Quantifying net CO2 exchange (NEE) of arctic terrestrial ecosystems in response to changes in climatic and environmental conditions is central to understanding ecosystem functioning and assessing potential feedbacks of the carbon cycle to future climate changes. However, annual CO2 budgets for arctic tundra are rare due to the difficulties of performing measurements during non-growing seasons. It is still unclear to what extent arctic tundra ecosystems currently act as a CO2 source, sink or are in balance. This study presents year-round eddy-covariance (EC) measurements of CO2 fluxes for an arctic heath ecosystem on Disko Island, West Greenland (69 degrees N) over five years. Based on a fusion of year-round EC-derived CO2 fluxes, soil temperature and moisture, the process-oriented model (CoupModel) has been constrained to quantify an annual budget and characterize seasonal patterns of CO2 fluxes. The results show that total photosynthesis corresponds to -202 +/- 20 g C m(-2) yr(-1) with ecosystem respiration of 167 +/- 28 g C m(-2) yr(-1), resulting in NEE of -35 +/- 15 g C m(-2) y(-1). The respiration loss is mainly described as decomposition of near- surface litter. A year with an anomalously deep snowpack shows a threefold increase in the rate of ecosystem respiration compared to other years. Due to the high CO2 emissions during that winter, the annual budget results in a marked reduction in the CO2 sink. The seasonal patterns of photosynthesis and soil respiration were described using response functions of the forcing atmosphere and soil conditions. Snow depth, topography-related soil moisture, and growing season warmth are identified as important environmental characteristics which most influence seasonal rates of gas exchange.

Permafrost is vulnerable to rapid changes in climate, and increasing air temperatures have recently resulted in the increase of active layer thickness, thaw subsidence and warming of the underlying permafrost. Such changes have important implications for geotechnical properties and the stability of infrastructures in permafrost-affected areas. Many studies focus on the sensitivity of the active layer with respect to changes in climate conditions, but few assess the sensitivity of active layer thermal properties in relation to sediment types and soil water contents, and the importance of direct measurements of thermal property sensitivity with respect to soil water content compared to default physical relationships incorporated in process-based models. In this study, we use on-she data and samples to measure thermal conductivity (TC) a different gravimetric water/ice contents (GWC) in frozen and thawed permafrost. The samples, obtained from an emerged delta and an alluvial fan in the Zackenberg Valley, NE Greenland, are characterized by contrasting grain-size distribution and mineralogy. We calibrated a coupled heat and water transfer model, the CoupModel, to simulate permafrost temperatures a two sites on the delta. The sites have different snow depth characteristics and were simulated using both observed and default values of TC, and observed liquid soil water content. The results show that depth- and sediment type-specific TC values are crucial for a successful model simulation, and that transfer function derived values of TC are useful for modeling permafrost temperatures as long as site- and depth-specific grain size distribution and ice contents are defined. A thicker snow pack increased ground surface temperatures and resulted in a 1 degrees C higher annual mean ground temperature a the depth of zero annual amplitude. Permafrost temperatures increased by 1.5 degrees C and 3.5 degrees C a the depth of 18 m with 3 degrees C and 6 degrees C ground surface warming, but warming combined with increased soil water content had no important additional effect on the thermal regime when ground surface temperatures were prescribed as upper boundary conditions. Precipitation in the form of snow, however, may have a larger effect on ground temperatures directly, due to the surface temperature changes, than will the subsequent changes in thermal properties following increase in soil water content.

Hydrothermal processes are key components in permafrost dynamics; these processes are integral to global warming. In this study the coupled heat and mass transfer model for (CoupModel) the soil-plant-atmosphere-system is applied in high-altitude permafrost regions and to model hydrothermal transfer processes in freeze-thaw cycles. Measured meteorological forcing and soil and vegetation properties are used in the CoupModel for the period from January 1, 2009 to December 31, 2012 at the Tanggula observation site in the Qinghai-Tibet Plateau. A 24-h time step is used in the model simulation. The results show that the simulated soil temperature and water content, as well as the frozen depth compare well with the measured data. The coefficient of determination (R (2)) is 0.97 for the mean soil temperature and 0.73 for the mean soil water content, respectively. The simulated soil heat flux at a depth of 0-20 cm is also consistent with the monitored data. An analysis is performed on the simulated hydrothermal transfer processes from the deep soil layer to the upper one during the freezing and thawing period. At the beginning of the freezing period, the water in the deep soil layer moves upward to the freezing front and releases heat during the freezing process. When the soil layer is completely frozen, there are no vertical water exchanges between the soil layers, and the heat exchange process is controlled by the vertical soil temperature gradient. During the thawing period, the downward heat process becomes more active due to increased incoming shortwave radiation at the ground surface. The melt water is quickly dissolved in the soil, and the soil water movement only changes in the shallow soil layer. Subsequently, the model was used to provide an evaluation of the potential response of the active layer to different scenarios of initial water content and climate warming at the Tanggula site. The results reveal that the soil water content and the organic layer provide protection against active layer deepening in summer, so climate warming will cause the permafrost active layer to become deeper and permafrost degradation.