OK---so you can't read very well. Lots of people can't read very well. So I copied it and pasted it and highlighted in bold for you the parts that were relevant to the use of isotopic characteristics of methane. Its not a very complicated idea, really. The fact that there are distinctive isotopic characteristics that indicate the provenance of CH4 derived from fossil fuel sources has been known for decades.
Obviously, if you can't understand this simple idea, then you won't understand the rest of the report, and you won't understand what it means.
Read the damned paper which I am now convinced you never did but instead probably got your information from a news release (as is your normal modus operandi)
Worden, J. R. et al, Reduced biomass burning emissions reconcile conflicting estimates of the post-2006 atmospheric methane budget. Nature communications. DOI: 10.1038/s41467-017-02246-0
Here is what actually the paper is about (versus what you are trying to characterize).They got all of their estimates of fossil fuel contributions from the paper I previously quoted (Schwietzke, S et al, 2016) that is quite obvious from their Table 1. They did not measure it themselves, in fact the entire paper deals specifically with looking at a decrease in biomass area burned and then figuring out how that impacts the CH4 mass balance based on previous estimates. Simply put because they assess that biomass burning has contributed less then the other sources (e.g. fossil fuels) must take up the slack. That is all they did. It isn’t a paper where they went out and measured isotopes trying to ascertain how much fossil fuels contributed, indeed what they did was to back out biomass burning CH4 using the following methodology:
As discussed in the following sections, we first quantify monthly CO fluxes and their uncertainties at monthly timescales on a 5 × 4° (longitude × latitude) grid using measurements of CO concentrations from the Terra Measurements of Pollution in the Troposphere (MOPITT) satellite instrument (V6J multi-spectral product40 and the adjoint version of GEOS-Chem31). CO fluxes are then re-partitioned to the CO emission types plus uncertainties on each 5 × 4° grid cell using a Bayesian Markov Chain Monte Carlo approach25,41 that accounts for the a priori and a posteriori uncertainties of the BB emissions and other CO emissions. Estimates of the CH4 emissions and their uncertainties are then calculated by multiplying BB CO emissions by the GFED-based estimate of each fire-type contribution, the expected CH4/CO emission factors for all fire types within each grid cell, and the uncertainties of the GFED-recommended emission factors. The emission factor uncertainties are tested with CH4 and CO measurements from the Aura TES instrument
And if you go to the source of their data on fossil fuel contribution to CH4 (which I previously gave the reference to).
Schwietzke, S, et al, 2016. Upward revision of global fossil fuel methane emissions based on isotope database. Nature. 538, pp 88-91
They talk about how they arrived at their data (hint no measurements were made).
The sample sizes of δ13Csource values used in published global CH4 budgets are either small (N < 100, based on cited original measurements) or unknown, uncertainties are rarely applied, and global representativeness is lacking (especially in the tropics and the Southern Hemisphere), but some δ13Csource values have nevertheless taken on canonical status with few references to primary sources (for example, refs 3, 4, 9 and 10; see full list of references in Supplementary Information section . We have compiled the most comprehensive δ13Csource database to date (see ref. 14 and Supplementary Information sections 3–5 for complete list of data, metadata and references) including 9,468 δ13CFF, δ13Cmic and δ13CBB original measurements from the peer-reviewed literature and other publicly available sources to define globally weighted average δ13CFF (time-dependent), δ13Cmic, and δ13CBB with well defined uncertainties. These data allowed us to revisit the source attribution of global CH4 emissions since the 1980s by applying an atmospheric box-model to global atmospheric CH4 and δ13Catm measurements (and avoiding the use of a priori FFtot and microbial source strengths), thus maximizing the CH4 and δ13Catm constraints.
And the manner by which they used this data that was compiled from literature was:
Our box-model applies Monte Carlo techniques to estimate global FFtot and microbial CH4 emissions and uncertainties as a function of δ13Csource, of isotope fractionation during oxidation (OH + CH4), of the uncertainties of both of these values, and of other factors (see Supplementary Table 1). We also estimated FFind emissions by subtracting FFgeo emissions from FFtot emissions. This allowed us to calculate global long-term trends in the Fugitive Emission Rate (FER), which is the fraction of natural gas production lost to the atmosphere through its lifecycle (production, processing, transport and use), and is a critical parameter for evaluating the climatic impact of natural gas as a fuel
given you never read any of this prior to pontificating about the measurement of CH4 isotopes let me summarize:
The NASA paper never measured CH4 for any reason other than determining biomass burning amounts through time. They did not directly measure fossil fuel originated CH4. They got all of that information from a previous paper and those authors got their information from a literature search and compilation (with no quality control, ie. assumed all values were equally relevant). The numbers that Worden et al (NASA) used were based on the box model analysis conducted by Schwietke et al. And the final numbers that Worden et al end up with regarding contribution of fossil fuels to current CH4 mass balance was arrived at through their own box model that took into account the lesser amount of biomass burning in the mix of what constitutes total CH4.
Please before you go off on pedantic rants it would help if you actually read the papers you refer to.