When the entire lifecycle of natural gas is considered, however, it is impossible to call it "clean." Sourcing natural gas, like petroleum, is becoming increasingly difficult. To increase yields and tap into areas not suitable for conventional extraction techniques, operations have turned to hydraulic fracturing, commonly known as "fracking."
Fracking has been shown to be extremely harmful to local environments. The nature of the process -- which consists of releasing gas from rock or shale with high-powered blasts of water -- is inherently difficult to control. This has led to groundwater contamination and earthquakes, to name just two of the many consequences.
Furthermore, there is some evidence that the emissions of gas extracted through fracking are actually higher than estimated.