Environmental Engineering Reference
In-Depth Information
Table 2 Estimates of comparative life-cycle climate impact of shale gas.
Life-cycle climate impact of electricity
generated from shale gas relative to:
Conventional
natural gas
Methane
GWP
Study
Coal
Howarth et al. (2011)
7
14 to 19% greater
18% lower to
more than 100%
greater
33 and 105
Jiang et al. (2011)
8
3% greater
20 to 50% lower
25
Skone et al. (2011)
9
3.4% greater
42 to 53% lower
25
Stephenson et al. (2011)
10
1.8 to 2.4% greater
30 to 50% lower
25
Burnham et al. (2011)
11
6% lower mean value
but statistically
indistinguishable
52% lower
Hultman et al. (2011)
12
11% greater
44% lower
25
JISEA (2012)
21
Very similar
Less than half
25
AEA (2012)
14
1 to 8% greater
41 to 49% lower
25
MacKay & Stone (2013)
15
0.5 to 22% greater
49 to 53% lower
25
all of the assessments are independent, the work by AEA (2012)
14
and
MacKay and Stone (2013)
15
relies upon data presented in a number of the
other studies.
GHGs have different radiative properties, lifespans in the atmosphere and
interactions with other atmospheric components; a number of metrics are
available for comparison of these differing impacts from emissions.
16
Global
Warming Potential (GWP) is the most commonly used metric for policy
appraisal and life-cycle comparison. It integrates the warming effect of an
instantaneous release of gas, relative to carbon dioxide, over a chosen time
period, typically 100 years. The use of different time periods and different
estimates of GWP can significantly affect the quantitative and qualitative
conclusions of comparative emissions-accounting studies.
Carbon dioxide and methane are the main GHGs arising from shale gas
production, with methane being the more potent of the two. It has an at-
mospheric lifetime of 12.4 years and is subsequently oxidised to CO
2
though
a series of reactions, with indirect effects on tropospheric ozone, strato-
spheric water and sulfate aerosols.
17
Most studies use the Intergovernmental
Panel on Climate Change (IPCC) Fourth Assessment Report, AR4 (2007),
18
estimates of GWP, with a tonne of methane taken to have 25-times the im-
pact of CO
2
over a 100-year period. The most recent IPCC report, AR5
(2013),
19
has up-dated the advised values for methane to 28-times on a 100-
year basis, if climate-carbon feedbacks are not accounted for, and 34-times if
they are. Using these values would tend to increase the impact of upstream
emissions relative to combustion emissions and increase the importance of
mitigation of these sources.
When ranking shale gas against coal or renewable sources of energy such
revision is unlikely to make a substantial difference. However, as can be seen
from Table 2, presenting a 20-year comparison period can make a much
larger difference and alter rankings between fossil fuel sources which have
