Long-term Trends and Confidence in Global Natural Gas Fugitive Emissions Rates Based on δ13C-CH4
Thursday, 18 December 2014: 10:20 AM
Numerous life cycle assessment (LCA) and field studies have estimated natural gas (NG) fugitive emissions rates (FER) – the fraction of produced NG, mostly CH4, emitted to the atmosphere, unintentionally or by design, during extraction, processing, transport, and distribution – at local, regional, and national scales. In a recent study, we estimated for the first time the global mean FER using long-term (three decades) atmospheric CH4, δ13C-CH4, and C2H6 measurements from global monitoring networks. As a further development, this work investigates the global mean FER uncertainty range (factor of 2) in more detail to increase confidence in the results. The objectives of this research are to (i) estimate probability distribution functions (PDF) of global mean FER, and (ii) identify long-term trends in global fossil fuel (FF) and other CH4 sources. In order to achieve these objectives, global atmospheric δ13C-CH4 measurements since the mid-1980s are analyzed using a box-model of the global CH4 sources and sinks. First, we derive PDFs of the key model parameters including literature isotopic source signatures, atmospheric lifetimes, natural and anthropogenic emissions, and FF hydrocarbon gas composition. Second, a Monte Carlo simulation of the box-model is performed to quantify FER confidence intervals. While our model attributes the majority of increased CH4 levels over the past three decades to microbial sources, FF sources have also increased slightly. However, FER – an indicator of NG life cycle efficiency – has decreased over the same period given the large NG production increase worldwide. Results are most sensitive to global average microbial isotopic signatures (weighted by source strength) and bottom-up estimates of biomass burning emissions, which will be discussed in more detail.