1

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

2

Outline

• Introduction into the global methane cycle

• Model examples• Presentation of the paper itself

3

Methane sources/sinks

Total source approx. 600 Tg/year

Source: NASA/GISS

Main sink: OH radical (90%)Further: Oxidation in soil, transport to stratosphere

4

What triggers methane formation

• Biological methane formation (70-80% of total source) is an anaerobic process, microbial digestion of organic matter (by methanogens)

5

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

6

7

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)

8

Methane steady state, whole earth

[CH4] = 4850 Tg/Earth (1750 ppb), tau=8.6 years Flux = 560 Tg/year

][

][][

4

44

CHF

CHF

dt

CHd

9

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

10

Assuming 75% on northern hemisphereand 600 Tg/year total flux

11

N-S difference [%] with respect to source strength

12

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

13

Methods

• Temperature sensitivity Q10:factor by which flux increases at a 10° temperature increase (Literature: 1.7-16)

Global constant

Wetland fraction Soil carbon

14

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)

15

Model-Measurement RMS

3.7

297

16

17

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

18

Results

Control run (CTL) total T increase 4.2°

19

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

20