Why measuring fugitive methane emissions from shale gas production matters
- 24 Jul 2014, 14:40
- Mat Hope
CC 2.0 Tim Evanson
As an ever-increasing number of countries
consider exploiting their shale gas resources, and researchers
scramble to understand what a production boom could mean for the
climate, two new pieces of research appear to come to opposite
What is the climate impact of shale gas?
Since gas has about half
the emissions of coal when it's burned for electricity, it has
been touted as
a 'bridging fuel' for countries seeking to decarbonise their
economies to use as a stop gap on the way to a low carbon
But as we've
explored before, scientists are struggling to establish the
full impact of increased shale gas production on the climate, due
to methane that escapes during the extraction process - known as
fugitive methane emissions.
Two papers released this month examine what the actual climate
impact of natural gas is. At first glance they seem to show
opposite things. The graph on the left, taken from a paper by
appears to show natural gas electricity generation emissions - the
towering left bar - can be much higher than coal's. The second
Heath et al, appears to show the opposite - that coal's
generation emissions (on the left) are much higher than those from
both conventional and shale gas.
Both papers examine the 'lifecycle emissions' of the fuels: the
amount of gas emitted from extraction to combustion. So why is
there such a large discrepancy between two papers?
In short, it's because Howarth's data includes an estimate of
fugitive emissions, while Heath et al's doesn't.
If you remove the fugitive emissions element (the red chunk on
the graph above) from Howarth's data, you get a very different
picture. Convert the data so both graphs use the same units, and
the paper's results look like this:
Sources: Howarth (2014) and Heath et al (2014).
Graph by Carbon Brief. Heath et al's median value is used for both
coal and gas.
Here we see the papers obviously agree that
without accounting for fugitive methane emissions, the emissions
from gas are much lower than coal.
This illustrates just how important
understanding fugitive methane emissions is to calculating the
climate impact of gas. So why doesn't Heath et al's graph include
them? In short, because fugitive emissions data remains
Fugitive emissions are a problem because methane
is a potent greenhouse gas - approximately
25 times more powerful than carbon dioxide
over a 100 year timescale.
There's currently a wide range of estimates
about how much methane escapes during gas production. Estimates of
gas leakage rates are expressed as a percentage of total
We've put some of the key estimates of methane
leakage rates in the chart below:
Source: Various, see
this Google Doc for details. Graph by Carbon Brief. Note: ^
means value is for unconventional - i.e. shale - gas wells only, *
means the value in the graph is the mid-estimate or mean of a range
where a 'best estimate' is not given.
There's a number of reasons the results have
such a wide range, which we've covered
in detail here. Perhaps the most important
is how the data was collected.
Broadly speaking, there are two approaches to
measuring fugitive emissions: bottom up and top down. Bottom-up
approaches, the blue bars on the chart above, are better are
measuring emissions from a particular well, but don't necessarily
accurately reflect the emissions of the whole production process.
Top-down approaches tend to come out with higher measurements, the
grey bars, as they potentially capture a wider source of
As you can see from our chart, Howarth's
research gives some of the highest estimates of fugitive emissions
for the studies using a bottom up approach - hence his contention
that gas can be more polluting than coal.
The main reason Heath et al don't include
fugitive emissions in their graph is because researchers are a long
way from agreeing a standard or average leakage rate.
So who's right? At the moment, the jury is out.
Gas is only cleaner than coal if the leakage rate is below 3.2 per
cent, according to
one estimate. Some
studies now suggest the amount of gas leaking from wells could be
as high as
nine per cent. But
opinions differ on which studies are more reliable.
Both Howarth and Heath et al's papers highlight
the need for better fugitive emissions data. Without it,
policymakers trying to curb emissions can't make informed choices
about whether to ramp up natural gas production.
The proliferation of studies since the issue
first hit the scene in 2011 means such data is gradually being
collated. As countries consider whether to emulate the US and
undertake their own dash for gas, the task becomes increasingly
urgent - as these papers' differences illustrate.
Howarth, R.W. 2014. A bridge to nowhere: methane
emissions and the greenhouse gas footprint of natural gas. Energy
and Science and Engineering, vol. 2, no. 2, pp. 47-60
Heath, , G.A., O'Donoughue, P., Arent, D.J., and Bazilian,
M. 2014. Harmonization of initial estimates of shale gas life cycle
greenhouse gas emissions for electric power generation. PNAS Early