wetland emissions, permafrost thawing, and destabilization of marine hydrates may affect the climate system. It is likely that methane wetland emissions will increase over the next century. Uncertainties arise from the temperature dependence of emissions and changes in the geographical distribution of wetland areas. Another major concern is the possible degradation or thaw of terrestrial permafrost due to climate change. The amount of carbon stored in permafrost, the rate at which it will thaw, and the ratio of methane to carbon dioxide emissions upon composition form the main uncertainties.

Large amounts of methane are also stored in marine hydrates, and they could be responsible for large emissions in the future. The time scales for destabilization of marine hydrates are not well understood and are likely to be very long for hydrates found in deep sediments but much shorter for hydrates below shallow waters, such as in the Arctic Ocean. Uncertainties are dominated by the sizes and locations of the methane hydrate inventories, the time scales associated with heat penetration in the ocean and sediments, and the fate of methane released in the seawater. Overall, uncertainties are large, and it is difficult to be conclusive about the time scales and magnitudes of methane feedbacks, but significant increases in methane emissions are likely, and catastrophic emissions cannot be ruled out. We also identify gaps in our scientific knowledge and make recommendations for future research and development in the context of Earth system modeling.

category: carbon