Disentangling Spatiotemporal Signals in Global Atmospheric Methane Columns

Citation

Saad, Katherine Michelle (2017) Disentangling Spatiotemporal Signals in Global Atmospheric Methane Columns. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z97W697G. https://resolver.caltech.edu/CaltechTHESIS:06052017-164408423

Abstract

Methane is a major driver of atmospheric chemistry and a powerful radiative forcing agent. Thus, explaining tropospheric methane trends over the last two decades is critical for the scientific understanding of the global carbon cycle as well as the ability to predict future consequences on the climate and biosphere. Tropospheric concentrations of methane have been increasing, but the growth rate in the last two decades has been extremely variable. Long-term trends in atmospheric methane concentrations and short-term fluctuations in its growth rate are not well understood because its surface emissions and chemical loss are poorly constrained. The ranges of uncertainties for estimates of methane sources and sinks are considerably broad due to the complexity of both natural and anthropogenic fluxes and the heterogeneity of their timescales. This research takes a multifaceted approach to constraining methane fluxes and determining the causes of interannual and long-term variability by developing and synthesizing measurements, integrating methane observations with tracers of tropospheric advection, and assessing systematic biases in chemical transport models. This work synthesizes satellite, aircraft, and surface measurements, including a newly developed dataset of tropospheric column-averaged dry-air mole fractions from Fourier Transform Spectrometer (FTS) instruments within the Total Carbon Column Observing Network (TCCON). Assimilating measurements into the GEOS-Chem chemical transport model, the sensitivities of the temporal and spatial distributions of methane to changes in the distributions of sources and sinks are evaluated. We demonstrate that systematic biases in model stratospheres alias into assimilation of total column methane, masking measurement-model mismatch in tropospheric seasonality. This work also investigates the influence of large-scale transport to the spatial distribution, and in particular the interhemispheric and meridional gradients, of methane.

Thesis Files