The Global Methane Cycle CH 4 in soil & atmosphere

Topics General Methane Information Sources & Sinks (general) CH 4 in the soil CH 4 in the atmosphere Conclusions

General Methane Information

Ins & Outs Most abundant organic trace gas in the atmosphere Concentrations have doubled since pre- industrial times (now ~1700 ppbv) After CO 2 and H 2 O most abundant greenhouse gas 20 to 30 times more effective greenhouse gas than CO 2 (carbon dioxide)

CH 4, what does it do? Helps control amount of OH (hydroxyl) in the troposphere Affects concentrations of water vapor and O 3 (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

CH 4 through Time Record of CH 4 from air bubbles trapped in polar ice (Antarctica and Greenland) CH 4 levels closely tied to glacial- interglacial records CH 4 ‘follows’ temperature Unprecedented rise since industrial revolution: CH 4 emissions

CH 4 Geographically 150 ppb Pole-to-pole gradient, indicating consistently large emissions in the northern hemisphere

Sources & Sinks (general)

Natural Sources Wetlands Oceans Hydrates Wild ruminants Termites + Total : 30% (~ TgCH 4 /year)

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

Sinks for tropospheric CH 4 Reaction with hydroxyl radical (~90%) Transport to the stratosphere (~5%) Dry soil oxidation (~5%) + Total : ~560 TgCH 4 /y

CH 4 in the Soil

General Information Atmospheric CH 4 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 CH 4 Upland (emerged) soils responsible for ~5% uptake of atmospheric CH 4

Methanogenesis in Soils Produced in anoxic environments, by anaerobic digestion and/or mineralisation of organic matter: C 6 H 12 O 6  3CO 2 + 3CH 4 (with low SO 4 2- and NO 3 - concentrations) Formed at low Eh (< -200mV) Formed by ‘Methanogens’ (Archaea)

Methanotrophy in Soils 2 Forms of oxidation recognized in soils: I) ‘High Affinity Oxidation’ in soils with close to atmospheric CH 4 concentrations (<12ppm), upland/dry soils II) ‘Low Affinity Oxidation’ in soils with CH 4 concentrations higher than 40 ppm, wetland/submerged soils

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

Low Affinity & Rice Fields More than 90% of methane produced in methanogenic environments is reoxidised by methanotrophs Variations in CH 4 emissions from ricefields mostly due to variations in methanotrophy Emission of CH 4 mostly through rice aerenchyma (‘pipes’) Soil oxidation through aerenchyma

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

High Affinity Upland forest soils most effective CH 4 sink Temporarily submerged upland soils can become methanogenic Arable land much smaller CH 4 uptake than untreated soils

Water Soil submersion allows methanogenesis Reduces methanotrophy Short periods of drainage decreases methanogenesis in ricefields dramatically (Fe, SO 4 )

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

Rice and Fertilizers Goal: High yield and less methane emission Organic fertilizers increase CH 4 (incorporation org. C)  Reduce CH 4 by raising Eh and competition (e.g. SO 4 )

Rice UP, CH 4 DOWN Fertilizers containing SO 4 may poison the soil Ammonium and urea decrease methanotrophy/CH 4 oxidation, especially in upland soils Calcium carbide significantly reduces CH 4 emission and increases rice yield by inhibiting nitrification

CH 4 in the Atmosphere

Major atmospheric CH 4 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 (CH 4, CO, NO x ) Amount CH 4 removed constrained by OH levels and reaction rate

Source of OH Formed when O 3 (ozone) is photo- dissociated: O 3 + hv  O( 1 D) + O 2 which in turn reacts with water vapor to form 2 OH radicals: O( 1 D) + H 2 O  OH + OH (OH is also formed in Stratosphere by oxidation of CH 4 due to high concentrations of Cl)

Sink of OH CH 4 mainly removed by reaction CH 4 + OH  CH 3 + H 2 O 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 CH 4 concentrations

Projections OH loss rates may increase due to rising anthropogenic emissions OH loss rates may be balanced by increased production through O 3 and NO x: :  Urban areas: NO x increase  NO x results in O 3 formation  O 3 dissociates to OH

Projections 2 Stratospheric ozone decreases as seen in recent years Due to decrease of stratospheric O 3, ultraviolet radiation in troposphere increases  increase OH Water vapor through temperature rise may either increase or decrease OH

Projections 3: Tropics Tropics: high UV, high water vapor  High OH High CH 4 production due to rice fields, biomass burning, domestic ruminants Future changes in land use / industrialization

NO x and OH Polluted areas  High NO x  OH production (temperate zone Northern hemisphere, planetary boundary layer of the tropics) Unpolluted areas  Low NO x  OH destruction (marine area`s, most of the tropics, most of the Southern hemisphere)

O 3 in Tropo- and Stratosphere Ozone (O 3 ) absorbs ultraviolet radiation, but is also a greenhouse gas 90% of O 3 in the Stratosphere Stratospheric production by photo- dissociation of O 2 and reaction with O 2 10% of O 3 in the Troposphere, through downward transport from the stratosphere and photolysis of NO 2 in the troposphere

Stratospheric Ozone O 3 destroyed by catalytic mechanisms involving free radicals like NO x, ClO x, HO x CH 4 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 CH 4 in stratosphere) reacts with HCl to form reactive Cl

Stratospheric Ozone 2 OH from the dissociation of methane can react with ozone (especially in the upper stratosphere) Conclusively: increasing CH 4 leads to net O 3 production in troposphere and lower stratosphere and net O 3 destruction in the upper stratosphere

CH 4 impact on Climate CH 4 absorbs infrared radiation  increases greenhouse effect Globally-averaged surface temperature 1.3 o C higher than without methane Dissociation of CH 4 leads to CO 2 : additional climatic forcing

CONCLUSIONS

CH 4 has increased dramatically over the last century and continues to increase Causal role of human activity Climate forcing by CH 4 confirmed, though not fully understood Future developments uncertain