The Global Methane CycleThe Global Methane Cycle

CHCH44 in soil & atmosphere in soil & atmosphere

TopicsTopics

• General Methane Information

• Sources & Sinks (general)

• CH4 in the soil

• CH4 in the atmosphere

• Conclusions

General Methane InformationGeneral Methane Information

Ins & OutsIns & 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)

CHCH44, what does it do?, 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

CHCH44 through Time 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

CHCH44 Geographically Geographically

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

Sources & Sinks (general)Sources & Sinks (general)

Natural SourcesNatural Sources

• Wetlands• Oceans• Hydrates• Wild ruminants• Termites

+Total : 30% (~100-200

TgCH4/year)

Anthropogenic SourcesAnthropogenic Sources

• Agriculture (ruminants)

• Waste disposal• Biomass burning• Rice paddies

+Total : 70%

Sinks for tropospheric CHSinks for tropospheric CH44

• Reaction with hydroxyl radical (~90%)

• Transport to the stratosphere (~5%)

• Dry soil oxidation (~5%)+

Total : ~560 TgCH4/y

CHCH44 in the Soil in the Soil

General InformationGeneral 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

Methanogenesis in SoilsMethanogenesis 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)

Methanotrophy in SoilsMethanotrophy in Soils

• 2 Forms of oxidation recognized in soils:

• I) ‘High Affinity Oxidation’ in soils with close to atmospheric CH4 concentrations (<12ppm), upland/dry soils

• II) ‘Low Affinity Oxidation’ in soils with CH4 concentrations higher than 40 ppm, wetland/submerged soils

Low Affinity OxidationLow 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 FieldsLow 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

More General InfoMore 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 AffinityHigh Affinity

• Upland forest soils most effective CH4 sink

• Temporarily submerged upland soils can become methanogenic

• Arable land much smaller CH4 uptake than untreated soils

WaterWater

• Soil submersion allows methanogenesis

• Reduces methanotrophy• Short periods of

drainage decreases methanogenesis in ricefields dramatically (Fe, SO4)

pH and TemperaturepH 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

Rice and FertilizersRice 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)

Rice Rice UPUP, CH, CH44 DOWNDOWN

• 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

CHCH44 in the Atmosphere in the Atmosphere

Major atmospheric CHMajor atmospheric CH44 sink: sink: OHOH

• 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

Source of OHSource 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)

Sink of OHSink 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

ProjectionsProjections

• OH loss rates may increase due to rising anthropogenic emissions

• OH loss rates may be balanced by increased production through O3 and NOx::

Urban areas: NOx increase

NOx results in O3 formation

O3 dissociates to OH

Projections 2Projections 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

Projections 3: TropicsProjections 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

NONOxx and OH 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)

OO33 in Tropo- and Stratosphere 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

Stratospheric OzoneStratospheric 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

Stratospheric Ozone 2Stratospheric 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

CHCH44 impact on Climate 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

CONCLUSIONSCONCLUSIONS

• 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