carbon dioxide cycling through the snowpack, implications of change
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Carbon dioxide cycling through the snowpack, implications of change. Gareth Crosby. CO 2 up north. Carbon dioxide (CO 2 ) largest component of carbon cycling between the biosphere and the atmosphere. approximately 45% of total greenhouse forcing - PowerPoint PPT PresentationTRANSCRIPT
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