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
Government ministers argue the UK could burn gas -
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
Natural gas releases
about half the carbon emissions that
coal does when burnt, so it's generally viewed as a much less
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
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
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
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
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.
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
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
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
fix the leaks as there aren't as many
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
"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
in this blog.