Transcript The Global Methane Cycle

The Global Methane Cycle
CH4 in soil & atmosphere
Topics
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General Methane Information
Sources & Sinks (general)
CH4 in the soil
CH4 in the atmosphere
Conclusions
General Methane Information
Ins & Outs
• Most abundant organic trace gas in the
atmosphere
• Concentrations have doubled since preindustrial times (now ~1700 ppbv)
• After CO2 and H2O most abundant
greenhouse gas
• 20 to 30 times more effective greenhouse
gas than CO2 (carbon dioxide)
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
CH4 through Time
• Record of CH4 from air bubbles trapped in
polar ice (Antarctica and Greenland)
• CH4 levels closely tied to glacialinterglacial records
• CH4 ‘follows’ temperature
• Unprecedented rise since industrial
revolution: CH4 emissions
CH4 Geographically
• 150 ppb Pole-to-pole gradient, indicating
consistently large emissions in the northern
hemisphere
Sources & Sinks (general)
Natural Sources
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Wetlands
Oceans
Hydrates
Wild ruminants
Termites
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Total : 30% (~100-200
TgCH4/year)
Anthropogenic Sources
• Agriculture
(ruminants)
• Waste disposal
• Biomass burning
• Rice paddies
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Total : 70%
Sinks for tropospheric CH4
• Reaction with hydroxyl radical (~90%)
• Transport to the stratosphere (~5%)
• Dry soil oxidation (~5%)
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Total : ~560 TgCH4/y
CH4 in the Soil
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
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)
Methanotrophy 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 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 CH4 emissions from ricefields
mostly due to variations in methanotrophy
• Emission of CH4 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 CH4
sink
• Temporarily
submerged upland
soils can become
methanogenic
• Arable land much
smaller CH4 uptake
than untreated soils
Water
• Soil submersion allows
methanogenesis
• Reduces methanotrophy
• Short periods of
drainage decreases
methanogenesis in
ricefields dramatically
(Fe, SO4)
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
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)
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
CH4 in the Atmosphere
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
Source of OH
• Formed when O3 (ozone) is photodissociated:
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 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
Projections
• 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 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: 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
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)
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 photodissociation 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 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 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
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
CONCLUSIONS
• 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