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12.842 / 12.301 Past and Present Climate
Fall 2008
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12.842
Climate Physics and Chemistry
Fall 2008
Atmospheric Chemistry II:
Methane
ogenic
CH4
and
CO2
330
1900
CO2
1700
CH4
1500
310
1300
290
1100
270
250
900
1750 1800 1850 1900 1950
CH4 Concentration (ppbv)
CO2 Concentration (ppmv)
350
700
Year
Reconstruction of the CO2 and CH4 increases since the preindustrial
time 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).
Figure by MIT OpenCourseWare based on Science Vol. 259, 1993.
Methane is a strong greenhouse gas and
contributes to global warming
RELEVANCE TO CLIMATE
Global-Mean Radiative Forcing
(Wm-2)
3
Halocarbons
N2O
CH4
2
CO2
1
Tropospheric
aerosols - direct effect
Fossil
fuel soot
Solar
0
Tropospheric
ozone
Sulphate
-1
Stratospheric
ozone
Biomass
burning
Tropospheric aerosols - indirect effect
-2
High
Low
Confidence Level
Low Low Very Very Very Very
Low Low Low Low
Estimates of the globally and annually averaged anthropogenic radiative forcing
(in Wm-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
1850 to the present day.
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 oxic
environment (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
CH3CC13
180
160
Mixing Ratio (ppt)
140
(
ALE/GAGE/AGAGE
k1
OH + CH3CCl3
CH2CCl3 + H2O
k2
OH + CH4
CH3 + H2O
(
120
100
Results
80
�i =
60
40
�[i]dV
�ki[i][OH]dV
=
{
_ 0.3yr (CH3CCl3)
4.6 +
_ 0.6yr (CH4)
8.9 +
{
�k [i][OH]dV =
_ 0.6)x 105 cm-3
(9.7 +
< [OH] < = �kii[i]dV
78
80
82
84
86
88
90
Ireland
Oregon
Barbados
Samoa
Tasmania
92
94
96
Year
Determination of OH by Inverse Method
Figure by MIT OpenCourseWare. Adapted from Prinn, et al. Science 269 (1995): 187-192.
•
•
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.
CH4 (ppb)
1800
1700
1600
1500
1984
1985
1986
1987
90
60
30
0
Latitude
-30
-60
-90
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)
2
0
b
-2
-4
-6
CH4 (p.p.b.v)
700
-8
600
c
500
400
0
50,000
100,000 150,000 200,000 250,000 300,000 350,000 400,000
Age (yr BP)
Figure by MIT OpenCourseWare based on Nature Vol. 399, 1999.
Methane in the Vostok Ice Core
Temperature (oC)
Methane has undergone large changes in the past due
to climate modulation of methane sources
CH4 budget
0
100
200
300
400
Turnover (Tg yr-1)
500
600
700
Total sinks
Reaction with OH radicals
Soil uptake
Transport into the stratosphere
Total sources
Natural wetlands
Rice fields
Enteric fermentation
Landfills
Biomass burning
Animal wastes
Domestic sewage
Coal mining
Oil and gas usage
Termites
Various
Atmospheric increase
Average 1983-1991
Expected 1992
Observed 1992
1992 anomaly
Figure by MIT OpenCourseWare.
CH4
budget
Image removed due to copyright restrictions.
Citation: see Table 4.2 IPCC report Climate Change 2001: The Scientific
Basis on page 250.
Methane during the past 12,000 years
1,600
1,500
1,400
1,300
1,200
1,100
1,000
100
200
300
400
500
600
700
800
900
Depth (m)
-34
-36
-37
800
CH4 (p.p.b.v.)
-38
750
-39
700
650
600
550
0
2,000
4,000
6,000
8,000
10,000
Figure by MIT OpenCourseWare.
12,000
�18O (
850
)
-35
High resolution
records of CH4 for
the past 110,000
years
Image removed due to copyright restrictions.
Citation: Figure 1. Brook, et al. Science 273 (August 23, 1996): 1089.
used for ice core
correlation
Inter-hemispheric CH4
gradient 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 highpressure/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 CH4
instead of other fossil fuels is that it emits half the
carbon per unit energy released. Is CH4 clathrate
recoverable 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
Image removed due to copyright restrictions.
Citation: Figure 4. Kvenvolden, K. A. Rev Geophys 31 (1993): 173-187.
Significant quantities of methane hydrates exist in Northern tundra/permafrost, so climate
warming may result in enhanced methane releases from their decomposition.
Catastrophic releases of methane hydrates:
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).
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).
Storegga
Slide
~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)
Image removed due to copyright restrictions.
Citation: Figure 12. Nature 353: 225-229.
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)
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.