It's a significant question because proponents of shale gas development using the controversial practice of high-volume hydraulic fracturing argue that natural gas is a cleaner-burning "bridge fuel" from the age of coal to an era of wind, solar and other sustainable energy sources.
The U.S. Energy Information Administration projects that unconventional gas, mainly from shale, will supply nearly half of U.S. gas production by 2035. A core benefit of tapping vast embedded gas reserves such as the Marcellus Shale beneath western New York, Pennsylvania, Ohio and West Virginia, is the belief that it produces less greenhouse gas than coal.
Opponents of shale development cite potential damage to health and the environment, especially water supplies, from hydraulic fracturing, or "fracking," which injects a well with chemically treated water to stimulate production. They also have seized on the greenhouse gas study published by Cornell's Robert Howarth in the journal Climatic Change last spring.
In that study, and in a follow-up released yesterday, Howarth said methane leakage at the well, along aging pipelines and at other points, give shale gas development a worse greenhouse gas footprint than that of coal. He estimated that as much as 8 percent of methane from shale gas production escapes into the atmosphere, where it is a far more potent greenhouse gas than carbon dioxide.
Howarth and colleagues Renee Santoro and Anthony Ingraffea wrote that the greenhouse gas footprint of shale gas is "perhaps more than twice as great" as coal when compared over a 20-year time frame, and comparable to coal over a 100-year period.
That flies in the face of previous estimates that the greenhouse gas impact of natural gas is about half that of coal.
But Cornell colleague Anthony Cathles countered the Howarth study in the same journal this month, challenging Howarth's calculations and conclusions. He noted that natural gas is widely considered to be cleaner than coal because it doesn't produce hazardous by-products such as sulfur, mercury and ash, and it provides twice the energy per unit of weight when it's burned.
"We argue that their analysis is seriously flawed in that they significantly overestimate the fugitive emissions associated with unconventional gas extraction," Cathles wrote. He said the Howarth study also undervalues the contribution of green technologies to reducing those emissions, and bases its gas vs. coal comparison on heat rather than electricity generation.
In his new study, Howarth stands by his analysis and conclusions.