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:

… we found that methane’s impacts increased even further since increasing methane lowers OH and so slows the formation of sulphate aerosol and, since sulphates are cooling, having less of them is an additional warming effect. This leads to an increase in the historical attribution to methane (by a small amount), but actually makes a much bigger difference to the GWP of methane (which increases to about 33 – though with large error bars).

For comparison, the GWP in IPCC (2007) was 25 – this is for a 100 year time frame. For shorter periods like 20 years, the relative increase in our numbers was somewhat higher (about 50%) over that given by in AR4.

Thus a combination of high fugitive emissions, and larger updated numbers for the impact of methane are the main components the Howarth conclusion, relating the impact of shale gas to coal. However, for an apples-to-apples life cycle comparison, one would need to also update the impacts of coal and oil to include their fugitive emissions, their impact on other short-lived components (black carbon, CO, etc). Thus, it’s not clear that the Howarth comparisons are exactly on a level playing field. Regardless, the uncertainties in some of these estimates are such that very clear conclusions are going to be elusive for some time to come.

A few further points are worth making. The estimates for fugitive emissions are uncertain because they are not being reported, either voluntarily by the industry or through regulation from the states. It is also worth stating that there is nothing inevitable about fugitive emissions. Better management (and/or regulation) can reduce these losses substantially (up to 90% in some situations) in very cost-effective ways (since lost methane is lost product in many cases).

Which brings me to the responses to this story. The industry website Energy in Depth was quick off the mark with a response that feigned surprise and shock that the emission estimates were uncertain (somewhat hypocritically since it is the same industry that has resisted almost any improvement in reporting standards). They also try to imply that the Shindell et al study was somehow suspect because it was different to the earlier IPCC GWP numbers, without any apparent interest or knowledge of why that was. Again, the industry would be better advised to deal with fugitive emissions (which also impact air pollution) rather than attacking inconvenient science. (Funnier still are the contrarian responses, for instance from “Bishop Hill” who completely agrees with the industry (again without any actual knowledge of the issues), and who can’t resist using their criticism of Howarth to condemn a whole University (and by proxy, the whole scientific enterprise). I mean, why bother with independent scientists when the industry can tell you exactly what you are supposed to think?).

Another frequent framing is the false dichotomy. Apparently, natural gas must either be perfect solve-all or worse than useless (see for instance, Keith Kloor’s take). One would think that the overwhelming consensus that there are no panaceas for decarbonising our energy supply might have at least started to make a little impact on the media. Any real policy initiative will have complex effects, and while scientists can certainly help quantify them, nothing at the scale we require is going to be completely neutral in all particulars – and the media should stop expecting it to be so. Since there will always be people who can be portrayed as having taken a black/white position on some issue, it is all too easy to frame any new result as undermining some over-optimistic idealist, which unfortunately buries the conversation related to the nuances of real issues.

Howarth et al is unlikely to be the last word on this subject, but it does highlight the need for more of this kind of research, and for further quantification of these emissions and their effects. For anyone interested in the larger issues of time-scales and the implications of combining emissions of short-lived and long-lived species in assessing impacts, I recommend reading the latest UNEP report on Black Carbon and Tropospheric Ozone mitigation (at least the summary). Another relevant read is the post by Ray Pierrehumbert on the same issue. This is not just an issue for fracking, but rather something that is far more general and affects almost all emitting activities.