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Are we underestimating natural gas emissions?

  • 13 Feb 2014, 17:10
  • Robin Webster

A new study threatens the conventional wisdom that natural gas emits half the greenhouse gases that coal does. The research, published in Science today, may have implications for plans to use shale gas as a major energy source in the future. 

Government ministers argue the UK could burn gas - including domestic shale gas - instead of coal as a way of cutting greenhouse gas emissions from the energy system. But gas leakage during the extraction and transportation process may mean gas is not as low carbon as official estimates presume. 

Leaking gas makes it hotter 

Natural gas releases about half the carbon emissions that coal does when burnt, so it's generally viewed as a much less carbon-intensive fuel. 

But when leakage is taken into account, the picture could change. Natural gas is mainly methane, a powerful greenhouse gas. When it escapes into the atmosphere, it adds to the warming effect of climate change. 

Today's paper reviews more than 200 studies, assessing leakage over the whole lifecycle of extracting, producing, processing and transporting natural gas from a variety of different facilities across the USA. The different stages are illustrated in the graphic below, created by the researchers: 

 Screen Shot 2014-02-11 At 12.17.48

US government regulator the Environmental Protection Agency (EPA) estimates the effect on emissions of using gas as a fuel in its greenhouse gas emissions inventory. It includes gas leakage in its assessment, but the studies reviewed suggest it's underestimating the effect. 

The actual leakage is about 1.25 to 1.75 times higher than the 'official' estimates from the EPA, the research concludes. And that means gas coud be more climate-polluting than the EPA thinks it is.

Same numbers, different conclusion

All this has implications for an academic debate about what burning shale gas means for the climate. In 2011, a study from Cornell University highlighted the effect of methane releases during the extraction and transportation of shale gas. 

The Cornell paper concluded that the effect was so extreme that shale gas's greenhouse gas footprint could be "even worse than coal's". 

Interestingly, today's study, authored by researchers from Stanford university, produces very similar numbers to the Cornell paper - but comes to the opposite conclusion. The 2011 Cornell study suggested that overall, 3.6 to 7.9 per cent of the gas leaks out during the process of extracting and transporting shale gas. Today's research produces almost identical figures- and for all gas natural gas facilities. It puts the range at 3.6 to 7.1 per cent. 

But today's study concludes that "system-wide leakage is unlikely to be large enough to negate climate benefits of coal to natural gas substitution" - or in other words, gas is less polluting than coal. 

Different timescales 

The reason for the different conclusions is the two studies compare the impact of gas and coal on the climate over different timescales. The Cornell study compares coal and gas over a 20 year timescale. Over 20 years, methane has a big impact on the climate - 72 times bigger than carbon dioxide. 

The Stanford university study compares the two fuels over a 100 year timescale. Methane doesn't stay in the atmosphere as long as carbon dioxide, so the effect over 100 years isn't as big. This means comparing the effect of leaking methane on a hundred year timescale makes it sound a lot less significant. 

Lots of caveats 

Today's paper has produced very similar figures to the 2011 Cornell study, and so could be used to make a similar argument - that over a 20 year time period, burning gas is more polluting than coal. 

The researchers say this isn't at all likely, however. There are two reasons for this. First, the argument only holds true if the amount of gas leaking is at the very top end of the range the paper suggests.

Second, the numbers the study gives are probably overestimates. That's because it's very hard to work out how much of the extra methane in the atmosphere comes from the natural gas industry, and how much of it comes from other sources. A significant portion of it could come from cows, for example. 

Lead author Professor Brandt tells Carbon Brief that for this reason the assessments of gas leakage in the research are "absolute worst case estimates". 

Diligence is needed 

Nonetheless, the study still implies that the US government could be significantly underestimating current emissions from extracting and burning gas as a fuel. It doesn't make a comparison to emissions over here, but a government spokesperson tells Carbon Brief UK and US methodology "should be broadly comparable". So it seems likely that the same issue may apply in this country.  

More positively, the paper argues that the problem could easily fixed. It concludes that just a few sites are responsible for most of the gas leakage measured. 58 per cent of methane emissions come from 0.06 per cent of the sources - so-called 'superemitter' sites. This means that is should be easy to fix the leaks as there aren't as many of them. 

But the issue can't be ignored. If anything, today's paper suggests that it may be even more significant than previously thought, because this analysis applies to all natural gas extraction - not just shale gas. 

The researchers emphasise this point in their final conclusion: 

"If natural gas is to be a "bridge" to a more sustainable energy future, it is a bridge that must be traversed carefully: Diligence will be required to ensure that leakage rates are low enough to achieve sustainability goals."

If politicians are going to promote natural gas as a low-carbon fuel, it's important that its effect on the climate is fully understood. Or the impact could be a lot more significant than they presume. 

UPDATE 14 Feb: In the  press release, the authors emphasise the difficulties of attributing methane emissions to a particular source. As discussed above, some of the difference between the EPA's estimate and the leakage observed will be the result of natural or other human-created sources, but it's hard to know how much at this stage - more research is needed. We have discussed the challenges of accurately measuring methane emissions from different sources  in this blog

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