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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)

Methane currently the 2nd largest contributor to anthropogenic greenhouse effect.

Wetlands currently largest single source of CH4 Wetland emissions sensitive to climate Currently high latitudes are a significant CH4 source Large changes in climate are predicted at high latitudes

Source: CEH Wallingford@British Crown Copyright 2004

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Overview

Rationale and introduction Wetland CH4 Emissions determinants Modelling techniques Modelling Studies:

– Present Day

– Last Glacial Maximum

– Future

– Present Day -> Future

Remaining issues and possible ways forward

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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

-> Confident in future projections

Where are we now? How far are we limited by observations? What are the likely next steps?

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Historical atmospheric methane concentrations

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).

Figure 4.1: (e) Atmospheric CH4abundances (black triangles) and temperature anomalies (grey diamonds)

(Petit et al., 1999).

Climate Change 2001. IPCC Third Assessment Report.Working Group I: The Scientific Basis. Figure 4.1

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Historical atmospheric methane concentrations

(b) Globally averaged monthlyvarying CH4(c) Instantaneous annual growth rate(Dlugokencky et al., 1998).

Climate Change 2001.IPCC Third Assessment Report.Working Group I: The Scientific Basis.Figure 4.1

Present Day sources:Anthropogenic: 200-350 TgCH4yr-1

Total: 500-600

Main Sink (tropospheric OH):CH4+OH→CH3+H2O

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Wetland CH4 Emissions determinants

Temperature:

– Usually described by Q10(T/10)

– Q10 1.5-16 (Walter and Heinmann, 2000)

» Northern wetlands: Q10 5 (Christensen et al. 2003)

» Rice: Q10 1.5-3 (Khalil et al. 1998)

Water table height

Substrate availability and quality

(Vegetation composition)

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Modelling Techniques

Bottom-up – process based estimates of the CH4 source

CH4 budget scenarios

Atmos chemistry

Modelled atmos

CH4 conc

Min

imise

erro

rs

Top-down – measured CH4 concs

-> CH4 sources

(Walter and Heimann 2000)

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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

(Dlugokencky et al. 2003)

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)

1997-1998 Indonesian fire anomaly: 1.2-3.6 Tg CH4 (Levine 1999)

5.0Tg CH4 (Duncan et al. 2003)

Other estimates >> (e.g. van der Werf et al. 2004)

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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

Obs:

both moistening and drying

with permafrost melt(Christensen et al. 2004, Stow et al. 2004)

Hypothetical tundra response (Christensen and Cox 1995)

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Using present day observed variability to reduce the uncertainty in future climate change

prediction(Gedney, Cox and Huntingford)

Met Office Land Surface Scheme (MOSES) extended to include interactive wetlands (soil moisture high resolution topography)

Wetland CH4 emissions parameterisation calibrated from observed inter-annual variability in atmos CH4 concentrations

Impact of wetlands emissions on simulated transient climate change studied using an “integrated climate change effects model”

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Fs

Fs

Zsoil Zw

TI

pdf(TI)

MOSES - LSH (Gedney and Cox 2003)

saturated fraction influences runoff

mean water table plustopographic index

determine saturated fraction

MOSES soil model updates

mean water table

subgrid orography determinessubgrid water table depths

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Simulated annual mean wetland fraction

Off-line (Gedney and Cox 2003)

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Estimating CH4 emissions from wetlands

FCH4=K. Csoil. fwetl.Q10(Tsoil) Tsoil/10

f(wetl) - wetland fraction Tsoil - soil

temperature Q10 - fn of temperature Csoil - soil carbon

content K - global constant

Calibrate global flux and Q10(T): – Force with observed monthly anomalies of T and precip.– Use simple global atmospheric chemistry lifetime model:

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)

Temperature data: Jones et al., 2001. Precip data: Xie and Arkin, 1998CH4 flux derived from atmos CH4 conc data: Dlugokencky (pers com)

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Calibrating the wetland CH4 parameterisation (1992-1999)

Minimum RMS Error:FCH4=295-325TgCH4yr-1, Q10(T0)=3.2-3.8

Modelled varying wetland:RMS Error

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Schematic diagram of IMOGEN(Integrated Model Of the Global Effects of

climate aNomalies or “GCM analogue model”

BaseClimate

AnomalyPatterns

Atmospheric Greenhouse Gas Concentrations

Land Surface Scheme

scale factor

Forcing:T, q, u, Sd, Ld

Fluxes: CO2, CH4

Anthropogenic emissions

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Transient climate change predictions(Annual mean, land average)

IS92A projected anthropogenic increases ~ 400TgCH4yr-1

FCH4=325TgCH4yr-1, Q10(T0)=3-4

Predicted wetlands emission increases are close to anthropogenic

3-5% increase in radiative forcing

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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

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)

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

Met Office