Promoters of shale gas view it as a plentiful bridge fuel that will allow society to continue to rely on fossil fuels while reducing greenhouse gas (GHG) emissions compared to other fossil fuels. In April of 2011, we published a study challenging the assumption that shale gas has a low GHG footprint.

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19 Sept 2012: Slides from Robert Howath presentation at ESA 2012, Greenhouse gas footprint of shale gas obtained by hydraulic fracturing are available from

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Greenhouse Gas Footprint of Shale Gas Obtained by High-Volume, Slick-Water Hydraulic Fracturing

11 April 2011

Natural gas is widely advertised and promoted as a clean burning fuel that produces less greenhouse gas emissions than coal when burned. While it is true that less carbon dioxide is emitted from burning natural gas than from burning coal per unit of energy generated, the combustion emissions are only part of story and the comparison is quite misleading. With funding from the Park Foundation, my colleagues Renee Santoro, Tony Ingraffea, and I have assessed the likely footprint from natural gas in comparison to oil or to coal. Our paper was published in the peer-reviewed journal Climatic Change Letters in early 2011.

shale gas 20 year GHG

Our analysis of methane losses from the shale gas life cycle is based in part on a November 2010 report from the EPA. The EPA report is the first significant update by the agency on natural gas emission factors since 1996, and concludes that emissions – particularly for shale gas – are larger than previously believed. Our research further supports this conclusion.

The summary figure from our research shown here to the left compares shale gas with two estimates of methane emissions to the atmosphere (low and high, two bars to the left), conventional natural gas with two estimates of methane emissions (high and low estimates, next two bars), coal from surface mines (3rd bar from right), coal from deep mines (2nd bar from right) and diesel oil. Note that particularly when viewed on the 20-year time horizon after emission, the greenhouse gas footprint of shale gas is considerably greater than that for coal or diesel oil, when the full effects of the methane emissions are considered.

We urge caution in viewing natural gas as good fuel choice for the future, particularly as a transportation fuel where natural gas is no more efficient than diesel oil or gasoline, and where additional fugitive methane emissions beyond those in our study or the EPA (2010) analysis seem likely during refueling operations. Note that both the National Academy of Sciences and the Council of Scientific Society Presidents have urged great caution before proceeding with the development of diffuse natural gas from shale formations using unconventional technology.

  • National Research Council (2009). Hidden Costs of Energy: Unpriced Consequences of Energy Production and Use. National Academy of Sciences Press.
  • Letter to President Obama and senior administration officials, May 4, 2009, from the Council of Scientific Society Presidents. pdf

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Click on the question of interest to display a drop-down answer.

Is your paper “peer-reviewed” and what exactly does that mean?

Yes, our study was independently and anonymously peer-reviewed prior to publication.

Our paper was published by Climatic Change Letters which, like most scientific journals, only publishes articles that are recommended for publication by anonymous peer experts. The review procedure they used is that followed by most journals:

  1. Authors submit draft manuscript;
  2. Journal editor chooses a set of independent experts and sends the draft manuscript to them for review and comment;
  3. Reviewers return the draft to the journal editor with comments and a set of notes;
  4. The journal editor at this point either:
    • forwards the reviewers comments and notes (stripped of identifying information, i.e. the process is anonymous) to the authors to revise the paper and address any concerns noted by the reviewers;
    • if the reviewers’ comments are too negative, the manuscript is rejected.

In this case of our paper, we received around 15 pages of single spaced comments. The comments were positive, although they called for many revisions. We spent a month or two studying the comments and revising our paper accordingly and sent a new draft back to the editor, who then sent it along again to the reviewers. The reviewers all agreed that we had met their concerns with our new draft, and advised the editor accordingly; the editor then decided to publish our paper. Note that more than half of the papers that are submitted are not published, but rather are rejected by this process.

Sometimes industry and government use the term "peer-reviewed" in a much looser way. For instance, an agency might send a report to a few friendly experts (rather than anonymous and more critical reviewers). The agency may or may not take the advice they receive and they are free to print their report regardless of reviewers concerns. Sometimes, the reviewers receive funding from the agency, and so have a conflict of interest. This kind of review is not the same as that carried out by independent science journals.

If flowback emissions were as high as reported in your study, wouldn't it cause safety concerns?

Yes, they really do vent that much, as documented in both our paper and the EPA 2010 report. A FLIR video from a Marcellus well flowback is available here

Note that methane is less dense than air, and so naturally buoyant (i.e. 16.04 g/mol for methane vs 28.02 g for N2 or 32 g for O2). When coming up with the flowback fluids from depth, it is also very hot, and so even more buoyant. It rises rapidly into the atmosphere, and is not likely to accumulate at surface, so the explosion risk is low.

Why haven't you responded to the criticisms of your paper in the blogosphere and the Google ad from America's Natural Gas Alliance (ANGA)?

There is no mechanism available to us to respond to a Google ad, and we lack the time and resources to respond to blogs. We are scientists, and not advocates or public relations personnel. Our paper was published in a peer-reviewed journal, and we believe the proper place to present alternative views would also be in the peer-reviewed literature, rather than in hastily assembled, sometimes anonymous statements that lack independent scrutiny and review regarding their accuracy.

Have you seen any information presented in the blogs that would make you reconsider the information in your paper?

No. We worked diligently over 18 months to prepare our paper, and sought outside advice and review by many experts, including critical ones. Further, when we submitted the paper to Climatic Change Letters, the editor sent our paper out for review by anonymous experts, and we included their guidance in our final publication. Many of the criticisms we have seen in blogs are way off the mark, often scientifically inaccurate, and none that we have seen so far would make us change anything fundamental in our paper.

What is your response to NETL’s 2011 Life Cycle Greenhouse Gas Study of Natural Gas?

The NETL study is a powerpoint presentation of their analysis given in an oral presentation at Cornell, and not an independently peer-reviewed paper. The Post-Carbon Institute (Hughes 2011a) has since released a comparative analysis of our peer-reviewed paper and the NETL presentation. We refer the reader to Hughes 2011a), which concludes that the NETL analysis is consistent with our paper, if one corrects for some of their questionable assumptions(Fig 1).

Shale gas study compare

Fig 1. Comparison of total shale gas GHG emissions of Howarth et al. to Barnett Shale emissions of Skone after adjustment to an EUR of 1.24 bcf, with and without an adjustment to match the EPA emissions inventory. This assumes a 20-year timeframe with a GWP for methane of 105. Note that the Howarth et al. estimates include distribution losses which the Skone estimates do not.(Source: Hughes 2011, figure 4)

Note that the NETL study focused only on producing electricity from natural gas. We took a broader view, focusing on all uses of natural gas, since most natural gas is used for heating and industrial processes and not for electricity. As discussed in our paper, natural gas is more efficient in generating electricity than are other fossil fuels. For heating and other uses, natural gas has no suchadvantage.

Why does the paper compare fuels based on energy content rather than exclusively electric power generation?

In the United States, only 30% of natural gas consumption is used to generate electricity. According to the Energy Information Agency of the U.S. Department of Energy, the use of natural gas for electricity will fall slightly in the coming couple of years, and then will remain relatively constant – both in terms of absolute amount of gas used and as a percentage of gas use – for the coming decade or so (Fig. 1). Most natural gas in the U.S. is used to generate heat for industrial processes and for commercial and domestic needs, and the Department of Energy predicts more growth in this use of natural gas for heat than for electricity. Shale gas is expected to largely replace falling production from conventional natural gas and imports.

natural gas end use

Fig 1. Only 30% of natural gas in the United States is used to generate electricity. The Department of Energy forecasts no major change in this picture over the coming decades. (Source: EIA, Annual Energy Outlook 2011 Early Release ppt

The major energy use globally and in developed countries is for heat, followed by transportation. Electricity is a distant third. The major fuel to generate heat is natural gas, making up more than half of all heat in developed countries (Fig 2). Many lower carbon-footprint alternatives for heat exist. Replacing conventional gas with shale gas for generating heat will significantly aggravate the carbon footprint.

Fig 2. Heat generation is the major use of energy worldwide with natural gas providing the largest share of the fuel mix. (Source: International Energy Agency (2011). Cogeneration and renewables: Solutions for a low-carbon energy future)

We therefore focused on comparing shale gas with conventional gas, oil, and coal in terms of total greenhouse gas emissions per unit of energy available for either heat or to generate electricity. As noted in our paper, the final efficiency of use of the energy matters. In the case of electricity, natural gas is used with somewhat greater efficiencies than coal. Nonetheless, the greenhouse gas footprint of shale gas when used to generate electricity is still far worse than that of coal, when considering the effects of methane over a 20-year integrated time period, and only slightly less than that of coal over a 100-year time period. For heating, the efficiency of use of natural gas is no greater than that of other fossil fuels. For a more detailed discussion on the greenhouse gas footprint of shale gas used to generate electricity, see J.D. Hughes (2011b).

Why does your paper use methane global warming potentials (GWP) for both 20- and 100-yr time horizons?

Methane is an incredibly powerful greenhouse gas, far more powerful than carbon dioxide as long as it stays in the atmosphere. However, methane has a 10-fold shorter residence time in the atmosphere than does carbon dioxide. As a result, methane has a global warming potential that is some 105-fold greater than that for carbon dioxide over a 20-year integrated time period following emission, and 33-fold greater over a 100-year time period.

As discussed in our paper, most previous peer-reviewed papers on the greenhouse gas footprint of conventional natural gas also used both shorter and longer time periods, as both are important to consider. We followed this example, and also presented both. If one is concerned about global warming over the coming several decades, it is essential to consider the shorter time scale. Many Earth systems scientists are deeply concerned that continued global warming over coming decades may push the planet past some tipping point, and move the global climate system into some alternative, highly undesirable state. But it is also important to consider the longer time frame.

In June of 2011, the United Nations issued a new report emphasizing the importance of reducing emissions of greenhouse gases other than carbon dioxide, pointing out the great gains that could be made by focusing on gases, such as methane, with shorter residence times (UNEP 2011).

Why did you not use the Intergovernmental Panel on Climate Change value for global warming potentials ( GWPs)?

The last report of the Intergovernmental Panel on Climate Change was published in 2007. They used the best available scientific information available to them. However, more recent evidence, published in the prestigious journal Science in the fall of 2009 (Shindell et al., 2009), demonstrated that the global warming potential of methane compared to carbon dioxide is greater than assumed by the IPCC (2007) due to the interaction of methane with other radiatively active components in the atmosphere, such as aerosols. We chose to use the most recent, most robust science available in our analysis.

As noted in our paper, both carbon-trading markets and the U.S. Environmental Protection Agency continue to use very old science for estimates of global warming potentials, relying on the lower values from the IPCC (1996) report rather than the IPCC (2007) report or the more recent paper in Science. We find this indefensible.


17 Jan 2012: Bob Howarth presents a review of recent research on the importance of methane emissions and climate impact of shale gas development for BC Sustainable Energy Association: watch video

Howarth et al presentation, 15 Mar 2011

Related Studies

D. Hughes. 2011 Lifecycle Greenhouse Gas Emissions from Shale Gas Compared to Coal: An Analysis of Two Conflicting Studies A comparative analysis of the Howarth et al 2011 paper and NETL (Skone et al 2011) study

D. Hughes. 2011 Will Natural Gas Fuel America in the 21st Century?

Osborn et al. 2011. Methane contamination of drinking water accompanying gas-well drilling and hydraulic fracturing Duke University study on methane contamination of drinking water from gas wells using hydraulic fracturing

Related Media Coverage

25 June, 2011. The New York Times. Insiders Sound an Alarm Amid a Natural Gas Rush