Environmental Engineering Reference
In-Depth Information
substantially different profiles. Coal has a significantly greater carbon
footprint that is dominated by CO 2 from combustion when compared with
gas over a 100-year timescale. This ranking is reversed for higher levels of
methane leakage if a 20-year comparison is made; it is this choice that ac-
counts for Howarth et al.'s (2011) report of higher impacts for gas, not the
assumed rates of methane venting. 7,10
The choice of comparison period is a value judgement, not borne from a
scientific principle but related to the period over which one is concerned
about impacts. 17 Given that equilibrium climate response depends upon
cumulative emissions and is insensitive to the timing or peak rate of emis-
sions, this shorter period does not seem appropriate to energy policy
analysis. 20
Substantial uncertainties remain in the precise life-cycle impact of shale
gas production, 6 even in cases where detailed field work has been con-
ducted. 21 The majority of the quantitative shale gas studies identified in
Table 2 take a 'bottom-up' approach, i.e. identifying and summing point
sources from direct measurement or by reference to existing inventories,
emissions factors and activity factors. However, a number of recent top-
down studies, taking total atmospheric measurements and then attributing
these to sources by atmospheric modelling, have found substantial dis-
crepancy with bottom-up inventory-based estimates. 22-24 In one case, a tight
oil and gas field with processing facilities, the US Environmental Protection
Agency inventory estimate of 1.7% leakage of methane, for the field as a
whole, was found to be a substantial underestimate of the 2.3 to 7.7%
leakage indicated by atmospheric monitoring. 22 In a second case, leakage of
6.2% to 11.7% was identified with a mean value 1.8 times the record for
field. 23 National US emissions inventories have also been found to under-
estimate total methane emissions by a factor of ca.1.5toca. 1.7 times, with
substantial regional discrepancies suggesting that emissions from fossil fuel
extraction and processing could be 4.9 2.6-times larger than that recorded
in inventories. 24 Further work is required to identify the reasons for this and
understand the contribution made by shale gas production. Such discrep-
ancies suggest that greater attention should be paid to monitoring, the
process of inventory production and background measurement prior to
development. 25 Nevertheless, the broad conclusion that shale gas has a life-
cycle climate impact that is within the range of other natural gas sources
appears to be robust.
3 Shale Gas in the Global Energy System
To assess the impacts of shale gas production on climate change, it is ne-
cessary to go beyond a simple assessment of the carbon intensity of the gas,
to consider the potential role of shale gas in the global energy system.
International agreements on climate change mitigation focus on the
avoidance of dangerous climate change, 26 defined and agreed as more than
2 1C increase in global mean surface temperature above pre-industrial
levels. 27 This ambition is repeated in EU and UK climate policy. 28 To
 
Search WWH ::




Custom Search