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A comprehensive new study of atmospheric levels of methane, an important greenhouse gas released by leaky oil and gas operations and livestock, has found much higher levels over the United States than those estimated by the Environmental Protection Agency and an international greenhouse gas monitoring effort. The paper, “Anthropogenic emissions of methane in the United States,” is being published in the Proceedings of the National Academy of Sciences.

The study, combining ground and aerial sampling of the gas with computer modeling, is the most comprehensive “top down” look so far at methane levels over the United States, providing a vital check on “bottom up” approaches, which have tallied estimates for releases from a host of sources — ranging from livestock operations to gas wells.

Read on for an excerpt from the news release issued by Harvard University, the home of two of the lead authors, Scot M. Miller and Steven Wofsy, which summarizes the main points well.

Later today, I’ll add answers to questions I posed to the authors as well as reactions from Robert Howarth, the Cornell environmental scientist who’s been studying methane levels and pushing for an end to hydraulic fracturing for natural gas, and Steven Hamburg of the Environmental Defense Fund, which is working with a host of academic and industry partners on a big emissions study, part of which was described here recently.

This new work clearly points to the importance of the E.P.A. updating how it tracks this important emission, and also bolsters arguments for the Obama administration to move forward faster with its proposed standards for cutting leaks and emissions from oil and gas operations. A spokeswoman for the agency, Alisha Johnson:

E.P.A. has not yet had the opportunity to fully review the PNAS study on methane emissions; however we are encouraged that more methane emissions measurement data are now available to the public. E.P.A. is committed to using the best available data for our Inventory and continually seeks opportunities to update and improve our estimates. Research studies like these will add to our knowledge base of GHG emissions and will help us refine our estimates going forward. E.P.A. is currently evaluating data received through the Greenhouse Gas Reporting Program [link], which provides valuable information on the location and magnitude of certain emissions sources from large facilities.

As early as 2009, as I explained in a front-page article co-written with Clifford Krauss, the E.P.A. was acknowledging that its estimates for methane leaks from gas wells at that time were likely far too low:

An E.P.A. review of methane emissions from gas wells in the United States strongly implies that all of these figures may be too low. In its analysis, the E.P.A. concluded that the amount emitted by routine operations at gas wells … is 12 times the agency’s longtime estimate of nine billion cubic feet. In heat-trapping potential, that new estimate equals the carbon dioxide emitted annually by eight million cars.

It’s important to note that the new study is a snapshot of conditions in 2007 and 2008, before concerns increased about the need for tighter standards for gas and oil drilling operations. The authors say a similar analysis is under way for more recent years. A comparison will be very helpful in clarifying the impact of changed practices by energy companies so far, and what remains to be done.

First, the basics. Here’s an excerpt from the Harvard news release:

Overall, according to the new study, total methane emissions in the United States appear to be 1.5 times and 1.7 times higher than the amounts previously estimated by the U.S. Environmental Protection Agency (E.P.A.) and the international Emissions Database for Global Atmospheric Research (EDGAR) [link], respectively. The difference lies in the methodology. The E.P.A. and EDGAR use a bottom-up approach, calculating total emissions based on “emissions factors” — the amount of methane typically released per cow or per unit of coal or natural gas sold, for example. The new study takes a top-down approach, measuring what is actually present in the atmosphere and then using meteorological data and statistical analysis to trace it back to regional sources. Generated by a large, multi-institutional team of researchers, the latest findings offer a robust and comprehensive baseline for assessing policies designed to reduce greenhouse gases. They also point to a few areas where the assumptions built into recent emissions factors and estimated totals may be flawed. “The bottom-up and top-down approaches give us very different answers about the level of methane gas emissions,” notes lead author Scot M. Miller, a doctoral student in Earth and Planetary Sciences through the Harvard Graduate School of Arts and Sciences. “Most strikingly, our results are higher by a factor of 2.7 over the south-central United States, which we know is a key region for fossil-fuel extraction and refining. It will be important to resolve that discrepancy in order to fully understand the impact of these industries on methane emissions.”… Miller is a 2007 graduate of Harvard College and earned a master’s degree in engineering sciences at the Harvard School of Engineering and Applied Sciences (SEAS) in 2013. He studies in the lab of Steven C. Wofsy, Abbott Lawrence Rotch Professor of Atmospheric and Environmental Science at SEAS. “When we measure methane gas at the atmospheric level, we’re seeing the cumulative effect of emissions that are happening at the surface across a very large region,” says Wofsy, a coauthor of the PNAS study. “That includes the sources that were part of the bottom-up inventories, but maybe also things they didn’t think to measure.” Miller and Wofsy, along with colleagues at the Carnegie Institution for Science, the National Oceanic and Atmospheric Administration (NOAA), and five other institutions, used a combination of observation and modeling to conduct their analysis. NOAA and the U.S. Department of Energy collect observations of methane and other gases from the tops of telecommunications towers, typically about as tall as the Empire State Building, and during research flights. The team combined this data with meteorological models of the temperatures, winds, and movement of air masses from the same time period, and then used a statistical method known as geostatistical inverse modeling to essentially run the model backward and determine the methane’s origin. The team also compared these results with regional economic and demographic data, as well as other information that provided clues to the sources—for example, data on human populations, livestock populations, electricity production from power plants, oil and natural gas production, production from oil refineries, rice production, and coal production. In addition, they drew correlations between methane levels and other gases that were observed at the time. For example, a high correlation between levels of methane and propane in the south-central region suggests a significant role for fossil fuels there.

Here are the questions I posed (with some e-mail shorthand fleshed out), with responses from various authors as indicated:

1. It’d be great to see the same analysis done on a more recent span.* Is that in the works?

Scot Miller, Harvard:

[A]n analysis of more current time periods is in the works. I think our study provides an important baseline if we want to understand how emissions are changing with time. Starting in 2006 and 2007, N.O.A.A. began collecting many more measurements of atmospheric methane in the United States. The years 2007 and 2008 are really the first time period when we could conduct such an extensive and detailed analysis of methane emissions.

2. More specifically, do your regional findings give a clue as to how much of what you see from energy sector is from drilling as opposed to other oil and gas operations?

Miller:

Our findings do not provide such detailed sector-by-sector information. We use measurements of methane gas in the atmosphere. These air samples are taken from aircraft and from telecommunications towers that are often the height of the Empire State Building. The measurements “see” the accumulated signal of all surface sources over a surrounding region. Hence, we get a strong idea of total methane emissions over all source sectors but get much less information about individual sectors. Both ruminants and the oil and gas industries may be important players in the large discrepancies we see over the south-central US. But our analysis does not give specific information about different components of the energy sector.

Anna M. Michalak, Carnegie Institution for Science:

The greatest strength of the methods that we applied in this study is to give a very robust indication of the total emissions. This is invaluable, because it tells us what bottom-up inventory approaches, such as those of E.P.A. and EDGAR, have to add up to. The key difference between the inventory approaches and the approach that we used is that inventories are essentially accounting-based methods where an attempt is made to put together a comprehensive list of source types, and each source type is estimated based on available data. These line items are then added up to give an estimate of the total. As such, the inventories are very detailed in itemizing emissions, but uncertainties compound when looking at the total. Conversely, the approach used here sees the total directly, based on the amount of methane present in the atmosphere. What our study is showing is that the total seen from the atmosphere is much higher than the sum of the parts in the inventories. There is now an opportunity to re-examine the inventories based on the individual source types that you list in your question, to see exactly where source items are either underestimated or missing entirely from the inventories.

3. It’d be incredibly valuable to have ongoing monitoring and analysis of this sort, or have these methods incorporated into E.P.A. and EDGAR methods. Is this doable as a running analysis or is it implicitly a snapshot?

Steven C. Wofsy, Harvard:

NOAA people…are committed to making the measurements continuously and doing the analysis, also. Optimally their network would be more frequent and blank spots filled in, but their budget has been very constrained since the Bush administration.

Miller:

I think that “top-down” approaches like our study are invaluable in estimating greenhouse gas emissions…. With that said, our ability to estimate greenhouse gas emissions is only as good as our atmospheric observation network. As Steve mentioned in his e-mail, our ability to measure atmospheric methane has been subject to tight budget constraints for many years. If we want to understand methane (and greenhouse gas) emissions with high certainty sufficient for regulation, we would need an expanded network of observations.

Here are reactions to the work from Steven Hamburg at the Environmental Defense Fund and Robert Howarth of Cornell.

First, Hamburg:

The PNAS paper that is coming out today by Miller et al. provides new insight into the methane budget for the United States. The study compliments the ongoing work at universities across the country to quantifying the methane emissions from the natural gas supply chain. The need for greater certainty in the methane budget, as well as the need for better information upon which to facilitate methane emissions reductions, has motivated the large effort EDF has been leading, one involving dozens of academic experts. EDF’s three-year effort involving 16 academic-lead studies will conclude at the end of 2014 and should provide the component emissions data to allow improved mitigation as well as to better resolve any differences between top-down and bottom-up estimates of emissions from the natural gas supply chain. The data presented by Miller et al. constrains the overall leak rate from the oil and gas supply chains – providing an independently derived aggregate estimate of fossil fuel sources of methane emissions. While Miller et al.’s estimate is higher than EPA’s current emissions estimate from the oil and gas sector it does suggest that the much higher emissions rates found in a couple of recent geographically specific studies are unlikely to represent mean national emissions rates, but rather important opportunities for mitigation. We will need to wait until the 16 academic-lead studies are completed to compare them with the national top-down emissions rate reported by Miller et al. for fossil fuel related emissions, including end uses and natural leakage, neither of which is being measured in these studies. [

Robert Howarth:

Using this new information as well as other independent studies on methane emissions published since 2011, and the latest information on the climate influence of methane compared to carbon dioxide from the latest synthesis report from the Intergovernmental Panel on Climate Change released in September of this year, it is clear that natural gas is no bridge fuel. When used to generate electricity, natural gas likely has a greenhouse gas footprint similar to that for coal. However, when used for domestic heating of water, the greenhouse gas footprint of natural gas is at least two-times larger than that of using modern electric-driven heat pumps. Society should move as quickly as possible away from using natural gas for water heating and domestic and commercial space heating – uses which are equal to the use of gas to generate electricity in the US. This is the low-hanging fruit for reducing the total greenhouse gas emissions from the United States. When you add up that there is more methane being emitted than E.P.A. has estimated, that methane is responsible for up to half of all the greenhouse gas emissions for the entire US, and that each unit of methane emitted is far more important in causing global climate change over the critical few decades ahead, it should be clear that bridge-fuel argument just doesn’t hold up. And the oil and gas industry is the major source of these methane emissions.

I invited the lead authors of the new paper to react to the commments from Hamburg and Howarth. Anna Michalak offered this reply:

I agree with Steve’s comment, and want to add the following, which is, I think, objectively representative of what we can say based on our studies. Re. Hamburg: Our study reinforces the fact that specific source types for methane are higher than current E.P.A. and EDGAR inventories suggest, and they do indeed need more attention because emissions in some regions of the US appear to be substantially higher than previously estimated. Our study says nothing about whether such sources can be reduced, or even specifically about what aspects of oil and gas production, refining, and transport is responsible for the elevated sources. Re. Howarth: Our study bolsters recent smaller-scale studies that hinted that methane emissions associated with energy production may be higher than previously estimated. Our study does not address how the magnitude of the source types that we present translate into an overall climate impact for natural gas vs. other fuel types, nor does it address implications for natural gas as a bridge fuel. I think that the one policy implication that I would feel comfortable drawing from our study is that any discussion of methane must be based on a robust understanding of the current situation with regard to emissions. What our study shows is that these emissions appear to be substantially higher than the most recent inventory estimates from E.P.A. and EDGAR suggest.