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N. Gedney

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

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N. Gedney

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  1. Climate feedback from wetland methane emissionsGEOPHYSICAL 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

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

  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. 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)

  7. Methane steady state, whole earth [CH4] = 4850 Tg/Earth (1750 ppb), tau=8.6 years  Flux = 560 Tg/year

  8. 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

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

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

  11. 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

  12. Global constant Wetland fraction Soil carbon Methods • Temperature sensitivity Q10:factor by which flux increases at a 10° temperature increase (Literature: 1.7-16)

  13. 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)

  14. Model-Measurement RMS 3.7 297

  15. 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

  16. Results Control run (CTL) total T increase 4.2°

  17. 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 CO2AND CH4 is necessary

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