Last week, a new paper  on methane emissions from natural gas development made headlines in a variety of prominent outlets, reviving the argument that methane emissions from shale gas development in the United States are very high. Presidential candidate Jay Inslee even cited the study to support his call for a nationwide ban on “fracking.” The paper in question is by Professor Robert Howarth of Cornell University, whose previous work on methane emissions  has been critiqued by numerous independent researchers, including me. [3-5]
Before digging into the substance, a quick note on Professor Howarth’s contribution to the field. His 2011 paper  helped spark public concern over methane emissions from oil and gas development in the US. Although concern over methane emissions was rising independently, this work helped push the methane question to the top of the regulatory agenda, which helped spur numerous US states and the federal government to take action to reduce the potent greenhouse gas. For this, Professor Howarth deserves credit and thanks.
Now onto the topic at hand.
Atmospheric Methane is Rising: Why?
Natural gas is comprised mostly of methane, a powerful greenhouse gas. The amount of methane in the atmosphere, and its contribution to global warming, has been increasing steadily for most of the past 200 years. This growth was interrupted by a plateau from 1999 to 2006, followed by a resumption in growth from 2007 through today.  Numerous papers have been published in recent years grappling with how explain this stagnation and subsequent increase, with a variety of sometimes conflicting results. Howarth’s new paper argues that roughly one-third of the growth since 2007 is due to shale gas development in the US, roughly one-third from all other fossil fuels (coal, oil, and “conventional” natural gas), and roughly one-third from biogenic sources such as agriculture and wetlands.
What’s new here is the explicit estimate of the role of shale gas, whereas previous work focused on broader categories including natural gas (both shale and “conventional” natural gas), oil, and coal. This existing body of research presents mixed results, with some researchers arguing that the recent growth in concentrations has been driven primarily by biogenic sources (e.g., agriculture), [6,7] while others have estimated that oil, gas, and coal development likely play a larger role. [8,9]
In the US, one study found that methane emissions were roughly flat from 2000 to 2012,  while another study estimated that emissions increased by more than 30 percent from 2002 to 2014.  (Notably, US natural gas production increased by 38 percent from 2002 to 2014, indicating that the rate of methane emissions may well have declined during the onset of shale gas development around 2005).
At the global level, recent studies have estimated that methane emissions from the natural gas system actually decreased between 2000 and 2011—a notable result given the fact that global gas consumption increased by more than 35 percent during that period.  A follow-on study from some of the same authors reinforced this finding, estimating that global methane “leakage” (methane emitted into the atmosphere per unit of natural gas produced) declined from 7.6 percent to just 2.2 percent between 1985 and 2013. 
But emissions aren’t the whole story here. We also have to consider the global role of methane sinks. That is, methane that degrades through chemical reactions in the atmosphere or that is absorbed by soils, reducing the amount of methane remaining in the atmosphere. If methane sinks weaken, atmospheric concentrations of methane may continue to rise even if emissions are constant. Indeed, a couple of recent papers have discussed the potential role that weakening methane sinks could be playing in the recent rise in atmospheric concentrations. [14,15]
In short, this is a complex issue, and the evidence is mixed as to the relative contributions that fossil fuel development, biogenic sources, and sinks are playing in the rise of global methane concentrations. Fossil fuels are clearly a substantial source, but a review of the literature published just a couple of months ago compellingly makes the case that existing data don’t provide clear answers. 
Once More Unto the Breach
Into this uncertain terrain enters the new paper from Professor Howarth. The analysis includes a number of important assumptions, including a crucial one about the isotopic composition of methane from shale gas wells that I won’t go into here (atmospheric scientists are better positioned to discuss this issue, and the review paper I noted above  argues that considering the isotopic ratios alone cannot produce definitive conclusions about the relative role of different sources). Instead, I’d like to focus on several problematic accounting assumptions used by Howarth to estimate the relative contribution of different sources.
First, and most importantly, the analysis assumes that shale gas development is responsible for 63 percent of the growth in methane emissions from natural gas systems, while “conventional” sources account for 37 percent. This assumption is based on Howarth’s claim that shale gas has accounted for 63 percent of the growth in natural gas production between 2005 and 2015. The assumption is central to the findings in the paper, and is neither based on observed data nor any detailed modeling. Instead, it simply assumes that any increase in production of natural gas inevitably leads to a commensurate increase in methane emissions.
The problem with this assumption is that new methane emissions may or may not coincide with new oil and gas development. New sources of methane may come from newly drilled wells, 20-year old wells, abandoned wells, old pipelines, new pipelines, and many other potential sources. In fact, research in the US has found that shale gas wells are often far less leaky than their conventional brethren.  A more careful analysis would use real-world data and detailed estimates, such as those available from the International Energy Agency’s “Methane Tracker,”  which provides global estimates of methane emissions from various sources.
Second, and nearly as important, are the questionable estimates for methane emissions from coal mining and oil development. While Howarth pays close attention to distinguishing between emissions from shale gas and conventional gas, he takes no such care with methane emissions from other fossil fuel development. Instead, he assumes a constant emissions rate of 870 grams of methane per ton of coal production, which he describes as “well accepted,” citing his own 2011 paper,  which in turn cites estimates from 2008 and 2009.
This figure assumes that all new methane emissions from coal occur through surface mining operations, rather than any new emissions from underground mines. Howarth states in the article that “almost all” growth in global coal production occurred at surface mines in China during the relevant times period, but the citations offered in the article [19,20] provide no such evidence. 2018 data from the International Energy Agency  show that China accounted for roughly 75 percent of the net growth in global coal production from 2005 to 2016 (outside of China, production fell in OECD nations and grew in non-OECD nations). Neither the citations provided by Howarth nor the 2018 report provide data on the share or levels of coal mined from the surface or underground mines.
For oil-related methane emissions, the analysis again assumes a constant emissions rate based on a 2008 estimate. In the last several years, multiple analyses have produced detailed estimates of methane (and other “upstream”) emissions from oil production around the world. [22,23] These more recent and more comprehensive estimates are not referenced or utilized in Howarth’s analysis.
Finally, the analysis ignores the potential role of methane sinks. As noted above, a change in the rate of methane uptake from sinks could have a major effect on the change in atmospheric concentrations of methane.
The Bottom Line
Atmospheric scientists have been debating the cause of the recent rise in methane concentrations since roughly 2007 and—before that—were debating the reasons behind the earlier pause. A variety of detailed analyses from dozens of authors have come to different conclusions about the relative contributions of fossil fuel development, biogenic sources, natural sinks, and more.
This latest entry into the field from Professor Howarth falls short on a number of fronts. In my view, the conclusion that shale gas is a key driver in the growth in global methane concentrations is not supported by the evidence presented in the paper, which relies on overly simplistic and, in some cases, unsupported assumptions. Instead, I would view the claim as a hypothesis that can, and should, be tested with more detailed data.
One area where there is little disagreement is the need to reduce methane emissions across the energy system, whether from shale gas development in the US, long-distance gas transmission in Russia, oil production in the Middle East, or elsewhere. There are a wide variety of cost-effective abatement opportunities,  and lawmakers could incentivize operators to take advantage of these opportunities with smart policy. Developing smart policy, in turn, depends on the availability of evidence, informed by the best available data.
- Howarth, R. W. Ideas and perspectives: is shale gas a major driver of recent increase in global atmospheric methane? Biogeosciences 16, 3033–3046 (2019).
- Howarth, R. W., Santoro, R. & Ingraffea, A. Methane and the greenhouse-gas footprint of natural gas from shale formations. Climatic Change 106, 679 (2011).
- Cathles, L. M., Brown, L., Taam, M. & Hunter, A. A commentary on “The greenhouse-gas footprint of natural gas in shale formations” by R.W. Howarth, R. Santoro, and Anthony Ingraffea. Climatic Change 113, 525–535 (2012).
- Burnham, A. et al. Life-Cycle Greenhouse Gas Emissions of Shale Gas, Natural Gas, Coal, and Petroleum. Environ. Sci. Technol. 46, 619–627 (2012).
- Raimi, D. The Fracking Debate. (Columbia University Press, 2017).
- Schaefer, H. et al. A 21st century shift from fossil-fuel to biogenic methane emissions indicated by 13CH4. Science aad2705 (2016). doi:10.1126/science.aad2705
- Wolf, J., Asrar, G. R. & West, T. O. Revised methane emissions factors and spatially distributed annual carbon fluxes for global livestock. Carbon Balance and Management 12, (2017).
- Hausmann, P., Sussmann, R. & Smale, D. Contribution of oil and natural gas production to renewed increase in atmospheric methane (2007–2014): top–down estimate from ethane and methane column observations. Atmospheric Chemistry and Physics 16, 3227–3244 (2016).
- Worden, J. R. et al. Reduced biomass burning emissions reconcile conflicting estimates of the post-2006 atmospheric methane budget. Nature Communications 8, 2227 (2017).
- Bruhwiler, L. M. et al. U.S. CH4 emissions from oil and gas production: Have recent large increases been detected? Journal of Geophysical Research: Atmospheres 122, 4070–4083 (2017).
- Turner, A. J. et al. A large increase in U.S. methane emissions over the past decade inferred from satellite data and surface observations. Geophysical Research Letters 43, 2218–2224 (2016).
- Schwietzke, S., Griffin, W. M., Matthews, H. S. & Bruhwiler, L. M. P. Natural Gas Fugitive Emissions Rates Constrained by Global Atmospheric Methane and Ethane. Environ. Sci. Technol. 48, 7714–7722 (2014).
- Schwietzke, S. et al. Upward revision of global fossil fuel methane emissions based on isotope database. Nature 538, nature19797 (2016).
- Rigby, M. et al. Role of atmospheric oxidation in recent methane growth. Proc Natl Acad Sci USA 114, 5373 (2017).
- Turner, A. J., Frankenberg, C., Wennberg, P. O. & Jacob, D. J. Ambiguity in the causes for decadal trends in atmospheric methane and hydroxyl. Proc Natl Acad Sci USA 114, 5367 (2017).
- Turner, A. J., Frankenberg, C. & Kort, E. A. Interpreting contemporary trends in atmospheric methane. PNAS 201814297 (2019). doi:10.1073/pnas.1814297116
- Omara, M. et al. Methane Emissions from Conventional and Unconventional Natural Gas Production Sites in the Marcellus Shale Basin. Environ. Sci. Technol. 50, 2099–2107 (2016).
- International Energy Agency. Methane tracker: Reducing methane emissions from oil and gas operations. (2019). Available at: https://www.iea.org/weo/methane/database/. (Accessed: 19th August 2019)
- International Energy Agency. World Energy Outlook 2008. (2008).
- International Energy Agency. Key world energy statistics. (2017).
- International Energy Agency. Coal Information 2018. (2018).
- Gordon, D., Brandt, A., Bergerson, J. A. & Koomey, J. Know Your Oil: Creating a Global Oil-Climate Index. (Carnegie Endowment for International Peace, 2015).
- Masnadi, M. S. et al. Global carbon intensity of crude oil production. Science 361, 851–853 (2018).