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Climate feedback from wetland methane emissions
GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L20503, doi:10.1029/2004GL020919, 2004
N. GedneyHadley Centre, Met Office, Joint Centre for Hydro-Meteorological Research, Wallingford, UKP. M. CoxHadley Centre, Met Office, Exeter, UKC. HuntingfordCentre for Ecology and Hydrology, Wallingford, UK
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Outline
• Introduction into the global methane cycle
• Model examples• Presentation of the paper itself
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Methane sources/sinks
Total source approx. 600 Tg/year
Source: NASA/GISS
Main sink: OH radical (90%)Further: Oxidation in soil, transport to stratosphere
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What triggers methane formation
• Biological methane formation (70-80% of total source) is an anaerobic process, microbial digestion of organic matter (by methanogens)
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What triggers methane formation
• Most CH4 released by methanogens is oxidised by methanotrophs (less in wet conditions)
• There is still a temperature dependence since microbial activity strongly depends on T:higher T higher CH4 flux
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Methane cycle
• On a global scale in principle simple since the sinks are simple (90% oxidation by OH radicals)
• Total burden of the atmosphere: 4850 Tg (@1,745 ppb), lifetime approx. 8.6 years but dependent on [CH4] itself)
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Methane steady state, whole earth
[CH4] = 4850 Tg/Earth (1750 ppb), tau=8.6 years Flux = 560 Tg/year
][
][][
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44
CHF
CHF
dt
CHd
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Two hemispheres
• Tauhemisphere is approx 1 year
• The question is how much each hemisphere contributes to the total flux of methane and how this influences the N-S gradient
hemisphere
S
hemisphere
NSS
S
hemisphere
N
hemisphere
SNN
N
CHCHCHF
dt
CHd
CHCHCHF
dt
CHd
][][][][
][][][][
4444
4444
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Assuming 75% on northern hemisphereand 600 Tg/year total flux
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N-S difference [%] with respect to source strength
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Finally, the paper
• Idea: Methane flux is triggered by temperature and should thus exhibit a positive feedback on climate change
• Goal:1) parameter identification of this T-dependence from past climatological measurements2) extrapolation into the future until 2100
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Methods
• Temperature sensitivity Q10:factor by which flux increases at a 10° temperature increase (Literature: 1.7-16)
Global constant
Wetland fraction Soil carbon
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Model run
• Met office climate model coupled to land-surface scheme MOSES-LSH
• Methane emission scheme:
• Invert Q10 and total wetland flux from the methane time series of Dlugokencky (variability of human sources can be neglected, some major biomass burnig events taken into account)
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Model-Measurement RMS
3.7
297
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21st century projection
• Incorporation of the wetland model into the „Integrated Model of Global Effects on climatic aNomalies“ (IMOGEN)
• GCM model which allows climate feedback
• IPCC Scenario IS92a
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Results
Control run (CTL) total T increase 4.2°
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Conclusion
• Approximate doubling of wetland CH4 emission by 2004 (comparable to the IS92a projected increase)
• Radiative forcing accounts only for 0.14-0.2K (3.7-4.9% of total increase) small effect despite doubling of CH4
• Feedback of northern peatlands could be stronger but better knowledge of carbon cycling including CO2 AND CH4 is necessary
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