Methane Exchanges between Terrestrial Ecosystems and the Atmosphere in Response to Multiple Environmental Changes - A Process-Based Modeling Study
Type of DegreePhD Dissertation
Forestry and Wildlife Science
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Methane (CH4), the most abundant non-carbon dioxide (CO2) greenhouse gas, has a relatively shorter lifetime (approx. 9 years) and higher global warming potential (approx. 28 times) than CO2 at a 100-year time horizon. The changes in CH4 fluxes have immediate feedback on the climate system. Since the early 1990s, the rate of increment in atmospheric CH4 concentration experienced a temporary slowdown, pause, and resumption; however, the reasons for those significant changes are still unclear. Variation of the CH4 fluxes from biogenic and pyrogenic sources and sinks were proposed to explain those changes in the atmospheric CH4 growth rate. In this study, we applied a data-model integration approach to comprehensively quantify the CH4 fluxes from wetlands, rice field, ruminants, biomass burning and upland soil. Our results showed that the global CH4 flux from wetlands, rice fields, ruminants, biomass burning and upland soil was 163.9±6.4 Tg C/yr (Avg. ± 1 std. dev.), and exhibited substantial inter-annual variation during 1993-2014. Among all the CH4 sources, wetlands contributed almost half (~49.2%) of the global total CH4 emission, followed by ruminants (~36.8%), rice fields (~7.5%) and biomass burning (~6.5%). The upland soil offset ~13.2% of the total emitted CH4 from wetlands, ruminants, rice fields and biomass burning. Regionally, tropics accounted for the largest portion of the estimated net CH4 fluxes, followed by the northern middle latitude region, northern high latitude region and southern middle latitude region. The results further revealed that CH4 emission from wetlands dominated the atmospheric CH4 variation during 1993-2014. In addition, the contribution of ruminants to CH4 emission became increasingly important after 2006. Likewise, biomass burning played a critical role on CH4 emissions only during years of large peatland fires. By adopting different water management practices in the rice field, the estimated CH4 emissions could be reduced by 50.6% under intermittent irrigation when compared to continuous flooding from global rice field. Over the past 110 years, global CH4 emissions from rice cultivation increased by 85%. The expansion of rice fields was the dominant factor for the increasing trends of CH4 emissions, followed by elevated CO2 concentration, and nitrogen fertilizer use. Under the future scenarios, the magnitude of CH4 emission from wetlands in the arctic and boreal region is projected to increase by 2%~65%, when compared with the contemporary level (2001-2010). Seasonal analyses indicated that the change of CH4 fluxes exhibits great spatial variability over time throughout the 21st century. The projected CH4 fluxes in summer accounted for the largest portion of annual emission and showed the largest increasing trend during the 21st century. By feeding different wetland datasets into the dynamic land ecosystem model (DLEM), the results further suggested that tropical regions accounted for the largest portion (~72 ± 7%) of the estimated CH4 emission from wetlands and also exhibited the largest uncertainty. To reduce the uncertainty in estimating CH4 emission from global wetlands, it is urgent to develop robust datasets delineating dynamic wetland extent and the inter-annual and intra-annual variation of inundation patterns, particularly in the tropical region. It can be anticipated that the future atmospheric CH4 variation will be determined by the increasing demand for food production with the climate sensitive natural emissions.