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Summary - This study presents a ﬁrst effort to systematically compare published CH4 emissions estimates at scales ranging from device- level (>103 g/year) to continental-scale atmospheric studies (>1013 g/year). Studies known to us that (i) report measurement- based emissions estimates and (ii) compare those estimates with inventories or established emission factors (EFs) are shown in the ﬁrst chart.
We include in the second chart a range of excess CH4 from all sources (7 to 21 × 1012 g or Tg/year) based on normalized national-scale atmospheric studies from the inset in the ﬁrst chart. This excess is conservatively deﬁned as 1.25 to 1.75 times EPA GHGI estimates. This estimate is derived from national-scale atmospheric studies and includes all sources of CH4 emissions: It should not be expected that NG sources are responsible for all excess CH4.
The scenarios in the second chart for NG production and/or processing, distribution, and petroleum system emissions apply observed leakage rates from the literature that are higher than EPA GHGI estimates. The frequency of such high-emitting practices is unknown, so illustrative prevalence scenarios are plotted: 1, 10, or 25% of activity is represented by high-emitters; the remaining facilities emit at EPA GHGI rates. This evidence suggests that high leak- age rates found in recent studies are unlikely to be representative of the entire NG industry; if this were the case, associated emissions would exceed observed total excess atmospheric CH4 from all sources.
In general, the wide ranges in the second chart suggest a poor understanding of sources of excess CH4 and point to areas where improved science would reduce uncertainty. However, hydraulic fracturing for NG is unlikely to be a dominant contributor to total emissions. Also, some sources not included in the GHGI may con- tribute to measured excess CH4, e.g., abandoned oil and gas wells and geologic seeps (see SM).
Brandt, A. R., G. A. Heath, E. A. Kort, F. O’Sullivan, G. Pétron, S. M. Jordaan, P. Tans, J. Wilcox, A. M. Gopstein, D. Arent, Wofsy, N. J. Brown, R. Bradley, G. D. Stucky, D. Eardley, R. Harriss (2014) Methane Leaks from North American Natural Gas Systems, Science 343:733-735.