Assessing causes of global methane changes during 1979--2007 using measurements and models of mixing ratios and stable isotopes

Atmospheric methane (CH4) is a major greenhouse gas and plays a critical role in the chemistry of the atmosphere. Rapid increases in CH 4 from the pre-industrial period (-700 ppb) to the 1970s contribute substantial to global warming. Since the early 1980s, however, the growth rate of CH4 has declined, implying that sources and sinks are becoming increasingly balanced. Superimposed on the multi-decadal slowdown are large year-to-year variations that have been linked with El Nino-Southern Oscillation. The factors contributing to the CH4 slowdown and interannual variability in the CH4 growth rate are incompletely understood. This research applied long-term records of CH4 mixing and its isotope ratios, estimates of fire emissions, and decadal histories of rice cultivation to assess the causes of multi-decadal changes in the CH 4 growth rate with the help of modeling techniques. First, the mechanisms contributing to the interannual variations in the global CH4 growth rate were evaluated using CH4, delta13C-CH 4, satellite-derived estimates of fire emissions, and a three-dimensional chemical transport model (GEOS-CHEM). Previous competing hypotheses related to the increase in CH4 anomaly during the 1997/1998 El Nino were tested using stable isotope observations and simulations from the GEOS-CHEM model. Next, the causes of the multi-decadal CH4 slowdown were assessed using synchronous time series of atmospheric CH4 (mixing ratio and both 13C/12C and D/H ratios). As a part of this study, changes in the interhemispheric differences of CH 4 and its isotopic ratios (delta13C, deltaD) were quantified using long-term time series from the University of California, Irvine (UCI) and National Institute of Water and Atmospheric Research (NIWA). A 2-box atmospheric model was developed to examine various hypotheses formulated to explain the overall decline in the CH4 growth rate. Finally, an empirical process-based biogeochemical model was developed to estimate the global CH4 flux from rice paddies, one of the largest uncertainties among anthropogenic emissions. In this analysis, I used multi-national histories of rice cultivation to assess changes in CH4 emissions from rice paddies. The impacts of changes in agricultural practices in rice fields were represented using the empirical process-based model. Overall, I concluded that the major high CH4 anomaly events (i.e. the 1998 increase, the 2003 increase, and the 2007 increase event) were linked closely to major fire events around the world. For the slowdown in CH4 mixing ratio, I found that a reduction in an isotopically depleted source, such as agricultures, was needed to explain the atmospheric observations. Agricultural intensification (such as the practices of mid-season drainage, and reductions in organic amendments in rice fields) is consistent with isotope observations and has reduced CH 4 emissions from rice agriculture during the past 3 decades.