Quantifying exchanges of methane and carbon dioxide between terrestrial ecosystems and the atmosphere in the northern high latitudes

Atmospheric carbon dioxide (CO2) and methane (CH4), the two most important greenhouse gases (GHG), are able to trap a large amount of long-wave radiation, leading to surface warming. Terrestrial ecosystems play an important role in determining the amount of these gases into the atmosphere. The northern high latitudes are particularly sensitive to climate warming and crucial to the budget of these carbon-based GHGs because the region is rich in soil organic carbon while it has experienced more dramatic environmental changes compared to low latitudes. It has been an important research focus to identify the source/sink and controlling mechanisms of both CO2 and CH4 fluxes between high-latitude terrestrial ecosystems and the atmosphere. In this dissertation, I apply modeling approaches to quantify ecosystem carbon budgets and analyze how these two GHGs respond to contemporary and future climate change. I first use an artificial neural network approach to quantify CH 4 emissions from wetland ecosystems in the northern high latitudes based on field observations of CH4 fluxes (Chapter 2). The estimated wetland CH4 emissions show a large spatial variability over the northern high latitudes, as well as significant inter-annual and seasonal variations in the past two decades. Among six explanatory variables (air temperature, precipitation, water table depth, soil organic carbon, soil porosity and pH), water table depth (WTD) stands out as the most important control of wetland CH4 flux density (emissions per unit wetland area). To improve future assessment of wetland CH4 dynamics, research priorities should be directed to better characterizing hydrological processes of wetlands, including temporal dynamics of WTD and spatial dynamics of wetland extent. To assess the effects of spatial heterogeneity of WTD on large-scale land-atmospheric CH4 fluxes, I apply a coupled hydrology-biogeochemistry model to compare regional CH4 fluxes simulated at two various spatial resolutions (Chapter 3). The effects of sub-grid spatial variability of WTD on CH4 emissions are examined with a TOPMODEL-based parameterization scheme. The comparison indicates that both the magnitude and temporal variability of CH4 emissions are different at two various spatial resolutions, and CH4 fluxes are better simulated at a finer spatial resolution. This study suggests that previous macro-scale biogeochemical models using a grid-cell-mean WTD scheme might have underestimated the regional CH 4 emissions, and the spatial scale-dependent effects of WTD should be considered in future quantification of regional CH4 emissions. To evaluate regional carbon budgets of both CH4 and CO 2 and their respective impacts on climate radiative forcing, I further develop a coupled hydrology-biogeochemistry model framework to simultaneously simulate both CH4 and CO2 fluxes at daily time step, by explicitly considering the spatial heterogeneity of WTD and the effects of permafrost thawing (Chapter 4). The pan-Arctic is estimated to be a sink of CO2 and a source of CH4 under contemporary climate conditions, and the magnitudes of both CO2 uptakes and CH 4 emissions are projected to increase over this century under a no-policy climate change scenario (assuming no explicit climate policy). This study indicates that permafrost thawing with climate warming significantly affects soil carbon decomposition, and the CH4 emissions continuously play an important role in affecting regional radiative forcing in the pan-Arctic