Vulnerability of frozen carbon

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Vulnerability of frozen carbon

• D.V. Khvorostyanov1,2, G. Krinner2, P. Ciais1,
• S.A. Zimov3
• 1Laboratoire des Sciences du Climat et
  l'Environnement,
• Gif-sur-Yvette, France
• 2Laboratoire de Glaciologie et Géophysique de
  l'Environnement,
• St Martin dHères, France
• 3Northeast Science Station, Cherskii, Russia

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Permafrost

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Permafrost melting
AnisimovNelson 1997
Oelke et al, GRL 2004 Active layer depth
increase 1980 2002
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NH Cryosols
7.8 mln km2 268 Gt (16 world soil organic C)
Soil C estimates top 1m only!
North America 3.6 mln km2 (46) 107 GtC
(40) Mean C content 31 kgC m-2
Eurasia 4.2 mln km2 (54) 162 GtC (60) Mean C
content 39 kgC m-2
Tarnocai et al, 2003
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Yedoma Ice Northeast Siberia

• 1-million km2 area of carbon-rich loess sediments
• Presumably 400 GtC at mean depth of 12 m and 33
  kgC m-3 density

Alekseev et al, Soil Science Society of America
Journal (2003)
Zimov et al, Science 1997
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Temperature dependence of biomass decomposition
One question, two answers D.Powlson, Nature
2005
One C pool (GlardinaRyan 2000)
Three C pools (Knorr et al 2005)
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Atmospheric warming feedbacks
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Soil Model Processes

• Heat conduction with freezing/thawing
• Hydrology
• Soil carbon consumption
• Oxic decompostion
• Methanogenesis
• Methanotrophy
• Diffusion of O2 and CH4
• Transfer of gases due to pressure difference
• Methane ebullition

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Holocene configuration comparison with
observations

• Methane fluxes
• Cherskii, summer 2003

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One point in Siberia...
The region of interest is Northeast Siberia, but

• First we test the model sensitivity and study in
  some detail the key processes providing the
  feedback
• These are local climate conditions that matter
  for this part of the study
• So we choose a point in the central southern
  Siberia but with soil configuration of Yedoma Ice

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The surface forcing 1000 1000
Present-day climate
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Soil carbon balance
Indefinite integrals over time How much of the
soil carbon has been transformed in one of these
processes at a given time
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Some details
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Step forcing and soil response

• 3 types of simulations
• No oxygen limitation on the oxic decomposition
• Oxygen limitation, no methane
• Methanogenesis and methanotrophy included

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Step forcing and soil response

• Biomass decomposition and methanotrophy
• are accompanied by heat release to the soil
• occur without heat release

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Surface forcing 1000 125 1000
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Soil carbon consumption
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Model sensitivity analysis
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Sensitivity to respiration heat

• Threshold between 35 and 40 MJ kgC-1
• Very small changes in consumed C elsewhere
• Methane fraction grows very slightly

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Sensitivity analysis résumé
Control soil respiration and heat transfer
Control methanogenesis, methanotrophy
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Simulations for the Yedoma Ice region
About 2 GtC are consumed in the first 100 yrs, 4
GtC in 200 yrs
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Conclusions

• The model reasonably simulates methane fluxes on
  seasonal timescales
• The carbon consumption time scale is about a few
  centuries in response to 2xCO2 forcing
• Decomposition heat release can be essential for
  the positive feedback between the global warming
  and frozen soil response
• Availability of oxygen, methanogenesis, and local
  climate conditions determine its existence and
  parameters
• Model sensitivity is the largest with respect to
  the parameters determining soil heating,
  freezing/thawing, and respiration
• About 4 GtC are released in the atmosphere as CO2
  in the first 200 years after the rapid 2xCO2
  warming