Burning natural gas produces 50 to 60% less emissions than coal. However, there is growing evidence that the magnitude of fugitive emissions (venting and leaks) of methane (CH4), a much more potent greenhouse gas than carbon dioxide (CO2), from natural gas production, processing, transmission, and distribution has been considerably underestimated by the EPA in the United States and globally by the international EDGAR database of methane sources. The rapid growth in shale gas extraction and natural gas power generation in the US raise the risk of greater climate impacts than projected by the EPA and other environmental protection agencies. The evidence that methane emissions have been considerably underestimated by the EPA was recently reinforced by the initiation of an investigation into how the EPA estimates methane emissions by the EPA’s Inspector General. In addition there is independent evidence that global methane emissions from fossil fuel production are considerably underestimated. If it is found that emissions of methane from natural gas production facilities (venting and leaks) are considerably greater than current estimates, the advantage that natural gas has over coal would be eroded and the advantage of switching to natural gas as a cleaner form of energy diminished.
Introduction
To generate the same amount of energy, the emissions from burning natural gas are 50 to 60% less than burning coal. However, the overall climatic benefits of switching from coal to natural gas must also take into consideration the magnitude of fugitive emissions (venting and leaks) of methane (CH4) from natural gas production, processing, transmission, and distribution because methane is a much more potent warming greenhouse gas than carbon dioxide. Power plants, refineries, and shale gas extraction are the largest sources of fugitive methane. Robert Howarth, an environmental biology professor at Cornell University, has argued that the type of shale gas drilling taking place in Texas, New York and Pennsylvania generates particularly high emissions of methane. Howarth et al. estimate that between 3.6% to 7.9% of the methane from shale-gas production escapes to the atmosphere in venting and leaks over the lifetime of a well. If verified, as the United States increasingly extracts natural gas from shale formations, an increase in CH4 emissions from shale gas extraction is expected and could have serious climate effects.
The concentration of methane in the atmosphere plateaued from about 1999 to 2007. There is evidence that since 2007 it has begun rising again. It has been suggested that an increase in U.S. methane emissions could be responsible for half of the global increase.
Estimating methane emissions in the U.S.
The United States Environmental Protection Agency (EPA) estimates CH4 emissions from the natural gas supply chain by scaling up individual ground-level measurements, mostly collected by reporting from industry. The CH4 emissions measurements made by Allen et al. of the University of Texas, which are a primary source used by the EPA, were made with a Bacharach Hi-Flow Sampler (BHFS).
A subsequent study by Howard found clear evidence of sensor transition failure in the Bacharach Hi-Flow Sampler device in the measurements made in the University of Texas study. The occurrence of this sensor transition failure was corroborated by field tests with the University of Texas BHFS device. Comparing the BHFS measurements with CH4 concentrations determined by a different method (downwind tracer ratio) indicated that the BHFS measurements were low by at least a factor of three. Because BHFS measurements were the basis of 98% of the inventory developed for the EPA, the inventory clearly underestimates CH4 emissions from production sites. Howard was not only surprised that the BHFS sensor failure went undetected in the University of Texas study (even though secondary measurements made with the downwind tracer ratio technique confirmed the BHFS sensor failure), but also found it remarkable that such an obvious problem could escape notice in such a high profile, landmark study. Doubts about the EPA estimates have been reinforced by the announcement earlier this week that the EPA’s Inspector General has begun preliminary research to assess how the EPA estimates methane emissions from the oil and natural gas production sector.
Further corroboration of underestimates in the EPA data was reported in another recent study where the average methane emission rates were found to be larger than the operator-reported estimates by 21 to 120 times for power plants and by 11 to 90 times for refineries. The researchers were able to determine that the methane emissions were primarily from non-combustion processes thus suggesting leaks and venting as the sources. Scaling these results to the national level suggests that methane emissions from these types of facilities are 4.4 to 42 times larger than current estimates.
A recent study has assessed the spatial distribution of anthropogenic methane sources in the United States by combining comprehensive atmospheric methane observations, extensive spatial datasets, and a high-resolution atmospheric transport model. Based on the results of this analysis the authors conclude that the EPA underestimates methane emissions nationally by a factor of 1.5 X. The study found that methane emissions due to animal husbandry and fossil fuel industries were larger than indicated by existing inventories.
Robert Howarth, an environmental biology professor at Cornell University, has suggested that natural gas may even turn out to be worse than coal in contributing to global warming. Howarth has argued that the type of shale gas drilling taking place in Texas, New York and Pennsylvania generates particularly high emissions of methane and could be as dirty as coal. A study by Robert W. Howarth, Renee Santoro, and Anthony Ingraffea has concluded that the emissions of methane from shale gas wells are between 30% and 100% more than methane emissions from conventional natural gas wells. The study estimates that between 3.6% to 7.9% of the methane from shale-gas production escapes to the atmosphere in venting and leaks over the lifetime of a well.
A study of ambient methane in the Marcellus shale region of the U.S. found elevated methane levels compared to the Northern Hemisphere mid-latitude (30–60°N) mean. Since the Marcellus Shale region is primarily rural, the presence of concentrations more characteristic of urban levels suggest that emissions from Marcellus Shale development may be responsible for the enhanced concentrations in the region. Furthermore methane concentrations in 2015 were observed to be significantly greater than those observed in 2012, indicating that ambient methane concentrations have increased, possibly due to increased emissions in the region.
Global methane emissions
The concentration of methane in the atmosphere plateaued from about 1999 to 2007. However, there is evidence that since 2007 it began rising again.
A recent study which analyzed GOSAT satellite and surface observations of atmospheric methane suggests that U.S. methane emissions have increased by more than 30% over the 2002–2014 period. This contrasts with EPA estimates which indicate that emissions remained flat over this period. It has been suggested that the increase in U.S. methane emissions over the 2002–2014 period could account for 30–60% of the global growth of atmospheric methane seen in the past decade.
The global methane budget and the contribution of the fossil fuel industry to methane emissions has recently been re-evaluated. The ratio of carbon-13 to carbon-12 in atmospheric methane provides a signature which helps identify the source of methane. Researchers have analyzed carbon-13 to carbon -12 ratios methane in the largest global isotopic methane source signature database. The analysis reveals that total fossil fuel methane emissions from the fossil fuel industry plus natural geological seepage are 60 to 110 % greater than current estimates. After accounting for natural geological seepage, methane emissions from natural gas, oil and coal production and their usage were found to be 20 to 60 % greater than current estimates.
Methane as a greenhouse gas
There is a consensus that methane is much more potent as a greenhouse gas contributing to global warming than carbon dioxide, but there is disagreement about the magnitude of the difference. The U.S. EPA estimates that methane is 21X more potent compared to carbon dioxide, but Howarth argues that methane is actually 72X as powerful as carbon dioxide in contributing to global warming when considered over a 20 year period. With this number and assuming with Howarth et al. that 3.6% to 7.9% of the methane from shale-gas production escapes to the atmosphere in venting and leaks over the lifetime of a well, the greenhouse gas footprint of shale gas is estimated to be 20% to 100% greater than coal over a 20 year period.
Conclusion
There is strong evidence that methane emissions have been considerably underestimated by the EPA. This has been reinforced by the initiation of an investigation into how the EPA estimates methane emissions by the EPA’s Inspector General. In addition there is independent evidence that global methane emissions from fossil fuel production are considerably underestimated. Together this provides grounds for serious concern about the climate impacts of methane emissions. If it is found that emissions of methane from natural gas, especially shale gas, production facilities (venting and leaks) are considerably greater than current estimates, the advantage that natural gas has over coal would be eroded and the advantage of switching to natural gas as a cleaner form of energy diminished.
Sources:
Measurements of methane emissions at natural gas production sites in the United States, Allen, D. T., V. M. Torres, J. Thomas, D. Sullivan, M. Harrison, A. Hendler, et al., 2013, Proc. Natl. Acad. Sci. USA 110:17768–17773. doi: 10.1073/pnas.1304880110
