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Carbon dioxide cycling through the snowpack, implications of change

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

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Carbon dioxide cycling through the snowpack, implications of change

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  1. Carbon dioxide cycling through the snowpack, implications of change Gareth Crosby

  2. 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.

  3. 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 contributeas much as 60%-81% of annual release.

  4. 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.

  5. 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

  6. 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.

  7. 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.

  8. 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.

  9. 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

  10. Model inputs Vegetation Characteristics Snow Depth Snow Properties Hydrological Dynamics Based on WBM Soil Thermal Dynamics Based on a STM Biogeochemical Dynamics Soil Temperatures Freeze-thaw Dynamics Based on TEM Version 5.0 of the Terrestrial Ecosystem Model (TEM) Zhuang et al., 2004)

  11. 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.

  12. 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.

  13. 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

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