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Criticism leveled at natural gas emissions study, part II

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Back in February we wrote about a new estimate for methane emissions, based on air sampling, at a natural gas field in Colorado. Authored by scientists at the National Oceanic and Atmospheric Administration (NOAA) and the University of Colorado, Boulder, the study suggested that methane emissions from this particular natural gas field might be significantly larger than commonly assumed. But the results also came under fire, and today the Journal of Geophysical Research (JGR) posted a formal peer-reviewed comment by Michael Levi at the Council on Foreign Relations, who presents an alternative analysis that is in line with prior emissions estimates.

First a little background. It might seem odd to have methane emissions from a natural gas field, but a little leakage is inevitable. Depending on the operation, methane might be vented for a short period as operators bring a new well on line, and then there are occasional leaks, etc. Most estimates suggest that around 1–2% of the methane is lost because of this kind of leakage. These come from ‘bottom-up’ calculations by industry and government regulators, but there is very little hard data.

The NOAA paper presented a ‘top-down’ estimate based on field measurements and suggested that leakage rates could be roughly double published estimates — with a best guess of 4% — and maybe significantly higher.  This raised broader questions about the ongoing gas boom: methane is a powerful greenhouse gas, and seemingly minor leakage rates could undercut natural gas’s climatic edge over coal. The key is that the NOAA’s estimates were extrapolated from measurements taken in 2008 and based on various assumptions about the source of emissions, including composition of the natural gas being produced. In particular, the NOAA team looked at the ratio of methane to propane identified in a database of 77 natural gas wells and used those numbers to calculate the emissions that would be necessary to explain the atmospheric concentrations they were seeing in their measurements.

Levi’s comment is chock full of equations (and described in detail on his blog), but the gist of his criticism is that the methane-propane ratios are neither reflective of broader trends, necessarily, nor particularly useful in constraining the range of emissions once uncertainties are factored in. He goes on to perform an alternative analysis that incorporates methane-butane ratios, which are more consistent across the wells tested. He says that his estimates are both better constrained and, as it happens, in line with previously published estimates.

“I’m not trying to say that I have the final story on this,” Levi says. “All I’m claiming is that this particular set of field data, when you analyse it properly, is consistent with every other credible analysis using bottom-up numbers.”

Levi first submitted his comment to JGR back in the spring, but Gabrielle Petron, lead author on the NOAA study, says that her team is still working on a formal response.

Ultimately, more and better data will be needed to pin things down, both in Colorado and further afield. The NOAA team is continuing its investigations at the site, and others are taking note as well. This week the University of Texas at Austin announced that it will conduct a field study to analyse methane leakage in cooperation with the Environmental Defense Fund, an advocacy organization based in New York, and nine major natural-gas producers.

The good news is that leaks can be plugged. In April, the US Environmental Protection Agency issued new rules that should reduce methane emissions by requiring better drilling practices (environmentalists say they could have been stronger). And Levi points out that, since the original data were collected for the NOAA study, the state of Colorado has issued rules that should have reduced emissions there as well. As scientists home in on the problem, regulators and industry should be able to do the same with solutions.


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