C32A-01
Carbon Dioxide Gas Exchange Through the Snowpack and Its Contribution to the Ecosystem Carbon Budget in a High-Elevation, Subalpine Forest
Snow cover significantly affects microbial activity in the soil underneath by influencing both respiration and soil nitrification and denitrification processes. Past studies have shown how this can alter ecosystem carbon budgets as climate change continues. In our project, we measured winter carbon flux from the soil through the snowpack near treeline at a 3345 m asl site in the Niwot Ridge Long-Term Ecological Research area in the Colorado Rocky Mountains for 3 winter seasons (2004 – 2006). Fick's law of diffusion was applied to calculate the carbon flux from measurements of gradients in gas concentrations and snowpack density. We also analyzed the influence of wind-pumping (or pressure-pumping) on carbon flux calculation, which Fick's law ignores. Generally, flux calculations using Fick's law without incorporating wind-pumping effects gave us an underestimate of the true carbon flux. We measured maximum snow depth at our study site to be 1.9 m in 2005 and 2.1 m in 2006. The total wintertime seasonal CO2 loss was 8.89 mol m-2 for 2004 and 7.48 mol m-2 for 2005. These values are about 2 times larger than those observed at a lower elevation (3021 m asl) flux tower approximately 3 miles from our site (4.03 mol m-2 from 2003 winter season), which is within a closed canopy forest dominated by subalpine fir, Engelmann spruce, and lodgepole pine.1 This and similar research presented previously have been done in high altitude alpine regions. We are now continuing our work at the University of Michigan Biological Station Ameriflux site, which is a low elevation (219 m asl) lake-side region near Pellston, MI to further study the environmental factors that determine the CO2 gas exchange through the snowpack. 1 Monson, R. K., S. P. Burns, M. W. Williams, A. C. Delany, M. Weintraub, and D. A. Lipson (2006), The contribution of beneath-snow soil respiration to total ecosystem respiration in a high-elevation, subalpine forest, Global Biogeochem. Cycles, 20, GB3030, doi:10.1029/2005GB002684.
C32A-02
A Comparison of Summer and Winter CO2 Fluxes From a Seasonally Snow-Covered High Alpine Meadow at Niwot Ridge, Colorado
Studies concerning CO2 emission from soil to the atmosphere are essential to quantify and understand the carbon source or sink strength of ecosystems and to develop a global carbon balance. Seasonally snow-covered subalpine soils have been shown to be a non-negligible source of CO2 even during the winter season. These relatively high wintertime CO2 fluxes are due to the accumulation of the seasonal snowpack, which provides thermal insulation for subnivean soils and therefore allows biological activity during the winter. In high elevation sites in Colorado, such as the one we studied, the snow-covered season can last six months or more and therefore under-snow CO2 production can have a significant contribution to the annual ecosystem respiration. Moreover, at these sites, snowmelt is the main process providing water and solutes to soils, that usually dry out from June to September. In order to compare winter and summer fluxes, we measured CO2 fluxes continuously through the snow at a subalpine meadow in the Rocky Mountains, Colorado, during the winter 2006-2007 and made bi-weekly soil chamber flux measurements during the summer 2007. Ancillary measurements include soil temperature and soil moisture. During the winter, air samples were collected through the snowpack at different depths and fluxes were calculated using Fick's law. In the summer, measurements were done using an open chamber design. While the summer measurements currently are still underway, our first, preliminary data indicate a decreasing trend in CO2 fluxes during the drying-out period from July to September.
C32A-03
Tundra Soil-Water Content and Temperature Behavior and Implications for Winter Tundra Travel
Unfrozen soil-water content was monitored in the upper meter of tundra soils, using TDR sensors at several locations on the North Slope of Alaska and in the Brooks Range foothills. In addition, soil temperature was monitored to a depth of 1.5 m at these locations using thermistors. Particular attention was paid to soil water and temperature behavior during freezing and thawing conditions. The upper organic layer of soil often exhibited very wet conditions and showed much greater temporal variability than the lower mineral soil layers. Permafrost acts as a barrier to water flow, so the soils usually are wet as they thaw in the spring. Boundaries between soil layers usually are very irregular and the soil materials are mixed due to churning from frost heaving. Soil-water content sensors integrate soil-water content over a relatively large volume compared to the essentially point measurements of the thermistors used to measure soil temperature. Anyone who has worked in the field knows how difficult it is to place sensors at an exact depth. Soil-surface roughness and vegetation under tundra conditions make accurate placement almost impossible. Minor discrepancies between soil-water freezing and thawing behavior should be expected. However, an overall picture of the annual soil-freezing processes still can be described by these matched sets of sensor observations. In addition to this data, general meteorological and snow depth data is collected. Results of this study may be useful in improving tundra travel guidelines. Currently, tundra travel is allowed if the soil temperature in the upper 30 cm of soil is colder than -5C. Soil water content and resulting ice bonding in the soil matrix does impact soil properties and the resulting impacts of tundra travel.
C32A-04
Soil Temperature Reemergence in Permafrost
Soil temperature reemergence is the disappearance and subsequent reappearance of near surface soil temperature anomalies, driven by soil freeze-thaw processes. Reemergence of past soil temperature anomalies is a new class of time-delayed, land-atmosphere feedbacks influencing surface fluxes of latent and sensible heat. Anomalous energy is stored, isolated from diffusion processes, as variations in latent heat of fusion. Schaefer et al. [2007] found that past soil temperature anomalies in seasonally frozen soils are stored as variations in the amount of ground ice and can reemerge at the surface after soil thaw in spring. Schaefer et al. [2007] also hypothesized that temperature anomalies in permafrost would be stored as variations in the active layer depth, reappearing after the soil column completely freezes in winter. Essentially, a warm summer produces a deeper active layer, which requires more energy to freeze in autumn, resulting in warmer soils in winter. Here, we explore this hypothesis using statistical analysis of long-term, in situ soil temperature measurements at 37 permafrost hydro-meteorological stations across Siberia. The observations span 30-40 years at depths of 2-320 cm. We also use a simple soil thermodynamic model with phase changes to explore the detailed thermodynamic processes driving temperature reemergence in permafrost.
C32A-05
Modeling permafrost and permafrost-related climate-change feedbacks in a GCM: Sensitivity to soil column depth and representation of soil organic matter
The sensitivity of a global land-surface model projection of near-surface permafrost degradation is assessed with respect to explicit accounting of the thermal and hydrologic properties of soil organic matter and to a deepening of the soil column from 3.5 to 50 or more meters. Together, these modifications result in substantial improvements in the simulation of near-surface soil temperature in the NCAR Community Land Model (CLM) which is the land surface model for the Community Climate System Model (CCSM) and the Community Atmosphere Model (CAM). When forced offline with archived data from a fully coupled CCSM simulation of 20th century climate, the revised version of CLM produces a near-surface permafrost extent (10.7 million km2 north of 45°N) that is improved over the standard model (8.5 million km2) and compares reasonably (although still biased low possibly due to biases in soil temperature caused by CCSM3 air temperature and/or snow depth biases) with observed estimates for continuous and discontinuous permafrost area (11.2-13.5 million km2). The rate of near-surface permafrost degradation, in response to the strong simulated Arctic warming (~ +7.5°C over Arctic land, 1900 - 2100; A1B greenhouse gas emissions scenario), is slower in the improved version of CLM, particularly during the early 21st century (81,000 km2 yr-1 versus 111,000 km2 yr-1). Even at the depressed rate, however, the warming is enough to drive near-surface permafrost extent sharply down by 2100. Experiments with a deep soil column exhibit a larger increase in ground heat flux than those without due to stronger near-surface vertical soil temperature gradients. This appears to lessen the sensitivity of soil temperature change to soil depth. Additional improvements to and features of CLM that are relevant to permafrost degradation related climate- change feedbacks will also be reviewed including those to snow, wetlands and lakes, carbon-nitrogen cycling, and dynamic vegetation biogeography.
C32A-06
Simulating the Terrestrial Cryosphere in a Regional Climate Model
The Canadian Regional Climate Model has been coupled with version 3 of the Canadian Land Surface Scheme. The new scheme includes a number of improvements of relevance to the terrestrial cryosphere, including the treatment of snow density, canopy interception and unloading, and turbulent exchange with the atmosphere. The impact of these improvements on the regional climate is evaluated in a 10-year simulation over western Canada. The modeled precipitation and surface air temperature were evaluated against a gridded observed monthly surface climate dataset produced by Environment Canada. In addition, the simulated snow water equivalent and snow cover fraction are compared with Special Sensor Microwave/Imager derived estimates over two contrasting surface types (boreal forest and cropland). Modeled results also suggest a link between the Pacific-North American (PNA) teleconnection pattern and the variation of snow cover over western Canada. Composite analysis shows a negative correlation between the PNA phase and snow cover over the region. Local thermo-dynamic and dynamic processes associated with this relationship are also examined.
C32A-07
Thaw-Subsidence Measurements in the Circumpolar Active Layer Monitoring (CALM) Program
Vertical movement of the ground surface due to frost heave and thaw subsidence is a common phenomenon in permafrost regions. The magnitude of this movement varies both temporally and spatially, owing to interannual climatic variability at the ground surface and to local variations of soil moisture. Because measurements at point locations made using frost/thaw tubes at CALM sites during the 1990s indicated that penetration of thaw into the ice-rich transient layer may not be reflected in records of active-layer thickness, spatial sampling experiments were initiated early in this decade at several sites in Alaska and Russia. Differential Global Positioning Systems (DGPS) technology was employed at sites in the northern Brooks Range Foothills and on the Arctic Coastal Plain of northern Alaska. Traditional theodolite survey methods were used at sites in the European Russian Arctic. The resulting heave/subsidence records, in conjunction with temperature and active-layer measurements, were used to evaluate regional and site-specific factors affecting the spatial and temporal variability of frost heave and subsidence. Heave and settlement show patterns of spatial variation similar to those of active-layer thickness (ALT). Results from all locations indicate a monotonic increase in thaw penetration into the upper ice-rich permafrost during the period of observation. The CALM II (2004-09) measurement protocol accommodates long- term subsidence measurements, and instrumentation is being developed for deployment at most CALM observatories.
C32A-08
Thermokarst Distribution in the Noatak Basin, Alaska: Increased Frequency and Correlations with Local and Regional Landscape Variables
In arctic regions, climate warming is leading to permafrost melting and wide-scale ecosystem alteration. A prominent pathway of permafrost loss is through thermokarst processes, which includes the catastrophic loss of soil structure and rapid subsidence. Regional-scale distribution of thermokarst features is poorly documented throughout the arctic, and correlations with landscape variables is not well understood. The Noatak Basin in northwestern Alaska's Brooks Range mountains harbors a transitional landscape from arctic and alpine tundra to boreal forest within a 7,000,000 acre watershed. Field investigations augmented by photogrammetric measurements from 2005 to 2007 revealed patterns in the distribution of classifiable thermokarst failure types in the Noatak Basin, and provided data on the physical and chemical impacts these features have on aquatic systems. Distinct thermokarst classes show significant relationships with local site variables such as slope and vegetation, and with regional variables including lithology, glacial geology and landcover. Frequency of thermokarst features has increased markedly in several core study areas in the Noatak Basin within the past 30 years. Analysis of current and historical aerial photographs shows two to three fold increases in number of features present, and in total surface area of landscape affected. The core study areas are spread along a gradient from the upper to lower Noatak Basin covering a number of major land cover types. The majority of these features occur in headwaters of Noatak tributaries and can have marked impacts on small headwater streams, which have less capacity to buffer the effects of disturbance. These studies show that thermokarst processes and effects, especially in headwater regions, have been vastly under-reported in the Noatak Basin. These findings suggest that similar phenomena may be under-reported in other permafrost regions as well, due in part to the logistical difficulty of conducting quantitative surveys in remote areas with rugged topography.