The primary objective of this project is to better understand recent trends in the growth rate of atmospheric methane (CH4) through: (1) analysis of the isotopic composition of CH4 over two decades using samples from the Oregon Graduate Institute (OGI) air archive and (2) the interpretation of long-term and interannual variations in atmospheric CH4 in terms of changing CH4 sources and sinks. More than 380 samples originally collected at Cape Meares, Oregon (1978-1998), Palmer Station, Antarctica (1991-1997), and Point Barrow, Alaska (1995-1998) will be analyzed for their CH4 isotopic composition (13C/12C and D/H). The resulting dataset will provide the only northern hemisphere time series record of 13C/12C available prior to 1988, and the only one at monthly resolution in any case. Equally important, the D/H time series will be the only record available prior to 1996. Results will be merged with more modern records creating a 30 year composite record and interpret the results of the changing concentration and isotopic composition of CH4 in terms of changing biogenic, fossil, and pyrogenic sources of CH4 over this time period.
The hands-on research opportunities for Portland State University undergraduate and graduate students provided through this project in an area of cutting edge research will enhance their development and build a foundation for the next generation of Earth science research. Building a partnership with the Portland Apprenticeships in Science and Engineering Program will initiate and foster a relationship of mentoring with local high school students during summers and provide the framework for conveying the excitement and challenges of science exploration to promising young students before they commit to fields of focus in college.
Considerable research since the 1970s has established the role of methane in climate, as an infrared active greenhouse gas, and as a chemically reactive species in the atmosphere. At a globally averaged mixing ratio of 1.8 parts per million, the abundance of methane in the atmosphere has more than doubled since the industrial revolution as a result of population growth, agricultural practices and fossil fuel use. The rise in methane concentrations have contributed nearly a quarter of the radiative forcing from greenhouse gases since 1750; methane is 28 times more effective than carbon dioxide as a greenhouse gas (on a 100 year time horizon). The radiative forcing of methane is roughly double if indirect effects of methane emissions are considered. Early measurements of methane in the atmosphere during the late 1970s and 1980s found that methane was increasing rapidly in the atmosphere at approximately 1% per year. Unexpectedly, in the late 1980s and 1990s the methane growth rate slowed and the year 2000 was the first year to record a negative annual growth. The drivers of this slowdown are a subject of scientific debate and a source of uncertainty identified by the IPCC in 2013. Even more recently, since 2006 methane appears to be, once again, on a sustained rise in atmospheric abundance. This sponsored research seeks to address this outstanding scientific question on the growth of atmospheric methane. We focused on which source categories have changed during this period by measuring the isotopic composition of methane in more than 300 rare historical air samples from the Portland State University – Oregon Graduate Institute (PSU–OGI) air archive, which contains samples collected during the period 1977–1998 as part of a global monitoring program. This is a powerful approach as a direct means to interpret changes in the methane budget due to characteristic isotope ratios in major methane sources: wetlands, rice and ruminant agricultural emissions, fugitive fossil fuel emissions, and biomass burning. Despite the promise of isotopic methane to help constrain these changes, the scientific community is historically data-limited prior to the turn of the century. Our dataset helps to fill this void and provides detailed information on the changing carbon and hydrogen isotopic composition during the late 1970s, 1980s and 1990s. With the addition of new isotope data and advanced atmospheric inverse modeling of the results with global methane atmospheric records and more recent isotope data, our study makes real advances towards understanding changes in the methane sources over the past three decades. In particular, our study reinforces previous findings that while the methane growth rate went through a dramatic decline over the past three decades (1980s–current), a decline in the total methane source is not needed to balance sources and sinks. However, we find significant evidence that source apportionment shifted during this period. Overall, our study favors a historical scenario of: interannual variability dominated by wetland emissions with very little long-term change through 2000 and decreasing wetland emissions after 2000; roughly constant fugitive fossil fuel emissions during the 1980s and 1990s, increasing after ~2000; a small decline in biomass burning emissions over the period excluding a large 1997-1998 anomaly; increasing waste emissions; and declining rice emissions. A key aspect of this research and education grant is to inspire and mentor young scientists in the study of science in general, and societally-relevant environmental science, in particular. Our educational and outreach activities provided unique research-based opportunities for high school, undergraduate, and graduate students at Portland State University. This award fully supported the Ph.D. dissertation and M.S. thesis research for two graduate students. We also worked with Portland State undergraduate and NSF Research Experience for Undergraduates (REU) students who gained valuable research experience in the environmental sciences and received individual career mentoring. Finally, through a partnership with the Apprenticeships in Science and Engineering Program, we had the pleasure of working with and mentoring talented summer interns from regional high schools.
