Transcript Nonlinear DOAS - uni

Climate feedback from wetland methane
emissions
GEOPHYSICAL RESEARCH LETTERS, VOL. 31,
L20503, doi:10.1029/2004GL020919, 2004
N. Gedney
Hadley Centre, Met Office, Joint Centre for Hydro-Meteorological Research,
Wallingford, UK
P. M. Cox
Hadley Centre, Met Office, Exeter, UK
C. Huntingford
Centre 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
Main sink: OH radical (90%)
Further: Oxidation in soil,
transport to stratosphere
Source: NASA/GISS
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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
d [CH 4 ]
[CH 4 ]
F
dt

F  [CH 4 ]
[CH4] = 4850 Tg/Earth (1750 ppb), tau=8.6 years
 Flux = 560 Tg/year
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Two hemispheres
N
4 S
4 N
d [CH 4 ]N
[
CH
]
[
CH
]
[
CH
]
N
4
F 


dt

 hemisphere  hemisphere
S
4 N
4 S
d [CH 4 ]S
[
CH
]
[
CH
]
[
CH
]
S
4
F 


dt

 hemisphere  hemisphere
• 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
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Assuming 75% on northern hemisphere
and 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 measurements
2) 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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16
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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