Natural gas extracted from shale（页岩） formations has a greater greenhouse gas footprint - in the form of methane（甲烷） emissions - than conventional gas, oil and coal over a 20 year period. This calls into question the logic of its use as a climate-friendly alternative to fossil fuels, according to Robert Howarth and colleagues, from Cornell University in New York. Their work is published online in Springer's journal, Climatic Change Letters. Shale gas has become an increasingly important source of natural gas in the United States over the past decade. Howarth and team evaluated the greenhouse gas footprint of natural gas, obtained by high-volume hydraulic fracturing（水力压裂法） of shale formations, focusing on methane emissions. They analyzed the most recently published data - in particular, the technical background document on greenhouse gas emissions from the oil and gas industry (EPA 2010), as well as a report on natural gas losses on federal lands from the General Accountability Office (GAO 2010).

They calculated that, overall, during the life cycle of an average shale-gas well, between four to eight percent of the total production of the well is emitted to the atmosphere as methane, via routine venting and equipment leaks, as well as with flow-back return fluids during drill out following the fracturing of the shale formations. Routine production and downstream methane emissions are also large, but comparable to those of conventional gas.

Methane is a far more potent greenhouse gas than carbon dioxide, but methane also has a 10-fold shorter residence time in the atmosphere. As a result, its effect on global warming falls more rapidly. Methane dominates the greenhouse gas footprint for shale gas on a 20 year horizon, contributing up to three times more than does direct carbon dioxide emission. At this time scale, the footprint for shale gas is at least 20 percent greater than that for coal, and perhaps twice as great.

Robert Howarth concludes: "The large greenhouse gas footprint of shale gas undercuts the logic of its use as a bridging fuel over coming decades, if the goal is to reduce global warming. The full footprint should be used in planning for alternative energy futures that adequately consider global climate change."

Shale gas is extracted by a high-volume hydraulic fracturing (fracking) process. Large volumes of water are forced under pressure into the shale to fracture and re-fracture the rock to boost gas flow. A significant amount of water returns to the surface as flow-back within the first few days to weeks after injection and is accompanied by large quantities of methane.