Global warming and natural gas: The role of methane

Background/Question/Methods

Natural gas is widely promoted as a “clean fuel” that has a lower greenhouse gas footprint that other fossil fuels.  Is this true?  And as conventional natural gas is increasingly replaced by unconventional gas from shale formations, tight sands, and coal beds, what is the consequence on greenhouse gas emissions?  In this talk, we will evaluate all available information on greenhouse gas emissions from both conventional and unconventional gas, focusing on methane emissions.

Results/Conclusions

Most estimates for methane emissions from conventional gas systems, considering the total life cycle from well drilling to delivery of gas to consumers, estimate that between 1.6 and 3.9 of the lifetime production of gas is emitted to the atmosphere as methane.  For unconventional gas, most studies estimate that methane emissions are 40% to 60% greater than for unconventional gas.  Several lines of evidence indicate that most of the emission estimates are low, for both conventional and unconventional gas.  Nonetheless, when viewed on the time scale of an integrated 20-year period following emission, methane dominates the greenhouse gas footprint of both conventional gas and unconventional gas, using all reasonable estimates of emissions.  At present, when viewed at this 20-year time scale, methane emissions make up almost half of the entire greenhouse gas inventory of the US.  At longer time scales, the importance of methane is less, since the residence time of methane in the atmosphere is less than for carbon dioxide.