The Global Methane Cycle

1. The Global Methane Cycle CH4 in soil & atmosphere

2. Topics • General Methane Information • Sources & Sinks (general) • CH4 in the soil • CH4 in the atmosphere • Conclusions

3. General Methane Information

4. 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)

5. 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

6. 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

7. CH4 Geographically • 150 ppb Pole-to-pole gradient, indicating consistently large emissions in the northern hemisphere

8. Sources & Sinks (general)

9. + Total : 30% (~100-200 TgCH4/year) Natural Sources • Wetlands • Oceans • Hydrates • Wild ruminants • Termites

10. + Total : 70% Anthropogenic Sources • Agriculture (ruminants) • Waste disposal • Biomass burning • Rice paddies

11. Sinks for tropospheric CH4 • Reaction with hydroxyl radical (~90%) • Transport to the stratosphere (~5%) • Dry soil oxidation (~5%) + Total : ~560 TgCH4/y

12. CH4 in the Soil

13. 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

14. 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 (< -200mV) • Formed by ‘Methanogens’ (Archaea)

15. Methanotrophy in Soils • 2 Forms of oxidation recognized in soils: • I) ‘High AffinityOxidation’ in soils with close to atmospheric CH4 concentrations (<12ppm), upland/dry soils • II) ‘Low AffinityOxidation’ in soils with CH4 concentrations higher than 40 ppm, wetland/submerged soils

16. 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

17. 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

18. 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

19. 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

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

21. 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

22. 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)

23. Rice UP, CH4DOWN • 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

24. CH4 in the Atmosphere

25. 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

26. 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)

27. 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

28.  Urban areas: NOx increase  NOx results in O3 formation  O3 dissociates to OH Projections • OH loss rates may increase due to rising anthropogenic emissions • OH loss rates may be balanced by increased production through O3 and NOx::

29. 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

30. 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

31. 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 area`s, most of the tropics, most of the Southern hemisphere)

32. 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 transportfrom the stratosphere and photolysis of NO2 in the troposphere

33. 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

34. 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

35. 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

36. CONCLUSIONS

37. 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