.In a recent review, DECC concluded that emissions from UK shale
gas should be comparable with conventional gas and lower than
coal (Figure 3). However, it excludes post-production emissions,
which may be considerable. For example, a recent academic
study found that groundwater methane concentrations increased
as shale gas wells aged, indicating significant leakage77. The
DECC review also assumes methane to be just 25 times more
potent a greenhouse gas than CO2, based on the 100-year
timeframe previously recommended by the IPCC78. However, the
IPCC now estimates methane to be 34 times more potent, or 84-
86 times when assessed over a 20-year timeframe (accelerating
warming in the short-term)79, meaning that the figures given by
DECC represent a significant underestimate.
To put the fugitive emissions figures into context, it has been
suggested that new gas plants reduce climate impacts compared
with new coal plants only if leakage rates remain below 3.2%81.
This is based on methane being 25 times (not the newly
recommended 34 times) more potent than CO2
http://www.sgr.org.uk/sites/sgr.org.uk/files/SGR-CIEH-Shale-gas-bfg.pdf