Vulnerability of frozen carbon D.V. Khvorostyanov 1,2, G. Krinner 2, P. Ciais 1, S.A. Zimov 3 1 Laboratoire des Sciences du Climat et l'Environnement, Gif-sur-Yvette, France 2 Laboratoire de Glaciologie et Géophysique de l'Environnement, St Martin d’Hères, France 3 Northeast Science Station, Cherskii, Russia

Permafrost 22.8 millions km 2 or 23.9% NH land area Continuous permaforst as far as o N 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 increase 1980 – 2002

NH Cryosols 7.8 mln km Gt (16% world soil organic C) Soil C estimates: top 1m only! North America: 3.6 mln km 2 (46%) 107 GtC (40%) Mean C content: 31 kgC m -2 Eurasia: 4.2 mln km 2 (54%) 162 GtC (60%) Mean C content: 39 kgC m -2 Tarnocai et al, 2003

Yedoma Ice: Northeast Siberia 1-million km 2 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 consumption Oxic decompostion Methanogenesis Methanotrophy Diffusion of O 2 and CH 4 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: Present- day climate 2xCO 2

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:

Soil carbon consumption

Model sensitivity analysis Carbon (kgC m -2 ) releasead since the 2 CO 2 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 respiration and heat transfer Control methanogenesis, methanotrophy

Simulations for the Yedoma Ice region About 2 GtC are consumed in the first 100 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 2xCO 2 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 CO 2 in the first 200 years after the rapid 2xCO 2 warming