the global methane cycle
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The Global Methane Cycle. Methane. CH 4 Colorless gas Component of the atmosphere Main component of natural gas CH 4 + 2O 2 → CO 2 + 2H 2 0 . Why is methane important?. Clean fuel source It is a powerful greenhouse gas It removes Hydroxyls from the troposphere

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  • CH4
  • Colorless gas
  • Component of the atmosphere
  • Main component of natural gas
  • CH4 + 2O2→CO2 + 2H20
why is methane important
Why is methane important?
  • Clean fuel source
  • It is a powerful greenhouse gas
  • It removes Hydroxyls from the troposphere
  • Affects concentrations of water vapor and ozone in the stratosphere
  • Used in the industrial production of hydrogen, methanol, acetic acid, and acetic anhydride
methane as a fuel source
Methane as a fuel source
  • Used as a fuel source for electricity generation, heating, and cooking
  • Methanol is used in vehicles
  • Heat of combustion is about 802 kJ/mol
  • Highest heat to weight ratio
  • Produces the least CO2 per unit of heat
how much methane is there
How much methane is there?
  • ~350 ppb 18,000 years ago
  • Today atmospheric levels are ~1800 ppb and rising
  • The most abundant greenhouse gas after CO2 and H2O vapor
  • Most abundant trace gas in the atmosphere
where is methane from
Where is methane from?
  • It is produced by organisms and geological processes
  • Livestock produces 37% of all human-induced methane
  • Most is released from wetlands in the northern hemisphere and tropics
methane is cycled
Methane is cycled
  • Certain organisms and geological processes produce methane
  • Other organisms and processes use methane and release new products
  • Several organisms and processes are involved
  • The methane cycle is part of the larger carbon cycle
sources natural

Sources: Natural




Hydrates & Clathrates

  • Produces 225 Tg per year.
  • 50% of wetlands are peat rich.
  • Produced by methanogenic bacteria decomposing organic materials in anaerobic environments.
  • CH4 produced in sediments are diffused to the surface via the water column, gas bubbles, or plants.
  • C6H12O6 -> 3CO2 + 3CH4
  • Massive amounts of CH4 trapped in permafrost could be released if temperatures rise.
  • Produces 20 Tg per year
  • Concentrated mostly in grasslands or forests
  • Produced in the breakdown of cellulose by CH4 oxidizing bacteria
  • Produces 15 Tg per year
  • Sources are poorly known.
  • Coastal areas have higher but more variable concentrations.
  • Produced by seepage areas in seabed with organic rich nutrients.
  • Produces 10 Tg per year
  • CH4 Hydrates are ridgid water cages surrounding CH4 molecules.
  • Mostly on the continental shelf of all latitudes.
  • Hydrate stability requires high pressures and cold temperatures. Most hydrates occur at depths and regions insulated from climate change.
  • Climate warming would lead to destabilization of hydrates and release CH4 molecules.
sources anthropogenic
Sources: Anthropogenic
  • Rice Cultivation
  • Fossil Fuels
  • Biomass Burning
  • Landfills
rice cultivation
Rice Cultivation
  • Produces 100 Tg per year
  • Production has increased over 40% since the 1980’s.
  • Produced by anaerobic consumption of organic material by methanogenic bacteria
domestic animals
Domestic Animals
  • Produces 115 Tg per year
  • 75% are produced from cows
  • Produced from fermentation of carbohydrates in the rumen. Microbes in the rumen are capable of breaking down cellulose.
  • Quanity & Quality of feed, plus weight, age, activity level, and species affect how much CH4 is produced.
fossil fuels
Fossil Fuels
  • Produces 110 Tg per year
  • Coal Gas and Natural Gas consist almost entirely of methane.
  • Fossil Fuel sources are coal mining as well as exploration, production, transmissions.
biomass burning
Biomass Burning
  • Produces 40 Tg per year
  • CH4 is released when vegetation is burned.
  • Amount produced depends on the burning technology, temperature, moisture and carbon content in the vegetation.
  • Produces 40 Tg per year
  • Decomposition of biodegradable organic material in landfills produces CO2 and CH4.

Losses of CH4 occur in lithosphere and the atmosphere

Lithosphere: Dry soil oxidation

Atmosphere: Trophospheric reactions with OH and losses to the stratosphere

dry soil oxidation
Dry Soil Oxidation
  • A loss of 30 Tg per year
  • Can act as an sink for both atmospheric CH4 and CH4 produced in deeper soil layers.
  • Methanotrophs use CH4 as a source of carbon in a process called methane oxidation
  • Methanotrophs exist in two forms:

\'high capacity - low affinity\' methanotrophs

’low capacity - high affinity\' methanotrophs

Changes in land use practices, better fertilizer application, and land conversion may help prevent the loss of methane sinks.

tropospheric reactions with oh
Tropospheric Reactions With OH
  • A loss of 510 Tg per year
  • The most dominant form of CH4 loss.
  • OH + CH4 -> CH3 + H2O
  • CH4 is removed when it reacts with the hydroxyl radical OH. This happens when cosmic rays strike a water vapor molecule.
  • Hydroxyl Radicals are considered the “Detergent” of the atmosphere because they react with many pollutants.
losses to the stratosphere
Losses To The Stratosphere
  • A loss of 40 Tg per year
  • Plays a minor role in removing CH4 from the atmosphere
  • CH4 is loss to the stratosphere when it reacts with OH, Cl and O(1D)
  • The stratosphere is very dry and any water vapor produced from methane oxidation becomes a greenhouse gas itself.
general facts about methane
General Facts about Methane
  • Known as a ‘well-mixed greenhouse gas’ because of long lifetime so it is able to evenly distribute in atmosphere
  • More potent than CO2, but concentration of CO2 is higher so CO2 has greater effect on global warming

-Control of methane emissions turns out to be a more powerful lever to control global warming than anticipated

effects on the environment
Effects on the Environment
  • Methane is a greenhouse gas ->absorbs heat in the atmosphere->Increase in temperature
  • Since Industrial Revolution, atmospheric methane concentration has doubled
  • Contributes about 20% towards the greenhouse effect, second to CO2
  • Levels of Greenhouse gases can be determined by:

-measuring level of gases after mixing with other gases in the atmosphere (official IPCC)


-measuring level of gases before entering atmosphere.

prevention for the future
Prevention for the Future
  • Methane to Market (United States, the UK, China, Russia, Brazil, India, Italy, Japan, Australia and Nigeria)

Goal: To recover Methane and use it as clean energy.

Federal Clean Air Act

Collection of methane gas from landfills and treated

  • Processing manure using anaerobic digesters making methane available for conversion to useful energy
  • New rice harvesting method, reduce methane gas, increase yield
methanogenesis the big picture
Methanogenesis - The Big Picture

Why is this process interesting?

  • Terminus of anaerobic food chain.
  • Prevents sequestering of large amts or organic material in anaerobic ecosystems.
  • Potential natural gas source.
  • Methanogens are the most important CO2- reducing prokaryotes, but are also a source of greenhouse gas (CH4).
methanogenesis the process
Methanogenesis: The Process

Overall Reaction:

4 H2 + H+ + HCO3- CH4 + 3H2O

and who are these
And who are these?
  • Archaea, ubiquitous in highly reducing habitats, also found in some “oxic” habitats
  • Obligate anaerobes
  • H2 + CO2 --> methane (CH4)
  • Autotrophic (fix CO2), chemolithotrophic (use inorganic H2 as e- donor)
  • Some use endproducts of fermentation (acetate, etc.)
  • Some important genera: Methanococcus, Methanosarcina
  • Capable of using elemental Sulfur as a terminal e- acceptor (anaerobic respiration - sulfur reduction)
  • Bacteria
  • Aerobes
  • Utilize methane:
  • CH4 + O2 --> CO2 + H2O
  • Rxn happens at interface btwn anoxic and oxic layers of soil (as CH4 diffuses up).
  • Genus: Methylomonas
cool and current studies
Cool and current studies

A new biogeochemical cycling pathway couples anaerobic methane oxidation to denitrification. (Nature; vol. 440, 04/06).

  • Freshwater sediments receive high loads of anthropogenic NO3- and CH4.
  • Theoretically microbes should be able to use NO3-/NO2- to anaerobically oxidize CH4, but prior to this report it has not been shown.
  • This study showed direct, anaerobic oxidation of methane coupled to denitrification in the complete absence of O2.

CH4 + NO3-/NO2- + H+ ---> CO2 + N2 + H2O

    • consortium consisted of two microbes: an uncultured bacteria and an archaea related to marine methanotrophic Archaea.


    • new pathways fill in holes in our understanding of global biogeochemical cycling.
methanogens on mars fems micro ecol 2007
Methanogens on Mars? FEMS Micro. Ecol. (2007)

Greenland Ice


  • 10 ppbv CH4 on mars, estimated 300 tons lost/year - 270 tons/year must be generated to offset loss.
  • Is this biogenic or abiogenic methane?
  • Metabolic rates of methanogens in extreme cold on Earth can be used to estimate metabolic rates and cell densities of martian methanogens.
  • Methane is the simplest hydrocarbon
  • Methane is a clean fuel source
  • Methane is a powerful greenhouse gas
  • Methane is part of the carbon cycle
  • The major reservoirs of CH4 are fossil fuel reservoirs, hydrates and Clathrates, and the atmosphere
  • Wetlands are the dominant natural source of CH4
  • Domestic animals are the largest anthropogenic source of CH4
  • The Hydroxyl Radical OH, is the dominant form of CH4 loss.
  • There has been an increase in terrestrial sources and a decrease in sinks, leading to excess CH4 in the atmosphere
  • Reducing Methane will have a stronger influence in reduce Greenhouse effect
  • ‘Methane To Market’ and ‘Federal Clean Air Act’ hopes to reduce Methane output
  • New agriculture and use of anaerobic digesters also reduce Methane output
  • The methane cycle is composed of two groups: methanogens (produce methane) and methanotrophs (consume methane).
  • Methanogenesis is carried out anaerobically by Archaea, methanotrophy is carried out aerobically by bacteria.
  • Methanogens are ubiquitous in anoxic environments (wetlands, sediments, etc.) as well as some oxic ones.
  • Madigan, Martinko, and Parker. 1997. Brock Biology of Microorganisms. 8th ed. Prentice-Hall, New Jersey.
  • Stanier, R.Y., Ingraham, J.L., Wheelis, M.L., and Painter, P.R. 1986. The Microbial World. 5th ed. Prentice-Hall, New Jersey.
  • Strous, M. et al. 2006. A microbial consortium couples anaerobic methane oxidation to denitrification. Nature, 440:918-921.
  • Price, P.B. 2007. Microbial life in glacial ice and implications for a cold origin of life. FEMS Microbiol. Ecol. 59:217-231.