Methane cycle

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Methane cycle
Université de Versailles Saint Quentin en Yvelines
Laboratoire des Sciences du Climat et de
l environnement
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Cycle du méthane
U S Geological Survey
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Presentation sketch
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CH4 Id

• Methane - CH4
• Colorless, odorless. Molar mass 16.04 g/mol.
  Low solubility in water
• Principal component of natural gas (3/4)
• Combustion of methane is highly exothermic.
• CH4(g) 2 O2(g) ------ CO2(g) 2 H2O(l) H
  891 kJ
• Melting point (-183C), boiling point (-164C)
  at room temperature
• Mainly formed by anaerobic bacterial
  decomposition of organic material
• Explosive in air with mixtures from 5 to 15
• 500-600 TgCH4/yr emissions. 8-12 year lifetime
  in the atmosphere.
• 150 increase of atmospheric concentration
  since pre-industrial time
• Greenhouse gas, GWP 20-30 times larger than CO2.
  Tropospheric O3 precursor.

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Radiative forcings in the climate system
IPCC, 2001
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Presentation sketch
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Wetlands and rice cultivation

• Anaerobic digestion and/or mineralisation of
  organic matter with low SO42- and NO3-
  concentrations
• C6H12O6 ---gt 3CO2 3CH4
• Formed by methanogens (Archaea)
• Swamps, bogs, tundra areas (natural) and rice
  fields (anthropogenic)
• More than 90 of methane produced in
  methanogenic environments is reoxidised by
  methanotrophs
• Emission of CH4 mostly through rice aerenchyma
  (pipes)
• Variations in CH4 emissions from rice fields
  mostly due to variations in methanotrophy

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Wetlands and rice cultivation

• Methanogenesis most efficient around pH
  neutrality
• Methanotrophs more tolerant to variations in pH.
  Present in all soils with pH higher then 4.4
• Methanogenesis is optimum between 30 and 40 oC
• Methanotrophs are more tolerant to temperature
  variations
• Rice Goal high yield and less CH4 emissions
• - reduce org. Fertilizers
• - raise Eh potential
• - raise competition (e.g. NO3- )
• - use of calcium carbide (CaC2)

Natural wetlands 92 - 237 TgCH4/yr Rice
fields 29 - 61 TgCH4/yr
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Processes of methane emissions
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Ruminant animals livestock manure management

• Enteric fermentation of ruminants (cattle,
  sheep, goats) during normal digestive processes
• Solutions to reduce emissions improving food,
  animal management health
• Termites emissions 20 TgCH4/yr, bacterial
  activity
• Decomposition of animal waste in anaerobic
  conditions
• Solutions to reduce emissions recover and
  flare CH4, use it to produce energy.

100 - 135 TgCH4/yr
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Landfills wastewaters

• Anaerobic decomposition of organical material.
• Main source of CH4 of USA and EU
• Solutions to reduce emissions
• Collect burn gas
• Reduce landfilling
• Current situation
• - USA Lanfill rules
• - EU lanfill directive

35 - 73 TgCH4/yr
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Gas, oil and coal industries

• Major component of natural gas (75)
• Natural gas and oil systems CH4 is emitted
  during its production, processing, transportation
  and distribution.
• Solutions to reduce emissions Reduce fugitives
  through enhanced inspection and maintenance,
  capture/prevent vented emissions.
• Coal mining CH4 is trapped within coal seams
  and the surrounding rock strata during coal
  formation.
• Solutions to reduce emissions drain gas before
  mining

75 - 110 TgCH4/yr
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Biomass burning - biofuel use

• Greenhouse gas released during biomass burning,
  including CH4.
• Emission factor of 3-4 g/kg on average variable
  in the different ecosystems (CO21600g/kg)
• Detectable by satellite biogeochemical
  modelling (Van der Werf et al., 2004)

23 - 55 TgCH4/yr
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OH oxidation

• Main CH4 sink (90)
• CH4 OH ---gt CH3 H2O
• OH 106 molecules/cm3
• More important in the low-to-mid tropical
  troposphere
• Amplitude of OH variations highly disputed
• Loss of CH4 by reaction with stratospheric OH,
  Cl and O
• OH formed from ozone photolysis reaction with
  H2O
• O3 hv ---gt O2 O
• O H2O ---gt 2OH

450-520 TgCH4/yr
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Dry soil oxidation

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

10 - 46 TgCH4/yr
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Ocean emissions gas hydrates

• Two identified sources including the anaerobic
  digestion in marine zooplankton and fish, and
  methanogenisis in sediments and drainage areas
  along coastal regions.
• Methane hydrate is a crystalline solid
  consisting of gas molecules, each surrounded by a
  cage of water molecules.
• Methane hydrate is stable in ocean floor in
  continental margin (high pressure) and in Arctic
  permafrost (low temperature)
• Methane can be released from the hydrates with
  changes in temperature, pressure, salt
  concentrations

10 - 25 TgCH4/yr
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Presentation sketch
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Methane Earth history solar activity
Ramstein, pers. Comm.
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Methane Earth history

• Earth should have been covered by ice in the
  early ages
• Reconstructed temperatures show that Earth
  climate was hot on average during the first 2
  Ga.
• 2 major climate crisis
• Huronian glaciation (2.6-2.3 Ga)
• Neoproteozoric glaciation (750-570 Ma)
• Greenhouse gas have more than compensated the
  lower solar intensity
• - Sagan 80s ammoniac (NH3)
• not compatible with a O2 free atmosphere
• - CO2 Request such an amount of CO2 in the
  atmosphere that siderite (FeC) would have
  formed. Not found in cratons.
• - CH4 Emissions as today may have generated
  more than 1000 ppm of CH4 due to low
  atmospheric OH

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Methane Earth history Atmospheric composition
J. Kasting, Pour la Science, N 323 - septembre
2004
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Methane Earth history Organic Haze
J. Kasting, Pour la Science, N 323 - septembre
2004
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Methane Earth history Titan atmosphere
Distance from Saturn 1 221 870 km Distance from
Sun 1 427 000 000 km (9.54 AU) Diameter
(atmosphere) 5550 km Diameter (surface) 5150
km Mass 1/45 that of Earth Average density 1.881
times liquid water Surface temperature 94K (-180
degrees C) Atmospheric pressure at surface 1500
mbar

• Largest moon of Saturn
• TITAN atmosphere 95 nitrogen
• 5 methane

www.esa.int
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Methane Earth history

• CH4 may explain the relative hot climate in the
  early ages of the Earth
• High level of atmospheric CH4 is controlled by
  organic haze that forms when CO2/CH4 ratio
  exceeds 1
• Massive oxygen release in the atmosphere around
  2.5 Ga
• Thermophil methanogens niches reduced to
  anaerobic environments
• OH radical increase in the atmosphere
• Large drop of atmospheric CH4
• Huronian glaciation (Kasting Siefert, Science,
  2002) ?

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Presentation sketch
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Forage de Vostok (Antarctique)
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Forage de Vostok (Antarctique)
Signaux climatiques sur les 400 000 dernières
années Forage de Vostok
Source www.cnrs.fr
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Changes in CH4 abundance in the recent past
Figure 4.1. (a) Change in CH4 abundance (mole
fraction, in ppb 10-9) determined from ice
cores, firn, and whole air samples plotted for
the last 1000 years. Data sets are as follows
Grip, Blunier et al. (1995) and Chappellaz et al.
(1997) Eurocore, Blunier et al. (1993) D47,
Chappellaz et al. (1997) Siple, Stauffer et al.
(1985) Global (inferred from Antarctic and
Greenland ice cores, firn air, and modern
measurements), Etheridge et al. (1998) and
Dlugokencky et al. (1998). Radiative forcing,
approximated by a linear scale since the
preindustrial era, is plotted on the right axis.
(d) Comparison of Greenland (GRIP) and
Antarctic (D47 and Byrd) CH4 abundances for the
past 11.5 kyr (Chappellaz et al., 1997). The
shaded area is the pole-to-pole difference where
Antarctic data exist. (e) Atmospheric CH4
abundances (black triangles) and temperature
anomalies with respect to mean recent temperature
(gray diamonds) determined for the past 420 kyr
from an ice core drilled at Vostok Station in
East Antarctica (Petit et al., 1999).
From IPCC, 2001
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What about isotopes ?

• 13CH4 measurements show an atmospheric ?13C of
  -47 on average
• 13CH4 measurements can help partitionning
  emissions into three categories due to different
  fractionation
• - biogenic (wetlands, rmuinants, termits,)
• ---gt ?13C -60
• - fossil (energy related emissions)
• ---gt ?13C -40
• - Biomass burning
• ---gt ?13C -25 (C3) or -12 (C4)

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Changes in CH4 abundance in the recent past
Ferretti et al., Science, 2005
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Changes in CH4 abundance in the recent past
Ferretti et al., Science, 2005
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Changes in CH4 abundance in the recent past
Ferretti et al., Science, 2005
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Presentation sketch
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Atmospheric inversion
Modèle direct
Modèle inverse
Estimation de xa minimisant la distance entre
ymodel et yo, approche bayesienne
Estimation Erreur
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Atmospheric network used
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Anthropogenic emission inventory
EDGAR database
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Methane total emissions
Zonal mean Spatial patterns
Emissions in TgCH4/gridbox/year
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Regional inversion
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Regional inversion
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Regional inversion
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Satellite use ?
Model (TM5) compared to Sciamachy retrievals,
after inversion
SATELLITE MODEL
Bergamaschi et al., 2006
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Satellite use ?
Model (TM5) compared to Sciamachy retrievals,
after inversion
SATELLITE MODEL
Bergamaschi et al., 2006
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Satellite use ?
Model (TM5) compared to Sciamachy retrievals,
after inversion
SATELLITE MODEL
Bergamaschi et al., 2006
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Satellite use ?
Model (TM5) compared to Sciamachy retrievals,
after inversion
Bergamaschi et al., 2006
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Satellite use ?
Model (TM5) compared to Sciamachy retrievals,
after inversion
SATELLITE MODEL
Bergamaschi et al., 2006
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Global distrubution of atmospheric CH4
www.cmdl.noaa.gov, Dlugokencky et al., 2003
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Atmospheric accumulation

• 5 of global emissions
• 7 ppb/yr in the 1990s
• Tends to zero in the 2000s

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OH formation and destruction
Oxydant capacity of the troposphere
OH est le principal oxydant dans la troposphère
et contrôle le temps de résidence de nombreux
polluants dans latmosphère.
CO OH (O2) g CO2 HO2 (R4) CH4 OH
(O2) g CH3O2 H2O (R5) HCFC OH ? Produits
(R7) SO2 OH ? (...) ? sulfates (R8)
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Oxydant capacity of the troposphere
Methan oxydation chain
Physique et chimie de latmosphère, ed. Belin,
2005
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Oxydant capacity of the troposphere
Fig. 2 Coupe zonale de la concentration de OH
(106 molécules/cm3) calculée pour
juin-juillet-août et pour décembre-janvier-février
.
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Oxydant capacity of the troposphere
Fig. 3 Distribution de la concentration de OH
(106 molécules/cm3) à la surface calculée à
laide du modèle MOZART pour le 4 juillet à 12h
TU.
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Methyl chloroform inversion
Fig. 4 Evolution du rapport de mélange de
CH3CCl3.
Bousquet et al., ACP, 2005
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Iterative inverse procedure
Bousquet et al., 2005
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OH variability
Bousquet et al., ACP, 2005
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CH4 atmospheric growth rate
Growth rate of atmospheric methane
Growth rate of atmospheric methane presents large
year-to-year variations
Long-term trend of the growth rate has been
negative for 20 years
How to explain such fluctuations ?
Bousquet et al., Nature, 2006
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Model - data Fit
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Methane emissions
Methane emissions
Zonal mean Spatial patterns
Emissions in TgCH4/gridbox/year
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Inversion of methane emissions - a source-type
view
Wetlands are the largest contributor to
year-to-year variations of methane emissions
Their contribution of Biomass burning is smaller
except in 1997-98 (El Niño).
Since 1999, increasing anthropogenic emissions
(North Asia) compensate decreasing wetland
emissions (droughts) and maintain a small growth
rate..
Inversion results are in good agreement with
bottom-up models based on satellite retrievals
and process models.
The long-term reduction in growth rate is mostly
due to anthropogenic emissions
Bousquet et al., Nature, 2006
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Inversion of methane emissions - a geographical
view

• OH radicals, the main CH4 sink, contribute
  significantly to CH4 atmospheric year-to-year
  variations.
• Northern emissions dominate long-term evolution
  of CH4
• Tropical emissions dominate year-to-year
  variations of CH4
• Biomass burning contributed significantly to
  methane growth rate in 1997-98

Bousquet et al., Nature, 2006
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Geographical partition
European emissions have been decreasing
continuously for 20 years
North-Asia emssion increase again since 1999.
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Geographical partition
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Process partition
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Methane budget (from atmospheric inversion)

•   Global mean source is 525 10 TgCH4/yr over
  1984-2003.
• Global trend is -1.8 TgCH4/yr mainly due to a
  anthropogenic sources (energy landfills) north
  of 30N, in agreement with data analysis
• IAV is  20 TgCH4/yr, dominated by the tropics
  in the 1980s and more spread in the 1990s and the
  2000s

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Anthropogenic emissions
70 254-426 TgCH4/an
75-110
Energy
80-115
Ruminants animals
35-73
Landfills waste
29-61
Rice
Biomass burning biofuel
35-67
IPCC, 2001, updated Bousquet et al., 2006.
Emissions in TgCH4/yr
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Natural emissions
30 127-272 TgCH4/an
92-237
Wetlands
20
Termits
10-25
Oceans Hydrates
IPCC, 2001, updated Bousquet et al., 2006.
Emissions in TgCH4/yr
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Methane Sinks
95 500-615 TgCH4/an
450-520
OH oxydation
Stratosphere
40-46
Soils
10-45
IPCC, 2001 Bousquet et al., 2006.
Emissions in TgCH4/yr
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Presentation sketch
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Methane hydrates (clathrates)
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Methane hydrates (clathrates)

• Methane hydrate is a crystalline solid
  consisting of gas molecules, each surrounded by a
  cage of water molecules.
• Methane hydrate is stable in ocean floor in
  continental margin (high pressure) and in Arctic
  permafrost (low temperature)
• 1 cm3 of methane hydrate ice can release up to
  164 cm3 of CH4.

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A new source ?
62 - 236 TgCH4/yr Keppler, 2006 23 - 71
TgCH4/yr Lathiere, pers. comm.
Keppler et al., Nature, 2006
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Natural emissions
?? 190-508 TgCH4/an
92-237
Natural wetlands
20
Termits
10-25
Ocean
Hydrates
62-236
Plants ?
??
Keppler et al., 2006
IPCC, 2001, updated Bousquet et al., 2006.
Emissions in TgCH4/yr
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Inversion v.s. satellite, so what ?
Satellite columns of CH4 (Frankenberg et al.,
2005)
CH4 emissions as inferred by inversions
(Bousquet et al., in prep)
TgCH4/gridbox/yr
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Satellite use ?
Colonnes de CH4
Différence observations / modèle entre Sciamachy
et un scénario sans émissions par les plantes
Echelles différentes
Différence modèle/modèle entre un scénario avec
émissions par les plantes et un scénario sans
émissions par les plantes
Houweling et al., 2006
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Plant source in inversion ?
Introduction of an additional source of 100
TgCH4/yr due to plants (Keppler et al., 2006) is
compatible with surface CH4 observations Plant
source reduce IAV of anthropogenic
emissions Plant source does not change much IAV
of retrieved wetland emissions in the 1990s.
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Conclusions

• CH4 has increased dramatically over the last
  century
• Causal role of human activity
• Locations of CH4 emissions (satellite,
  inventories) might be better known than those of
  CO2, thus limiting aggregation error in
  large-regions inversions.
• Actions to reduce CH4 emissions are less
  constraining to implement than for CO2
• Satellite measurements should help apportioning
  surface emissions

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And
Thank you
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Variations du taux de croissance du méthane
atmosphérique
Le taux de croissance du méthane atmosphérique
présente de fortes variations interannuelles
On note aussi une diminution du taux de
croissance moyen depuis 20 ans
Comment expliquer ces fluctuations ?
Bousquet et al., Nature, 2006
77
Variations des émissions de méthane par inversion
atmosphérique Bilan par processus
Les zones inondées (wetlands) contribuent le plus
à la variabilité interannuelle du méthane.
Les feux de biomasse contribuent moins sauf en
1997-98 (El Niño).
Depuis 1999, une compensation entre une
augmentation des émissions anthropiques et une
diminution des émissions des wetlands
(sécheresses) expliquent le faible taux de
croissance observé.
Les résultats de linversion sont en bon accord
avec des modèles basés sur des reconstructions
satellites des processus (en rouge).
La diminution du taux de croissance moyen est
surtout dû aux sources anthropogéniques.
Bousquet et al., Nature, 2006
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Variations des émissions de méthane par inversion
atmosphérique Bilan par bande de latitude

• Les radicaux OH, principal puits de méthane,
  contribue significativement à sa variabilité
  atmosphérique.
• Les sources des régions de lhémisphère nord
  dominent plutôt la variabilité à long terme
• Les sources des régions tropicales dominent
  plutôt la variabilité interannuelle
• Les feux de biomasse ont eu une contribution
  significative en 1997-98

Bousquet et al., Nature, 2006