Update (24/04/12): In case anyone wasn't sure about how widely the Howarth paper is now cited by those opposed to fracking, without any recognition of how controversial, and how widely criticised it has been from the scientific community, check out how often it gets a mention on the Guardian letters page (link).
As promised to those of you who follow my twitter feed (@TheFracDoctor) for those who weren't aware, a blog about the Cornell/Howarth paper, does shale gas emit cause more global warming than coal?
The reason we're talking about this is because of a study by Howarth and Ingraffea, from Cornell, examining the likely greenhouse-gas footprint from shale development. Link to paper here. We all know that burning gas produces about 1/2 as much CO2 as coal, so swapping coal for gas should be a good thing for global warming, right? However, during shale gas production, a proportion of the methane gas is vented from the well-head. Methane is a potent greenhouse gas (about 20x more so than CO2). So, Howarth et al came up with some numbers for methane emissions, and found that shale gas could contribute significantly more to global warming than, for example, using coal for electricity generation.
This paper is now regularly cited by anyone who doesn't like shale gas. Examples here (otherwise a sensible, well balanced article in my opinion), and here, it features prominently on the shale gas wikipedia entry (here) and if I wasn't so lazy (just got back from a long day in London cheering my better half round 26.2 miles of London's streets) I could find plenty more examples. But how accurate is the study?
The Howarth et al paper has received a tonne of criticism. You can find a summary of criticisms here. Indeed, even Howarth's peers at Cornell have come out in criticism of the paper (here and here). The criticisms can be divided into several parts:
1. Overestimates of fugitive emissions. Howarth et al use an estimate of fugitive emissions of ~7% - i.e. for every 100 MSCF of gas produced, 7 will be leaked to the air. This is a very high number (most estimates would go for 1-2%). Simply from a commercial point of view, it's difficult to imagine a company allowing 7% of it's saleable resource go drifting into the atmosphere. Reduce the fugitive emissions from 7% to 2% and you go a long way towards showing that total global warming from shale gas is significantly less than from coal.
2. 20 year vs 100 year forcing. When comparing the global warming effect from methane with respect to CO2, Howarth et al use the 20 year forcing value. Basically, methane is a much more potent global warming gas than CO2, but it has a much smaller residence time. so methane emitted soon oxidises. Therefore, while it is potent over the first 20 years or so, over 100 years methane is much less significant for global warming. Common practice in these things is to use the 100 year forcings, yet Howarth et al chose to use the 20 year forcings. Had they used the 100 year forcings, the effect of the fugitive emissions becomes a lot less than the CO2 emitted by coal power plants.
There were a couple of smaller points related to whether we should consider emissions for electricity generation versus domestic heating (gas domestic heating is less efficient than electricity generation, but hardly a fair comparison given that noone uses coal to heat their houses any more) and ignoring methane emissions from coal mining, but this post is already getting pretty long. In short, in my opinion, this paper should be used with extreme caution, and I think whenever it's used by mainstream news outlets it needs to come with a large disclaimer.
Anyway, to finish off, is shale gas good for us with respect to global warming? Well, that very much depends on how it fits into our energy production landscape. If shale gas is used to push renewables off the grid, then no, it's not a good thing for global warming. However, if the cheap gas prices enabled by shale gas makes coal-fired power a less attractive option, then it can only be a good thing. The world has far too many coal-fired power-plants, both in the West and especially in China. The key is for governments to stay strong and ensure that backing and subsidies for renewables remain in place, gradually allowing renewables to increase their market share, while allowing the market to dictate a replacement of coal power plants by gas turbines.
In this blog, the Secretary of Pennsylvania Department of Environmental Protection (or somesuch), the cheap price of shale gas has lead to the retirement of 106 coal-fired power stations, and the cancellation of 168 planned (future) such plants. These numbers seem a little high to me, and I have no way of confirming them. However, the general trend of gas replacing coal is almost certainly true, If that trend can be replicated when China starts producing shale gas in significant volumes, then there's a lot of Chinese coal-fired power plants that need to be shut down.