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12.842 / 12.301 Past and Present ClimateFall 2008
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12.842Climate Physics and Chemistry
Fall 2008
Atmospheric Chemistry II:Methane
ogenic CH4 and CO2
2501750 1800 1850
Year
Reconstruction of the CO2 and CH4 increases since the preindustrialtime from data, respectively, from the Siple (6) and DE08 (7) ice cores. Note the good agreement between the ice core data and the direct measurements (solid lines) in the atmosphere starting from 1958 (CO2)and 1978 (CH4).
1900 1950700
900
1100
270
290
CO
2 C
once
ntra
tion
(ppm
v)
CH
4 C
once
ntra
tion
(ppb
v)
310
330
350
1300
1500
1700
1900CO2CH4
Figure by MIT OpenCourseWare based on Science Vol. 259, 1993.
Methane is a strong greenhouse gas and contributes to global warming
-2
-1
0
Glo
bal-M
ean
Rad
iativ
e Fo
rcin
g(W
m-2
) 1
2
CO2
CH4
N2O Halocarbons
Stratospheric ozone
ozone Sulphate
3
Biomass burning
Solar
High
Estimates of the globally and annually averaged anthropogenic radiative forcing-2) due to changes in concentrations of greenhouse gases and aerosols from
pre-industrial times to the present day and to natural changes in solar output from
Low Low Confidence Level
Low Low Low Low Low
Fossil fuel soot
Tropospheric
Tropospheric aerosols - indirect effect
Tropospheric aerosols - direct effect
RELEVANCE TO CLIMATE
(in Wm
1850 to the present day.
Very Very Very Very
Figure by MIT OpenCourseWare based on IPCC.
Main natural CH4 source to atmosphere: anaerobic microbial fermentation in environments where all other oxidants
(O2, MnO2, NO3-, Fe2O3, SO4=) are depleted:
2 "CH2O" -> CH4 + CO2
ΔG ~ -350 kJ/mole of glucose
[actual direct precursors of CH4 are fermentation of acetate and/or oxidation of hydrogen using CO2 as the electron acceptor]
In some environments, this methane can escape into the oxicenvironment (e.g., gas bubbles rising through water). In other environments where transport goes through less reducing environments, methane can be lost by methanotropic bacteria. In marine sediments, CH4 is consumed by the microbial reaction:
CH4 + SO4= -> HCO3
- + HS- + H2O
Tropospheric life cycles of climatically
important species
Figure by MIT OpenCourseWare.
Determination of CH4 lifetime from CH3CCl3
Figure by MIT OpenCourseWare. Adapted from Prinn, et al. Science 269 (1995): 187-192.
7840
Mix
ing
Rat
io (p
pt)
60
80
100
120
140
160
180
80 82 84 86Year
Determination of OH by Inverse Method
88 90 92 94 96
OH + CH3CCl3( (CH2CCl3 + H2Ok1
OH + CH4 CH3 + H2Ok2
�i = = [i]dV +_4.6 0.3yr (CH3CCl3)
Results
ALE/GAGE/AGAGE
CH3CC13
+_
+_8.9 0.6yr (CH4){ {�ki[i][OH]dV�
[OH] = (9.7 0.6)x 105 cm-3< < = ki[i][OH]dV�ki[i]dV�
IrelandOregon
BarbadosSamoa
Tasmania
CH4 sources:• Some methane is emitted from the seafloor by natural gas seeps (major source
of methane from this source is thermal cracking of buried organic matter) including “cold seeps” and “mud volcanos”. On some occasions methane can be emitted from methane hydrate decomposition. Usually, most this methane is oxidized before it is emitted to the atmosphere.
• CH4 dominantly enters the atmosphere from land. The main natural sources are natural wetlands and termites. At present, these natural sources are supplemented by several anthropogenic sources – rice fields, enteric fermentation (cows), oil and natural gas leakage, coal mining, landfills, sewage, animal wastes, and biomass burning. Because most of the land is in the Northern Hemisphere, methane is slightly higher in the northern hemisphere, and maximum concentrations occur at high northern latitudes.
1984
CH
4(p
pb)
-90-60
-30 Latitude030
90601500
1600
1700
1800
19851986
1987
Temporal and latitudinal variation of the tropospheric methane mixing ratio.
Figure by MIT OpenCourseWare based on Fung, et al., 1991.
Image courtesy of NASA.
Methane Tropospheric Chemistry
• Main CH4 tropospheric sink is reaction with OH radical: CH4 + OH --> CH3 + H2O
• Tropospheric OH source is:O3 + hν --> O2 + O ; O + H2O --> 2 OH
• OH is the main oxidizing molecule of the troposphere; it is involved in most tropospheric oxidation reactions. But its lifetime is short and its concentration is very low: ~104 atoms per cm3.
• "OH can react quickly with CO to yield CO2, and with CH4 and other hydrocarbons to form formaldehyde. In the process, OH is converted to HO2(hydroperoxyl), from which OH can be regenerated to start the chain all over again."
(Physics Today, Nov. 1996 p. 17)
• CH4 tropospheric lifetime is ~ 8 yrs (best established through global model of anthropogenic methylchloroform CH3CCl4)
Methane has undergone large changes in the past due to climate modulation of methane sources
0
400
500
600
700
b
c
CH
4 (p
.p.b
.v)
50,000 100,000 150,000 200,000Age (yr BP)
250,000 300,000 350,000 400,000
-8
Tem
pera
ture
(oC
)
-6
-4
-2
0
2
Figure by MIT OpenCourseWare based on Nature Vol. 399, 1999.
Methane in the Vostok Ice Core
CH budget4
0
Observed 1992Expected 1992
Atmospheric increaseVariousTermitesOil and gas usageCoal miningDomestic sewageAnimal wastesBiomass burningLandfillsEnteric fermentationRice fieldsNatural wetlandsTotal sources
Soil uptake
Total sinksReaction with OH radicals
Transport into the stratosphere
Average 1983-1991
1992 anomaly100 200
Turnover (Tg yr-1)300 400 500 600 700
Figure by MIT OpenCourseWare.
CH4budget
Image removed due to copyright restrictions. Citation: see Table 4.2 IPCC report Climate Change 2001: The ScientificBasis on page 250.
Methane during the past 12,000 years
5502,0000 4,000 6,000 8,000 10,000 12,000
600
650
-39
�18 O
(
)
-38
-37
-36
-35
-34
100
200
300
400
500
600
700
800
900
1,00
0
Depth (m)
1,10
0
1,20
0
1,30
0
1,40
0
1,50
0
1,60
0
700
CH
4 (p
.p.b
.v.)
750
800
850
Figure by MIT OpenCourseWare.
High resolution records of CH4 for the past 110,000
years
used for ice core correlation
Image removed due to copyright restrictions. Citation: Figure 1. Brook, et al. Science 273 (August 23, 1996): 1089.
Inter-hemispheric CH4gradient existed for most of
the past 10,000 years(but gradient changed a bit)
Image removed due to copyright restrictions. Citation: Figure of "Interpolar CH4 gradient over the holocene." Chappellaz, J., T. Blunier, Kints S., A. Dälenbach, J. M. Barnola, J. Schwander,D. Raynaud, and B. Stauffer. “Changes in the Atmospheric CH4 Gradient between Greenland and Antarctic During the Holocene.” J Geophys Res 102 (1997): 15987-15997.
CH4 hydrates• Methane forms a solid cage-like compound (clathrate) with water under certain high-pressure/low-temperature regimes. It is estimated that a very large reservoir exists in this form globally - possibly larger than all other fossil fuel reserves. A significant advantage to using CH4instead of other fossil fuels is that it emits half the carbon per unit energy released. Is CH4 clathraterecoverable economically, or it similar to fusion power (always 50 years away)? One source estimated that the first commercial CH4 hydrate gas production may begin in 10-15 years (Science 303:944-947, 2004).
• Below the zone of methane hydrate stability (due to geothermal gradient), methane reverts to a gas phase and forms bubbles. The transition from solid methane hydrate to bubbles results in a strong acoustic impedance transition and a “bottom simulating reflector” (BSR).
Methane hydrate stability field:cold T, high P
Image removed due to copyright restrictions. Citation: Figure 1. Miles, P. R. “Potential Distribution of Methane Hydrate Beneath the European Continental Margins.” Geophys Res Lett 22 (1995): 3179-3182.
known methane hydrate deposits
Significant quantities of methane hydrates exist in Northern tundra/permafrost, so climate warming may result in enhanced methane releases from their decomposition.
Image removed due to copyright restrictions. Citation: Figure 4. Kvenvolden, K. A. Rev Geophys 31 (1993): 173-187.
climate, earthquake, and tsunami connections?
• There is some evidence that methane hydrates can decompose rapidly due to rising temperature and/or lower pressure due to falling sea level or glacial melting.
• Some modeling (C. Ruppel) suggests that it is difficult to decompose the methane hydrate quickly (because the reaction is endothermic and counteracts the diffusion of heat. So climate warming by itself may not be sufficient.
• There is evidence for a link between massive submarine landslides and methane hydrates. Could methane hydrate decomposition set off these landslides (and tsunamis)?
• A major submarine landslide in the Norwegian Sea 8200 years ago (“Storegga Slide”, which caused drastic tsunamis) occurred at the same time as a significant climate cooling seen in the Greenland ice cores (chicken, egg, or coincidence?). Earthquake triggering of slope failure (involving hydrate amplification) also has been suggested.
• It has been suggested that methane hydrate releases may have produced abrupt climate change during the past 60,000 years (J. Kennett).
Catastrophic releases of methane hydrates:
Seafloor “pockmarks” in the Arctic Ocean have been attributed to catastrophic methane hydrate decomposition
Image removed due to copyright restrictions. Citation: Figure 4. Henriet, J. P., and J. Mienert. Gas Hydrates,Relevance to World Margin Stability and Climatic Change.The Geological Society, London, 1998.
Some people have suggested that ships might have been lost at sea during hydrate releases (methane bubbles having reduced the density of water and hence the buoyancy of the ship).
StoreggaSlide
~8200 yrBP
Sent a 10 m high tsunami into the northern coast of Europe
? Triggered by earthquake and/or catastrophic CH4 hydrate decomposition?
Palecene/Eocene Thermal Maximum (PETM)
Clay layer occurring in deep sea sediments 2.7-4.8 indicates acidification of ocean by the CO2 released during this event (Zachos et al., 2005, Science 308:1611)
Image removed due to copyright restrictions. Citation: Figure 12. Nature 353: 225-229.
Runaway methane?
Image removed due to copyright restrictions.Citation: Figure 1. Prather, M. J. “Time Scales in Atmospheric Chemistry: Theory, GWPs for CH4 and C, and Runaway Growth.” Geophys Res Lett 23 (1996): 2597-2600.
