Soil temperature response to global warming: implications for carbon content from thawing permafrost soils in North America

Dominik Wisser1, Sergei Marchenko2, Claire Treat1, Julie Talbot1, Steve Frolking1

1 Complex Systems Research Center, University of New Hampshire, Durham, NH, USA. 2 Geophysical Institute-Permafrost Lab, University of Alaska Fairbanks, Fairbanks, AK, USA.

Funding was provided by NSF through Collaborative Research: Identifying Hydroclimatic Regimes of Carbon Stability in Northern Peatlands: Holocene Data Analysis and Process-Based Modeling' (award NSF-ATM #0628399)

Key References:Marchenko, S., Romanovsky, V., and Tipenko, G.: Numerical Modeling of Spatial Permafrost Dynamics in Alaska, Ninth International Conference on Permafrost, Fairbanks, Alaska, 2008, 1125-1130Tarnocai, C., J. G. Canadell, E. A. G. Schuur, P. Kuhry, G. Mazhitova and S. Zimov (2009) :Soil organic carbon pools in the northern circumpolar permafrost region, Global Biogeochem. Cycles 23(2): GB2023.Webb, R. W., C. E. Rosenzweig and E. R. Levine (2000). Global Soil Texture and Derived Water-Holding Capacities, Oak Ridge National Laboratory Distributed Active Archive Center Wisser, D., B. M. Fekete, C. J. Vörösmarty, and A. H. Schumann, Reconstructing 20th century global hydrography: a contribution to the Global Terrestrial Network- Hydrology (GTN-H), Hydrol. Earth Syst. Sci. ., 13, 1-23, 2009

Results-Snow

Numerical simulationsWe used a modified version of GIPL 2.0 (Marchenko et al. 2008), a 1-D thermodynamic model that simulates soil temperature dynamics and the depth of seasonal freezing and thawing by solving 1D non-linear heat equations with phase change numerically. A grid cell resolution of 0.5 degrees (lat x lon) was used at a daily time step. Grid cell was partitioned into a peat and mineral soil fraction (Fig. 2). For peat soil, a wet and dry scenario with different water table depth was used, for mineral soils, soil moisture was simulated using the WBMplus hydrological model (Wisser et al. 2010). Simulations were driven by air temperature and precipitation from ECHAM5 A1B scenario data (ΔT = +5°C; ΔP = + 20% by 2100).

Conclusions• Peatlands have unique thermal and hydraulic properties that need to

be explicitly considered in a coupled hydrologic/ thermodynamical model

• Despite small changes in peat areas underlain by permafrost, a considerable volume of peat could be thawed

• An additional 670km3 (~20% of total peat volume of 3480 km3) of peatlands containing 33 Pg of C could be thawed by 2100

• An additional 10,000 km3 (~27% of the total min soil volume) could be thawed by 2100; assuming carbon content of 5 kgm-3, these contain ~50 Pg C

Future Work• Extend analysis to all Northern peatlands• Couple thermodynamic model with dynamic hydrology model to

simulate dynamics of water table and soil moisture variations in peatlands

Fig. 1: Peat volume in North America based on the NSCID database (Tarnocai et al. 2009), gridded to 50x50km raster cells. Assumed peat bulk density BD = 100kgm-3, carbon density =50%; average depth = ~3m, total peat volume 3480 km3 (98% in Canada). One third of peat areas underlain by permafrost under current conditions

Abstract• Northern peatlands cover 10% of the land north of 45°N and contain

~500 Pg carbon; a considerable fraction of this carbon pool is currently in permafrost and is biogeochemically relatively inert

• Almost one third of the peatland areas in North America are in permafrost regions

• A geospatially explicit representation of peat areas and peat depth from a recently-compiled database and a geothermal model was used to estimate northern North America soil temperature responses to predicted changes in air temperature

• Model output shows that 21st century temperature trends lead to an increase in the soil frost free season length (~40 days or 25%)

• Mean annual snow depth decreases over the next 100 years and partly offsets effects of warmer air temperatures on soil temperatures

• Active layer depth increases in the entire region. The insulating properties of peat lead to a smaller increase in peat soils compared to mineral soils; this is particularly the case for drier peat

• An additional 670 km3 of peat soils in North America, containing ~33 Pg C, could be seasonally thawed by the end of the century, representing ~20% of the total peat volume in the region; 50 Pg could be newly thawed in mineral soils

Data and Methods

Humic

Humic

Fibric

Fibric

Mesic

Mesic

Humic

Fig 2.: Conceptual overview of the water level and water content (θ) in peatlands (wet/dry scenario). Organic layer on top of mineral soil is estimated based on land cover and varies between 0 and 0.25 m). G0 is the geothermal heat flux, red circles indicate computation nodes. Mineral soil layer depth, snowpack depth, and bedrock depth are not drawn to scale

Soil moisture content in peat soil was set to 20% of saturation for fibric peat, and 50% for mesic peat. Water table for the dry scenario at 0.3 m depth (‘bog’), and 0.07 m (‘fen’) for the wet scenario, assumed to be constant over the simulation period (2001-2100).

Changes in the Active Layer Thickness (ALT)

Increase in the frost free period in peatlands

Area underlain by permafrost and thawed soil volume

Fig. 4: Time series of the domain mean simulated start and end of the freezing period at the surface for grid cells with peat soils and one standard deviation around the mean. Averaged over all grid cells containing peat. Start date defined as the day when 5-day average temperature at 5 cm depth changes to positive values. Frost free season increases by ~40 days (25%) over the next 100 years .

Fig. 5: Projected changes in the area underlain by permafrost at different depths (top) and seasonally thawed soil volumes (bottom) in the model domain. Peatland area underlain by permafrost at 0.5 m declines sharply but is relatively stable for permafrost at 5m. Seasonally thawed volume of peat increases from ~1850 km3 (47% of total peat volume) to 2500 km3 (64%) by 2100. Maximum thawed mineral soil increases from 20000 km3 to 30000 km3,i.e., from 50% to 80% of total mineral soil volume – mineral soil depth is taken from Webb et al (200) and varies between 0 and 9.1m (average 4.25m.

Fig 3.: Projected changes in the ALT for three different 10year time slices for dry peat, wet peat and mineral soils as a result of warmer air temperatures and decreasing snow pack depth.ALT increases in peat soils are smaller due to the insulating properties of peat. ALT > 2.0 m is masked out.

For average C content in peat (Fig. 1), these volumes contain 33 Pg of C; C content in mineral soils is highly variable. Assuming 5 kgm-3, additionally thawed mineral soil contains ~50 Pg of C. Changes C cycling are not only controlled by the transition from frozen to unfrozen but also by temperatures increases (even below freezing)