Carbon dioxide cycling through the snowpack, implications of

changeGareth Crosby

CO2 up north

• Carbon dioxide (CO2)

– largest component of

carbon cycling between the

biosphere and the atmosphere.

– approximately 45% of total

greenhouse forcing

– industrial revolution = more than a 30% increase in atmospheric concentration

• Approximately 40% of the world’s soil carbon is stored in high-latitude ecosystems.

Seasonality

• CO2 exchange during the growing season only

represents 4 to 5 months of the year at most.

• growing season CO2 data alone have been found to

underestimate the actual magnitude of CO2 flux

from northern soils

• Cold season CO2 emissions through the snowpack

can contribute as much as 60%-81% of annual release.

• Soil thermal dynamics influence the exchange of CO2 between terrestrial ecosystems and the atmosphere.

• Field-based studies indicate both the importance of winter decomposition and freeze-thaw dynamics in the annual carbon budget in northern ecosystems.

Snow Characteristics: Insulation

• covers 44 to 53% of the northern latitudes during most of the year

• Insulation = structure +

depth of snowpack

• Deeper = greater insulation

• Less packed = greater

insulation

Snow Characteristics: CO2 release

• Snowpack properties – depth, density, and layering dictate the rate and amount

of CO2 evolution from the soil.

• Release = structure and depth• layering = compactness, ice lenses and crusts • trapping the gas below lenses makes calculating

fluxes difficult. • In a homogeneous snowpack, gas transport by

diffusion through the snow profile is linear.

Snow Characteristics: CO2 release

• Intense wind and high temperature gradients can cause mass transport of gas by convection

• Windpumping – three types of windpumping:

• barometric pumping

• turbulent pumping

• topographic pumping.

Soil structure and composition also influence rate and distribution of CO2 production and

movement in the soil layer.

• Porous soils = more CO2 to move upwards

– frozen soils will become better traps for CO2

produced in lower soil layers

• soils with high levels of organic matter will produce more CO2 than soils depleted in

organic matter at a given temperature.

Models

• Most models use monthly air temperature in the simulation of of seasonal dynamics of net primary production and decomposition.

• But many of these models have been predicting substantially different results

Model inputs

Hydrological Dynamics

Biogeochemical Dynamics

Soil Thermal Dynamics

Vegetation Characteristics

Snow Depth

Snow Properties

Soil Temperatures

Freeze-thaw Dynamics

Version 5.0 of the Terrestrial Ecosystem Model (TEM) Zhuang et al., 2004)

Based on WBM

Based on TEM

Based on a STM

Freeze-thaw• In the temperate soils are prone to freezing. • microbes are killed by freezing • snow cover regulates temperatures that enable microbes that

survived the freezing in lower soil layers to multiply• use the dead microbes in the upper layers to produce a pulse of

CO2 under the snow

• though early season development of snowpack allows production of CO2 to continue for most of the winter, sites that undergo hard freezes and then are covered with a consistent snowpack and allowed to thaw produced the highest fluxes.

Implications

• Lower average snowpack will increase the rate and depth of freezing in northern soils with longer lasting effects than simple wintertime gas fluxes.

• Frozen soils will decrease the production and movement of CO2 up from the soil thus possibly

becoming less of a source and more of a sink for CO2 during the winter months.

Further Implications

• At the same time the freezing and melting of soils in the spring could produce a larger spring pulse of CO2 emission as microbes that lasted the winter in deeper soils began to use the dead microbes as substrate for decomposition.

• Depending on the degree to which each of these processes affect the production or trapping of CO2

northern ecosystems could become less of a source in the winter and spring or less of a source in the winter but more of a source in the spring