Slide order and Notes
Atmospheric Methane: How well can we apportion present sources and predict future
Presented at University of Washington Program on Climate Change Summer Institute,
Sept 11 – 13, 2007, Friday Harbor, WA
Introduction: Know that methane is an important greenhouse gas (2nd to carbon
dioxide; 3rd if we consider water vapor), that it has a Greenhouse Warming
Potential of 25 times that of CO2, and that the atmospheric concentration was
increasing at a rate of about 1% per year until the late 1990’s, when it leveled off
and even decreased. We don’t really know the reasons for the increase and the
leveling off. This talk deals with the global methane budget, which has no
predictive power, approaches that have been used to attribute, and finally, new
findings that have been published within the last year or so. Emphasize that there
is no methane cycle – methane’s fate is to be oxidized.
2. IPCC 2004
3. IPCC 2006
4. GWP from Wahlen’s review
5. Global average concentration increase from Blake/Rowland lab at UCI
6. NOAA flying carpet – rises in winter due to respiration; falls in summer due to
7. NOAA global average slide – note similarity to slide 5. The increase is genuine.
Rate of change panel below – time derivative of concentration time series
8. Carritt’s four R’s of geochemistry; Broecker’s chemical plant analogy.
9. Carbon cycle schematic – aim is to know the fluxes and transfers between
10. Rate measurements: Fluxes measured with chambers, eddy flux systems.
Methanogenesis not measured too often. Sulfate reduction and methane oxidation
measured with radioisotope tracers.
11. Reactions: – microbial methane sources. Competitive substrates require strict
anaerobic conditions. Non-competitive substrates may produce methane under
12. Reactions: abiotic methane sources. Thermal cracking/pyrolysis. Some 14CH4
produced by pressurized water reactors.
13. Reactions: microbial methane sinks
14. Reactions: photochemical sink.
15. Constraints. Ehhalt (1974) produced first global methane budget; did a
remarkably good job with literature values. Various workers continued to add
terms and amounts of methane to the budget, so Cicerone and Oremland (1988)
place constraints on the budget to limit the “my source is bigger than your source
syndrome”. Know the size of the budget well; task is to apportion the source
terms within the total.
16. Posible use of 13C-CH4 to discriminate between sources. Unfortunately, ranges
are large and various sources aren’t that distinctive. There is also isotope
fractionation that blurs things further.
17. Cicerione and Oremland (1988). Solurce terms and ranges. Large source terms
also have large ranges of uncertainty. Total is 540 Tg/yr.
18. Constraints: net budget; all methane (90% is oxidized by OH).
19. Inversions. Drive with winds and atmospheric transport model to determine OH
(principal sink) then adjust or tune source terms and locations until we obtain a
match or fit to flying carpet diagram.
20. Fung et al (1991) methane budget from inversion. Natural sources are in yellow,
anthropogenic sources are in red. Scenario 7 –they seemed happy with this one
due to a good match with seasonal changes in tundra signal.
21. Söhngen Cycle. Methane is oxidized close to where it is formed, a way to
estimate sizes of pre-atmospheric sinks. Unfortunately, framework of constraints
can’t be applied to consumption or total production terms.
22. E + C = P: sources where C=0; animals, biomass burning, coal production,
venting and flaring, methane goes directly to atmosphere with no opportunity to
Sources where C is large: wetlands, rice, ocean. Present 75 Tg/yr for Ocean C
could be much larger than 300. Consumption has a big impact on additions to
atmosphere, especially rice term and ocean.
23. Recently reported methane sources
24. Keppler et al. 2006. Huge source (62-235 Tg/yr. Even with variability
permitted by C&O budget, can’t accommodate term this large.
25. Lowe’s perspective piece. Raised spectre of making things worse with methane
additions by planting trees to remove CO2.
26. Australian challenge. Accepted measurements as valid, showed that scaling up
was incorrect, resulting in units of mass per time squared. Leaf mass and
photosynthesis based estimates were much more reasonable, 10 – 60 Tg/yr. CH4
production from trees not a problem.
27. Dueck et al showed rates were only 0.3% or those reported by Keppler.
Still not dead. Keppler wrote Scientific American article. Keppler remains
unrepentant, stands by small arithmetic error as the cause. Believes process is
real and important.
28. Thaw lakes: Katey Walter was on NPR day before yesterday. 3.8 Tg/yr is a
small term in the 500
Tg/yr global budget; make much of it being a large fraction of the 6 – 40 Tg/yr
tundra term. Carbon is old: 35K to 42.8K BP.
29. Lost City vents. Off-axis from spreading centers, brucite towers of 30 – 70 m.
Serpentinization source. Proscurowski showed methane is radiocarbon-dead.
30. Methane-consuming benthic communities. Live by oxidizing sulfide. Sulfide
results from anaerobic oxidation of methane. Consumption is 50% efficient in
Beggiatoa; 86% efficient in Calyptogena clam beds.
31. Black Sea shelf chimney. Made of bacteria and methane-derived calcium
carbonate spicules. Reach heights of 3 – 5 meters. Michaelis indicates that this
must be what life was like in the pre-oxygenated Earth. Russians tried to date like
32. FISH (Fluorescent In Situ Hybridization image of Archaea (red) and sulfate
reducer (green) association.
33. Structure I methane hydrate. Cell consists of two 6-sided faces and twelve 5-
sided faces. An ice-like water cage. When cavities are fully-filled, holds 106
volumes of methane per volume of water.
34. Marine hydrate stability. Hydrates are a dymanic reservoir, forming at bottom,
decomposing at top. Have no idea of decomposition rate. This is a major
35. Mud volcano cross section. Decomposing hydrates are believed to be source.
Estimate between 1000 and 100,000. Don’t know how frequently they erupt.
Milkov estimated 13 Tg/yr release under quiescent conditions and an additional
17 during eruptions. Almost none introduced at depths greater than 70 m makes it
36. Terrestrial methane hydrates. Not much gets out. Mallik well in Mackenzie delta
is joint industrial/government venture. Produced by evacuating to decompose
37. Fossil additions – Cariaco Basin
38. Fossil additions – Black Sea
39. Future work
42. Ocean surface maximum.
What can we do about reducing methane fluxes? Which terms can be
Animals – experiments with chromophores, which inhibit methanogenesis.
Turnover is so high and generation times are so low that effects don’t last long.
Emission decreases when food quality increases.
Wetlands – salt marshes don’t emit methane. Sulfate reducers outcompete
AMO may be occurring. Can add traces of sulfate to inhibit methanogenesis.
Rice – water management. Japanese rice emits much less methane because they
drain and dress fields in mid-season. In many places, water is too expensive mid-
season to do this.
Sulfate addition may also work.
Biomass Burning – not much can be done
Landfills – recovery of biogas is common in Europe, where waste is sorted,
inoculated. U.S. law allows no changes once waste is covered. Can’t add
water to sustain
Hydrates – under study as a possible energy source, particularly in India and
Coal production – a likely large term in the future as petroleum availability
declines and coal
gasification or liquefaction begins.