The Global Methane Cycle

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The Global Methane Cycle

• CH4 in soil atmosphere

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Topics

• General Methane Information
• Sources Sinks (general)
• CH4 in the soil
• CH4 in the atmosphere
• Conclusions

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General Methane Information

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Ins Outs

• Most abundant organic trace gas in the atmosphere
• Concentrations have doubled since pre-industrial
  times (now 1700 ppbv)
• After CO2 and H2O most abundant greenhouse gas
• 20 to 30 times more effective greenhouse gas than
  CO2 (carbon dioxide)

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CH4, what does it do?

• Helps control amount of OH (hydroxyl) in the
  troposphere
• Affects concentrations of water vapor and O3
  (ozone) in the stratosphere
• Plays a key-role in conversion of reactive Cl to
  less reactive HCl in stratosphere
• As a greenhouse gas it plays a role in climate
  warming

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CH4 through Time

• Record of CH4 from air bubbles trapped in polar
  ice (Antarctica and Greenland)
• CH4 levels closely tied to glacial-interglacial
  records
• CH4 follows temperature
• Unprecedented rise since industrial revolution
  CH4 emissions

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CH4 Geographically

• 150 ppb Pole-to-pole gradient, indicating
  consistently large emissions in the northern
  hemisphere

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Sources Sinks (general)

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Natural Sources

• Wetlands
• Oceans
• Hydrates
• Wild ruminants
• Termites

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Anthropogenic Sources

• Agriculture (ruminants)
• Waste disposal
• Biomass burning
• Rice paddies

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Sinks for tropospheric CH4

• Reaction with hydroxyl radical (90)
• Transport to the stratosphere (5)
• Dry soil oxidation (5)

Total 560 TgCH4/y
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CH4 in the Soil
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General Information

• Atmospheric CH4 is mainly (70-80) from
  biological origin
• Produced in anoxic environments, by anaerobic
  digestion of organic matter
• Natural and cultivated submerged soils contribute
  55 of emitted CH4
• Upland (emerged) soils responsible for 5 uptake
  of atmospheric CH4

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Methanogenesis in Soils

• Produced in anoxic environments, by anaerobic
  digestion and/or mineralisation of organic
  matter
• C6H12O6 ? 3CO2 3CH4
• (with low SO42- and NO3- concentrations)
• Formed at low Eh (lt -200mV)
• Formed by Methanogens (Archaea)

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Methanotrophy in Soils

• 2 Forms of oxidation recognized in soils
• I) High Affinity Oxidation in soils with close
  to atmospheric CH4 concentrations (lt12ppm),
  upland/dry soils
• II) Low Affinity Oxidation in soils with CH4
  concentrations higher than 40 ppm,
  wetland/submerged soils

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Low Affinity Oxidation

• Performed by methanotrophic bacteria
• Methanotrophs in all soils with pH higher than
  4.4 in aerobic zone
• Methane oxidation in methanogenic environments is
  Low Affinity Oxidation
• Methane oxidation is Aerobic ? the amount of
  oxygen is the limiting factor

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Low Affinity Rice Fields

• More than 90 of methane produced in methanogenic
  environments is reoxidised by methanotrophs
• Variations in CH4 emissions from ricefields
  mostly due to variations in methanotrophy
• Emission of CH4 mostly through rice aerenchyma
  (pipes)
• Soil oxidation through aerenchyma

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More General Info

• Methanotrophy is highest in methanogenic
  environments
• Both methanogens and trophs prevail under
  unfavorable conditions (high/low water etc)
• Methane emission is larger from planted rice
  fields than from fallow fields, due to higher C
  availability and aerenchyma

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High Affinity

• Upland forest soils most effective CH4 sink
• Temporarily submerged upland soils can become
  methanogenic
• Arable land much smaller CH4 uptake than
  untreated soils

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Water

• Soil submersion allows methanogenesis
• Reduces methanotrophy
• Short periods of drainage decreases
  methanogenesis in ricefields dramatically (Fe,
  SO4)

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pH and Temperature

• Methanogenesis most efficient around pH
  neutrality
• Methanotrophs more tolerant to variations in pH
• Methanogenesis is optimum between 30 and 40 oC
• Methanotrophs are more tolerant to temperature
  variations

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Rice and Fertilizers

• Goal High yield and less methane emission
• Organic fertilizers increase CH4 (incorporation
  org. C)
• ? Reduce CH4 by raising Eh and competition (e.g.
  SO4)

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Rice UP, CH4 DOWN

• Fertilizers containing SO4 may poison the soil
• Ammonium and urea decrease methanotrophy/CH4
  oxidation, especially in upland soils
• Calcium carbide significantly reduces CH4
  emission and increases rice yield by inhibiting
  nitrification

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CH4 in the Atmosphere
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Major atmospheric CH4 sink OH

• Reaction with hydroxyl (OH) radical (90) in the
  troposphere
• OH is formed by photodissociation of tropospheric
  ozone and water vapor
• OH is the primary oxidant for most tropospheric
  pollutants (CH4, CO, NOx)
• Amount CH4 removed constrained by OH levels and
  reaction rate

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Source of OH

• Formed when O3 (ozone) is photo-dissociated
• O3 hv ? O(1D) O2
• which in turn reacts with water vapor to form 2
  OH radicals
• O(1D) H2O ? OH OH
• (OH is also formed in Stratosphere by oxidation
  of CH4 due to high concentrations of Cl)

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Sink of OH

• CH4 mainly removed by reaction
• CH4 OH ? CH3 H2O
• OH concentrations not only affected by direct
  emissions of methane but also by its oxidation
  products, especially CO
• Increase in methane leads to positive feedback
  build-up of CH4 concentrations

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Projections

• OH loss rates may increase due to rising
  anthropogenic emissions
• OH loss rates may be balanced by increased
  production through O3 and NOx

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Projections 2

• Stratospheric ozone decreases as seen in recent
  years
• Due to decrease of stratospheric O3, ultraviolet
  radiation in troposphere increases ? increase OH
• Water vapor through temperature rise may either
  increase or decrease OH

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Projections 3 Tropics

• Tropics high UV, high water vapor ? High OH
• High CH4 production due to rice fields, biomass
  burning, domestic ruminants
• Future changes in land use / industrialization

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NOx and OH

• Polluted areas ? High NOx ? OH production
  (temperate zone Northern hemisphere, planetary
  boundary layer of the tropics)
• Unpolluted areas ? Low NOx ? OH destruction
  (marine areas, most of the tropics, most of the
  Southern hemisphere)

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O3 in Tropo- and Stratosphere

• Ozone (O3) absorbs ultraviolet radiation, but is
  also a greenhouse gas
• 90 of O3 in the Stratosphere
• Stratospheric production by photo- dissociation
  of O2 and reaction with O2
• 10 of O3 in the Troposphere, through downward
  transport from the stratosphere and photolysis of
  NO2 in the troposphere

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Stratospheric Ozone

• O3 destroyed by catalytic mechanisms involving
  free radicals like NOx, ClOx, HOx
• CH4 acts as source and sink for reactive
  chlorine
• Sink direct reaction with reactive Cl to form
  HCl (main Cl reservoir species)
• Source OH (oxidation of CH4 in stratosphere)
  reacts with HCl to form reactive Cl

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Stratospheric Ozone 2

• OH from the dissociation of methane can react
  with ozone (especially in the upper stratosphere)
• Conclusively increasing CH4 leads to net O3
  production in troposphere and lower stratosphere
  and net O3 destruction in the upper stratosphere

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CH4 impact on Climate

• CH4 absorbs infrared radiation ? increases
  greenhouse effect
• Globally-averaged surface temperature 1.3oC
  higher than without methane
• Dissociation of CH4 leads to CO2 additional
  climatic forcing

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CONCLUSIONS
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• CH4 has increased dramatically over the last
  century and continues to increase
• Causal role of human activity
• Climate forcing by CH4 confirmed, though not
  fully understood
• Future developments uncertain

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