We talked about the relative GHG strength of natural gas compared with other fuels at a recent meeting. I’d like to share with you recent report and the method which I think is most useful to think about it, which comes from the Environmental Defense Fund. They do some very good research in this area.
The main issues which muddy the water with natural gas are:
a) the leakage of natural gas from storage facilities, pipelines and other infrastructure is not accurately known, but is quite a bit higher than estimates made a few years ago, and which are used by the EPA. The following report summary (https://www.edf.org/media/new-study-finds-us-oil-and-gas-methane-emissions-are-60-percent-higher-epa-reports-0) says that the EPA estimated leakage (1.4%) is only 60% of the value from recent survey data (2.3%). Since methane is a much stronger greenhouse gas, the effects of leakage can dominate over the CO2 from combustion.
b) since methane has a short atmospheric lifetime (around 12 years) compared with CO2 (>100 years), the relative warming effect depends on the time scale chosen, which is somewhat arbitrary. The EPA uses a 100 year timespan, for which Methane is 30-35 times more warming, while for a 20 year timeframe it is 85-105 times more warming. It is clear that for our existence the 20 year timeframe makes some sense to avoid self reinforcing climate change (a “tipping point”) while we reduce emissions. But it is still an arbitrary choice, which number to use. The EDF researchers used a concept called “Technology Warming Potential” to resolved this, which is described in the attached paper. The TWP is the relative warming of a choice between one fuel and another over the years, and it uses the example of natural gas powered buses compared with diesel. For a moderate amount of natural gas leakage, and taking into account engine efficiencies, a choice to go with a natural gas bus over a diesel causes more warming than the diesel for the first 90 years, after which the natural gas would have less warming impact. It would be a similar calculation for a natural gas heating system, though perhaps not as extreme.
In fact, the EPA uses an older estimate of the Methane 100yr GWP of 21 (compared to 30), which is I think is biased. Because of the underestimated consequences of natural gas, it would be good if the GHG inventory that Concord uses includes an alternate weighting for natural gas use. I think of this as an ‘error bar’ – essentially the amount we may be underestimating our climate impact as a community.