Atmospheric methane levels are of interest due to its impact on climate change.

Methane levels (green curve) in 420,000 years of ice core data from Vostok, Antarctica research station. Current period is at the left.

[edit] Atmospheric methane

Methane concentrations graph.

Early in the Earth's history—about 3.5 billion years ago—there was 1,000 times as much methane in the atmosphere as there is now. The earliest methane was released into the atmosphere by volcanic activity. During this time, Earth's earliest life appeared. These first, ancient bacteria added to the methane concentration by converting hydrogen and carbon dioxide into methane and water. Oxygen did not become a major part of the atmosphere until photosynthetic organisms evolved later in Earth's history. With no oxygen, methane stayed in the atmosphere longer and at higher concentrations than it does today.

In present times, due to the increase in oxygen, the amount of methane has decreased. The average mole concentration of methane at the Earth's surface in 1998 was 1,745 ppb.^[1] Its concentration is higher in the northern hemisphere as most sources (both natural and human) are larger. The concentrations vary seasonally with a minimum in the late summer mainly due to removal by the hydroxyl radical.

Methane is created near the surface, and it is carried into the stratosphere by rising air in the tropics. Uncontrolled build-up of methane in Earth's atmosphere is naturally checked—although human influence can upset this natural regulation—by methane's reaction with hydroxyl radicals formed from singlet oxygen atoms and with water vapor.

[edit] Methane as a greenhouse gas

Methane in the Earth's atmosphere is an important greenhouse gas with a global warming potential of 25 kg CO₂ over a 100-year period. This means that a methane emission will have 21 times the impact on temperature of a carbon dioxide emission of the same mass over the following 100 years. Methane has a large effect for a brief period (a net lifetime of 8.4 years in the atmosphere), whereas carbon dioxide has a small effect for a long period (over 100 years). Because of this difference in effect and time period, the global warming potential of methane over a 20 year time period is 72. The Earth's methane concentration has increased by about 150% since 1750, and it accounts for 20% of the total radiative forcing from all of the long-lived and globally mixed greenhouse gases.^[2] Usually, excess methane from landfills and other natural producers of methane are burned so CO₂ is released into the atmosphere instead of methane because methane is such a more effective greenhouse gas. Recently methane emitted from coal mines has been successfully converted to electricity.

[edit] Emissions accounting of methane

Computer models showing the amount of methane (parts per million by volume) at the surface (top) and in the stratosphere (bottom).

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Global average methane concentrations from measurement (NOAA).

The balance between sources and sinks is not yet fully understood. The IPCC Working Group 1 stated in chapter 2 of the Fourth Assessment Report that there are "large uncertainties in the current bottom-up estimates of components of the global source", and the balance between sources and sinks is not yet well known. The most important sink in the methane cycle is reaction with the hydroxyl radical, which is produced photochemically in the atmosphere. Production of this radical is not fully understood and has a large affect on atmospheric concentrations. This uncertainty is exemplified by observations that have shown between the year 2000 and 2006 increases in atmospheric concentration of methane ceased without reduction in anthropogenic sources, showing that methane accounting does not accurately predict methane observations.

Houweling et al. (1999) give the following values for methane emissions (Tg/a=teragrams per year):^[1]

Slightly over half of the total emission is due to human activity.^[2]

Living plants (e.g. forests) have recently been identified as a potentially important source of methane. A 2006 paper calculated emissions of 62–236 Tg a^-1, and "this newly identified source may have important implications".^[3][4] However the authors stress "our findings are preliminary with regard to the methane emission strength".^[5] These findings have been called into question in a 2007 paper which found "there is no evidence for substantial aerobic methane emission by terrestrial plants, maximally 0.3% of the previously published values".^[6]

Long term atmospheric measurements of methane by NOAA show that the build up of methane has slowed dramatically over the last decade, after nearly tripling since pre-industrial times.^[7] It is thought that this reduction is due to reduced industrial emissions and drought in wetland areas.

Very recent data now suggests that methane concentrations may be rising again.^[8]

[edit] Removal processes

The major removal mechanism of methane from the atmosphere involves radical chemistry; it reacts with the hydroxyl radical (·OH), initially formed from water vapor broken down by oxygen atoms that come from the cleavage of ozone by ultraviolet radiation:

This reaction in the troposphere gives a methane lifetime of 9.6 years. Two more minor sinks are soil sinks (160 year lifetime) and stratospheric loss by reaction with ·OH, ·Cl and ·O¹D in the stratosphere (120 year lifetime), giving a net lifetime of 8.4 years.^[1] Oxidation of methane is the main source of water vapor in the upper stratosphere (beginning at pressure levels around 10 kPa).

The methyl radical formed in the above reaction will, during normal daytime conditions in the troposphere, usually react with another hydroxyl radical to form formaldehyde. Note that this is not strictly oxidative pyrolysis as described previously. Formaldehyde can react again with a hydroxyl radical to form carbon dioxide and more water vapor. Note that sidechains in these reactions may interact with nitrogen compounds that will likely produce ozone, thus supplanting radicals required in the initial reaction.^[9]

[edit] Sudden release from methane clathrates

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At high pressures, such as are found on the bottom of the ocean, methane forms a solid clathrate with water, known as methane hydrate. An unknown, but possibly very large quantity of methane is trapped in this form in ocean sediments. The sudden release of large volumes of methane from such sediments into the atmosphere has been suggested as a possible cause for rapid global warming events in the Earth's distant past, such as the Paleocene–Eocene Thermal Maximum of 55 million years ago, and the Great Dying.

Theories suggest that should global warming cause them to heat up sufficiently, all of this methane could again be suddenly released into the atmosphere. Since methane is twenty-five times stronger (for a given weight, averaged over 100 years) than CO₂ as a greenhouse gas; this would immensely magnify the greenhouse effect, heating Earth to unprecedented levels (see Clathrate gun hypothesis).

[edit] Release of methane from bogs

Although less dramatic than release from clathrates, but already happening, is an increase in the release of methane from bogs as permafrost melts. Although records of permafrost are limited, recent years (1999 to 2007) have seen record thawing of permafrost in Alaska and Siberia.

Recent measurements in Siberia show that the methane released is five times greater than previously estimated.^[10] Melting yedoma is a significant source of atmospheric methane (about 4 Tg of CH4 per year).

[edit] See also

• Climate change
• Global warming

[edit] References

[edit] External links

• Methane in tundra and oceans to be released in atmosphere