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Global modelling of methane and wetlands:
Past, present and future.
Nic Gedney (Met Office, Hadley Centre (JCHMR))
(Pete Cox, Hadley Centre and Chris Huntingford, CEH Wallingford)
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
Methane currently the 2nd largest contributor to anthropogenic
greenhouse effect.
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Wetlands currently largest single source of CH4
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Wetland emissions sensitive to climate
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Currently high latitudes are a significant CH4 source
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Large changes in climate are predicted at high latitudes
Source: CEH Wallingford
@British Crown Copyright 2004
Overview
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Rationale and introduction

Wetland CH4 Emissions determinants

Modelling techniques
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Modelling Studies:
– Present Day
– Last Glacial Maximum
– Future
– Present Day -> Future
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Remaining issues and possible ways forward
Rationale

We need:
– Accurate estimate of global wetlands CH4 budget – past and
present
– Understand role of wetlands in CH4 variability –past and present
– Model necessary physical processes
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-> Confident in future projections
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Where are we now?

How far are we limited by observations?

What are the likely next steps?
Historical atmospheric methane concentrations
Climate Change 2001. IPCC Third Assessment Report.
Working Group I: The Scientific Basis. Figure 4.1
Figure 4.1: (e) Atmospheric CH4
abundances (black triangles) and
temperature anomalies (grey diamonds)
(Petit et al., 1999).
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Figure 4.1: (a) Change in CH4 abundance
(mole fraction, in ppb = 10-9)
Blunier et al. (1995) and Chappellaz et al. (1997);
Blunier et al. (1993); Chappellaz et al. (1997);
Stauffer et al. (1985);
Etheridge et al. (1998), Dlugokencky et al. (1998).
Historical atmospheric methane concentrations
(b) Globally averaged monthly
varying CH4
(c) Instantaneous annual growth rate
(Dlugokencky et al., 1998).
Present Day sources:
Anthropogenic: 200-350 TgCH4yr-1
Total:
500-600
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Climate Change 2001.
IPCC Third Assessment Report.
Working Group I: The Scientific Basis.
Figure 4.1
Main Sink (tropospheric OH):
CH4+OH→CH3+H2O
Wetland CH4 Emissions determinants

Temperature:
– Usually described by Q10(T/10)
– Q10  1.5-16
(Walter and Heinmann, 2000)
» Northern wetlands: Q10  5
» Rice: Q10  1.5-3
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(Christensen et al. 2003)
(Khalil et al. 1998)

Water table height
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Substrate availability and quality
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(Vegetation composition)
Modelling Techniques
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Bottom-up – process based
estimates of the CH4 source
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Top-down – measured CH4 concs
-> CH4 sources
CH4 budget scenarios
Modelled atmos
CH4 conc
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(Walter and Heimann 2000)
Minimise errors
Atmos chemistry
Modelling Studies:
Present Day source and sink estimates
(Walter et al 2001)
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Present Day zonal mean CH4 flux estimates
(Walter et al. 2001)

25-45% from > 30o N
(Walter et al. 2001, Hein et al. 1997,Fung et al. 1991)
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Present day global inter-annual CH4 variability:
wetland flux vs biomass burning

1997-98 global source anomaly ~ 24 Tg CH4
Significant contribution from wetlands
(Dlugokencky et al. 2001)

!998 Boreal fire anomaly:
 2.9-4.7 Tg CH4 (Kasischke and Bruhwiler 2003)
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1997-1998 Indonesian fire anomaly:
 1.2-3.6 Tg CH4 (Levine 1999)
(Dlugokencky et al. 2003)
 5.0Tg CH4 (Duncan et al. 2003)
Other
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estimates >> (e.g. van der Werf et al. 2004)
Modelling Studies:
Last Glacial Maximum source and sink estimates
(Kaplan 2002)
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Climate Change

Model sensitivity studies:
Critical play-off between
temperature and moisture
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Obs:
both moistening and drying
with permafrost melt
(Christensen et al. 2004, Stow et al. 2004)
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Hypothetical tundra
response (Christensen and Cox 1995)
Using present day observed variability to reduce the uncertainty in future climate
change prediction
(Gedney, Cox and Huntingford)
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
Met Office Land Surface Scheme (MOSES) extended to include
interactive wetlands (soil moisture high resolution topography)
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Wetland CH4 emissions parameterisation calibrated from
observed inter-annual variability in atmos CH4 concentrations
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Impact of wetlands emissions on simulated transient climate
change studied using an “integrated climate change effects
model”
MOSES - LSH (Gedney and Cox 2003)
saturated fraction
influences runoff
MOSES soil
model updates
mean water table
Zsoil
Fs
Zw
mean water table plus
topographic index
determine saturated
fraction
Fs
pdf(TI)
subgrid orography determines
subgrid water table depths
TI
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Simulated annual mean wetland fraction
Off-line (Gedney and Cox 2003)
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Estimating CH4 emissions from wetlands
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FCH4=K. Csoil. fwetl.Q10(Tsoil) Tsoil/10
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f(wetl) - wetland fraction
Tsoil - soil temperature
Q10 - fn of temperature
Csoil - soil carbon content
K - global constant
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 Calibrate global flux and Q10(T):
– Force with observed monthly anomalies of T and precip.
– Use simple global atmospheric chemistry lifetime model:
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d(CH4)/dt =  FCH4 – CH4/
– Include estimate of biomass burning variability
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Modelled inter-annual variability in CH4 emissions
(FCH4 ~ 325TgCH4yr-1, q10(T0)~3-4)
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Temperature data: Jones et al., 2001. Precip data: Xie and Arkin, 1998
CH4 flux derived from atmos CH4 conc data: Dlugokencky (pers com)
Calibrating the wetland CH4 parameterisation (1992-1999)
Modelled varying wetland:
RMS Error
Minimum RMS Error:
FCH4=295-325TgCH4yr-1, Q10(T0)=3.2-3.8
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Schematic diagram of IMOGEN
(Integrated Model Of the Global Effects of climate aNomalies or “GCM
analogue model”
Base
Climate

Anomaly
Patterns

scale factor
Atmospheric
Greenhouse Gas
Concentrations
Forcing:
T, q, u, Sd, Ld
Fluxes:
CO2, CH4
Land Surface Scheme
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Anthropogenic
emissions
Transient climate change predictions
(Annual mean, land average)
FCH4=325TgCH4yr-1, Q10(T0)=3-4
IS92A projected anthropogenic increases ~ 400TgCH4yr-1
Predicted wetlands emission increases are close to anthropogenic
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3-5% increase in radiative forcing
Remaining issues and possible ways forward

Observations:
– Lack of observations over tropics
– Isolating effects of obs drivers on flux (T, Zw etc)
– Separate flux components (production, oxidation not just net)
– Substrate availability
-> Parameters for process-based models
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Suitable models:
– High lats: peat soils, non-vascular plants
– Tropics: peat soils, seasonal flooding
– Wetlands hydrology
– Permafrost
-> reproduce current wetlands and recent regional responses to
permafrost melting
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Remaining issues and possible ways forward –contd.

Further constrain process based models:
– Present Day global mean budget uncertainty
– Present Day variability: relative roles of biomass burning and
wetlands
– LGM: disagreement over sources
-> Top down studies incorporating more process based models
(include isotopic signatures)
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Studies on future climate change:
– GCM-analogue model?
– And finally……
Fully coupled Earth Systems Model (wetlands + atmos chemistry)
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Acknowledgements
DEFRA

Copyright

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Met Office