We evaluate the greenhouse gas footprint of natural gas obtained by highvolume hydraulic fracturing from shale formations, focusing on methane emissions. Natural gas is composed largely of methane, and 3.6% to 7.9% of the methane from shale-gas production escapes to the atmosphere in venting and leaks over the lifetime of a well. These methane emissions are at least 30% more than and perhaps more than twice as great as those from conventional gas. The higher emissions from shale gas occur at the time wells are hydraulically fractured-as methane escapes from flow-back return fluids-and during drill out following the fracturing. Methane is a powerful greenhouse gas, with a global warming potential that is far greater than that of carbon dioxide, particularly over the time horizon of the first few decades following emission. Methane contributes substantially to the greenhouse gas footprint of shale gas on shorter time scales, dominating it on a 20-year time horizon. The footprint for shale gas is greater than that for conventional gas or oil when viewed on any time horizon, but particularly so over 20 years. Compared to coal, the footprint of shale gas is at least 20% greater and perhaps more than twice as great on the 20-year horizon and is comparable when compared over 100 years.

Reengineering of the aircraft structural life prediction process to fully exploit advances in very high performance digital computing is proposed. The proposed process utilizes an ultrahigh fidelity model of individual aircraft by tail number, a Digital Twin, to integrate computation of structural deflections and temperatures in response to flight conditions, with resulting local damage and material state evolution. A conceptual model of how the Digital Twin can be used for predicting the life of aircraft structure and assuring its structural integrity is presented. The technical challenges to developing and deploying a Digital Twin are discussed in detail.

CORRESPONDENCE Pros and cons of 24/7 working stir up debate p.280 CULTURE Martin Kemp muses on 15 years of artists in lab schemes p.278 MUSIC In conversation with climate-change composer Paul D. Miller p.279 HISTORY Copernicus biography from Dava Sobel mixes fact and fiction p.276 Natural gas from shale is widely promoted as clean compared with oil and coal, a 'win-win' fuel that can lessen emissions while still supplying abundant fossil energy over coming decades until a switch to renewable energy sources is made. But shale gas isn't clean, and shouldn't be used as a bridge fuel.Shale rock formations can contain vast amounts of natural gas (which is mostly methane). Until quite recently, most of A fter a career in geological research on one of the world's largest gas supplies, I am a born-again 'cornucopian' . I believe that there is enough domestic gas to meet our needs for the foreseeable future thanks to technological advances in hydraulic fracturing. According to IHS, a business-information company in Douglas County, Colorado, the estimated recoverable gas from US shale source rocks using fracking is about 42 trillion cubic metres, almost Should fracking stop?Extracting gas from shale increases the availability of this resource, but the health and environmental risks may be too high. POINT Yes, it's too high riskNatural gas extracted from shale comes at too great a cost to the environment, say Robert W. Howarth and Anthony Ingraffea. COUNTERPOINT No, it's too valuableFracking is crucial to global economic stability; the economic benefits outweigh the environmental risks, says Terry Engelder.

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