Recently, atmospheric CH4 became more abundant but also depleted in 13C, as δ13C decreased from about −47.1‰ in 2007 to −47.3‰ in 2017. If shale gas (with δ13CCH4 around −40‰ as documented in this study) and conventional oil and gas (with δ13CCH4 around −43‰16) were conceived to collectively dominate recent emissions of CH4 to the atmosphere, then atmospheric CH4 would very simply become more enriched in 13C relative to the current global mean δ13C, which is not consistent with global observations. While we agree that shale developments (and fossil fuel in general) represent an important CH4 source, and that emissions from those sources have been likely increasing due to growing production, we conclude that the increases in global atmospheric CH4 concentrations since 2008 are not as strongly attributable to shale gas and conventional oil and gas emissions as some studies claim2,5, based on our global observations of isotopic fractionation.
Additionally, we must emphasize that the measured atmospheric δ13CCH4 signal is the sum-total of all CH4 source and sink terms. For example, a decrease in biomass burning emissions (significantly enriched in 13C (δ13CCH4 −22.3 ± 1.9‰4), and an increase in fossil fuel emissions (including shale gas), could in principle result in the same global average atmospheric δ13CCH4 signal over time as if both sources had no trend4,5. The biomass burning category includes fires and solid biofuels (e.g., for use in cook stoves). Data on global CH4 emissions from fires is not entirely conclusive. Remote sensing data of CH4 and CO (and assuming (i) biomass burning CH4/CO emission ratios and (ii) a partitioning of CO emissions across sectors) suggests decreased fire CH4 emissions of ~3.7 Tg/yr from the 2001–2007 to the 2008–2014 periods5. In contrast, remote sensing of burned fire area suggests no such trend20 (no trend over this period apart from inter-annual variation; Fig. S1). Furthermore, CH4 emissions from solid biofuels are reported to have increased from 12.2 to 13.6 Tg/yr from 2000–201221 (latest time series available). While this data does not indicate an immediately apparent decrease in global biomass burning CH4 emissions, more research is needed. Potential trends in the various CH4 sink processes such as the soil sink22 and the tropospheric OH sink11 can further complicate the diagnosis of source trends. As a result, it is important to account for these processes, as well as other existing evidence such as latitudinal and seasonal CH4 trends, when attributing the global signal1,9.
From the above, it follows that attributing ~1/3 of the global CH4 increase to North American shale gas production and another ~1/3 to conventional gas and oil with a simple mass balance approach2 is not supported by observations because of unconstrained uncertainties. Based on long-term airborne CH4 measurements over the US, previous analysis concludes that oil and gas industry CH4 emissions (shale and conventional) over the past decade have increased at about the same rate as natural gas production volume7. The existence of unaccounted and poorly characterized emission sources within the oil and gas industry has also been demonstrated through intensive field studies in the USA23, and additional international studies paint a similar picture24,25, although little independent measurement data exist for many world regions including the Middle East, the Former Soviet Union, and Africa. Further research targeted for these areas, in addition to changing biogenic sources and sinks, will serve to further constrain the conclusions made in this work.
Based on existing knowledge of CH4 source and sink terms and isotopic signatures, additional CH4 emissions associated with increased shale gas development in the USA cannot account for a large fraction of the recent increase in atmospheric CH4. Yet, oil and gas industry expansion remains a significant factor in the complex patterns of global atmospheric CH4 emissions and concentrations4,23,24,25. And, of equal importance, fossil fuel CH4 sources may be mitigated with policy and best (or better) industrial practice that can effectively reduce emissions. We suggest that the rise in global CH4 concentrations is most effectively seen not through a lens of what is the most important or dominant source of emissions, but rather understanding all sources and how they can collectively explain the observed patterns of atmospheric increases. Indeed, a reduction in emissions from any major source (such as fossil fuels or cattle husbandry) would be expected to lead to a reduction in the global CH4 concentration1. Therefore, although our analysis indicates that shale gas and conventional gas and oil production has not played a dominant role in the increase in atmospheric CH4 since 2008, we should not lose sight of the powerful impact of interventions to reduce emissions from sources we have.
Source: Ecology - nature.com
