Fracking methane

The Howarth et al paper estimating the climatic impact of shale gas extraction by hydraulic fracturing (fracking) has provoked a number of responses across the media. Since the issue of natural gas vs. coal or oil, and the specifics of fracking itself are established and growing public issues, most commentary has served to bolster any particular commenter’s prior position on some aspect of this. So far, so unsurprising. However, one aspect of the Howarth study uses work that I’ve been involved in to better estimate the indirect effects of short-lived emissions (including methane, the dominant component of shale gas). Seeing how this specific piece of science is being brought into a policy debate is rather interesting.

The basic issue is that for any real economic or industrial activity there are a variety of emissions associated with the life cycle of that activity – from construction, transport of fuels, operating emissions, end products etc. In deciding whether one activity is ‘better’ or ‘worse’ than an alternative, people need to have an assessment of the cost, the carbon footprint, other impacts etc., over that whole life cycle. There are of course different elements to this (cost, pollution, social issues) that need to weighed up, but one piece that is amenable to scientific analysis is the impact on climate drivers.

Calculating the net climate impact of an activity requires tracking many different emissions (not just CO2), and accounting for their (time-varying) impact on radiatively active components of the atmosphere or the properties of the affected land surface. While straightforward in conception, this can be complex and, inevitably, there are uncertainties in assessing all the knock-on effects. Over the years, many of the complexities have become better acknowledged which, in some cases, increases the total uncertainty, but the alternative of assuming that the indirect effects have zero impact with zero uncertainty is not tenable.

For shale gas extraction, (and indeed for most fossil fuel extraction), a big issue is fugitive emissions. These are emissions that arise by accident – mostly consisting of methane, but also other volatile organic compounds – as a function of the mining, refining, transport, or incomplete combustion. Since methane is a relatively powerful greenhouse gas whose source is dominated by anthropogenic activities at present, the impact of the fugitive emissions can be a significant component of the climate forcing associated with any activity.

The Howarth study, using admittedly poor observations (for lack of anything better), has come up with a relatively large potential for fugitive emissions from the fracking process itself – up to a few percent of the extracted gas. Converting this into an equivalent CO2 amount (for comparison with the impact of the gas once it is combusted), they have used Global Warming Potentials (GWPs) from Shindell et al (2009) (a paper I co-authored). A GWP is a kilo-for-kilo comparison of the radiative forcing associated with the emission of particular substance compared to CO2, integrated over a specific time frame. For a long-lived gas like CO2, forcing persists over a long time, while for a shorter lived species (like methane), the forcing goes down faster with time. Therefore the time frame for the GWP calculation matters a lot for the relative importance of the two gases. Methane is relatively more important for a 20 year time frame, than it is for a 100 year time frame, by about a factor of 3.

There are indirect effects from methane emissions because it is chemically reactive in the atmosphere. It contributes to increases in tropospheric ozone and stratospheric water vapour (increasing the warming impact), and by changing the oxidising capacity of the atmosphere, affects it’s own lifetime, and that of SO2 and NOx – which in turn affects aerosol formation, and indeed aerosol-cloud interactions. The IPCC (2007) report had acknowledged the potential for these indirect issues, but had not given any numbers. The Shindell et al paper was an attempt to fill that gap. As we discussed previously:

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