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, France2Laboratoire de Glaciologie et Géophysique de l'Environnement,

St Martin d’Hères, France3Northeast Science Station, Cherskii, Russia

Permafrost

22.8 millions km2 or 23.9% NH land area Continuous permaforst as far as 50-60oN to the northeast of Lake Baikal 63% mainly in Siberia, Russian Far East, Northern Mongolia, Northeastern China

Continuous (90-100% area)Discontinuous (50-90% area)Sporadic (10-50% area)Isolated Patches (<10% area)

Permafrost melting

•12-22% all types•12-34% continuous

Area decrease by 2050:

Anisimov&Nelson 1997

Oelke et al, GRL 2004:Active layer depth increase1980 – 2002

NH Cryosols7.8 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

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

Zimov et al,Science 1997

Alekseev et al, Soil Science Society of America Journal (2003)

Temperature dependence of biomass decomposition

One C pool (Glardina&Ryan 2000)

Three C pools (Knorr et al 2005)

«One question, two answers»D.Powlson, Nature 2005

Goulden et al (1998) measurements: permafrost thaw =>

10-fold increased decomposition

Atmospheric warming feedbacks

Soil Model Processes

Heat conduction with freezing/thawing

Hydrology

Soil carbon consumptionOxic decompostion

Methanogenesis

Methanotrophy

Diffusion of O2 and CH4

Transfer of gases due to pressure difference

Methane ebullition

Holocene configuration: comparison with observations

Methane fluxes Cherskii, summer 2003

One point in Siberia...

• 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

The region of interest is Northeast Siberia, but…

The surface forcing: 1000 + 1000

Present-day climate

2xCO2

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

Some details

Step forcing and soil response

3 types of simulations: No oxygen limitation on the oxic

decomposition Oxygen limitation, no methane Methanogenesis and

methanotrophy included

Step forcing and soil response

Biomass decomposition and methanotrophy

➔ …are accompanied by heat release to the soil

➔ …occur without heat release

Surface forcing: 1000 + 125 + 1000

Soil carbon consumption

Model sensitivity analysis

Carbon (kgC m-2)

releasead since

the 2 CO2 warming

Accumulated surface

methane flux over

the same time

Sensitivity to respiration heat

Threshold between 35 and 40 MJ kgC-1

Very small changes in consumed C elsewhere

Methane fraction grows very slightly

Sensitivity analysis résumé

Control soil respirationand heat transfer

Control methanogenesis,methanotrophy

Simulations for the Yedoma Ice region

About 2 GtC areconsumed in the first100 yrs, 4 GtC in 200 yrs

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