<b>QUANTIFYING REGIONAL GREENHOUSE GAS EMISSIONS ACROSS ARCTIC LANDSCAPES BY COUPLING TERRESTRIAL AND AQUATIC ECOSYSTEMS MODELS</b>

<p dir="ltr">Atmospheric methane (CH₄) is the second most important greenhouse gas only after carbon dioxide (CO₂). Continuous CH₄ emissions have caused its atmospheric concentration rising 2.6 times since the beginning of the industrial age. Arctic land ecosystems play a significant role in the regional and global methane cycle, as they store approximately one-third of global terrestrial organic carbon, a pool highly sensitive to climate change. These Arctic landscapes contain extensive inland water systems (e.g. wetlands and lakes), which are considered as a major source of atmospheric methane in northern high latitudes. Although great efforts have been made to quantify CH₄ emissions from these landscapes and assess their uncertainties, significant discrepancies persist among different estimation methods, and even among studies applying the same method due to variations in models, input data, and/or operating protocols. This dissertation advances the methane emission quantification in the Arctic area by synthetically combining different types of advanced models, more precise datasets, and different accounting methods. First, this study quantifies the methane emissions from both land and freshwater bodies in the pan-Arctic using both terrestrial and lake biogeochemistry models. Two non-overlapping dynamic areal change datasets were used to drive the models, minimizing potential double accounting at the landscape scale. We estimate total pan-Arctic methane emissions at 36.46 ± 1.02 Tg CH₄ yr⁻¹ for 2000–2015. Our result narrows the gap between previous bottom-up (53.9 Tg CH₄ yr⁻¹) and top-down (29 Tg CH₄ yr−1) estimates. Subsequently, to investigate the impact of lateral flow on Arctic landscapes, this study combined a 3D ecosystem model with a 1-D vertical soil thermal model to simulate the soil moisture and lateral flow effects on soil thermal dynamics in central Alaska. Our coupled model improves soil temperature simulations by 43.5% compared to observational data, underscoring the importance of incorporating dynamic soil moisture and lateral flow effects in modeling soil thermal dynamics in permafrost regions. Finally, this study integrated the 1-D and 3-D modeling frameworks for both terrestrial and aquatic ecosystems to assess the effects of lateral water and carbon transport on regional CH₄ emissions in the same study area. Overall, our results indicate that lateral flow enhanced methane emissions by 0.36 and 1.35 g CH₄ m^-2 yr^-1 from wetland and lake ecosystems, respectively, leading to a total increase of 6.75 Gg CH₄/yr (6.7 from wetlands and 0.05 Gg CH₄/yr from lakes) across the study area. By extrapolating these findings and combining them with previous pan-Arctic quantifications, we estimate that lateral carbon transport could increase CH₄ emissions from wetlands and lakes across the entire Arctic landscape by 2.43 and 0.57 Tg CH₄ yr^-1, respectively.</p>