The study shows that, in general, natural gas produces less short-term climate change than coal only if there is little methane leakage associated with its extraction and if the efficiency of generating the electricity is high. It also shows that natural gas cannot deliver the depth of cut in emissions that’s needed to avoid a big contribution to global warming, unless carbon-capture is employed.

“We tried very hard to keep our own values out of the analysis and present a technical piece of work,” said Ken Caldeira at the Carnegie Institution for Science, US.

Over the past few years, many researchers have analysed the potential environmental impact of a more widespread transition to natural gas from coal, but conclusions have been mixed. Natural gas produces less carbon dioxide per unit energy than coal does, making it appear at first glance as a cleaner fuel. But natural gas is predominantly composed of methane, a much more potent greenhouse gas than carbon dioxide, and it is thought that leaks at drilling fields could outweigh its potential benefit.

To make matters more complicated, studies of natural gas have employed different metrics, and assumed different conditions, to assess the fuel’s impact on the climate. “If you choose a really bad coal plant as your point of comparison, natural gas looks relatively good,” said Caldeira. “If you pick the most efficient experimental coal plant, then benefits from natural gas look much smaller.”

There is also the problem of timescales, Caldeira says. Although potent, methane is a short-lived greenhouse gas that will strongly affect the climate for 10 years but produce less of an effect in 100 years. “Climate metrics that emphasize the next decade strongly penalize natural-gas plants that are associated with methane leakage to the atmosphere,” he said. “Climate metrics that emphasize the century timescale strongly reward the decreased carbon-dioxide output from natural gas plants relative to coal plants.”

“Rather than pick a timeframe to emphasize,” Caldeira said, “we just show the temperature change as a function of time and allow the reader to make their own judgments about what time scale is most important.”

Caldeira, together with colleagues at the Carnegie Institution and the patent-licensing company Intellectual Ventures, US, began by creating a model to assess the benefits of a variety of coal- and gas-fired plants under different conditions. However, the researchers quickly found that most of the differences in emissions came from just two factors: power-plant efficiency and methane leakage.

Taking these into account, Caldeira and the team found that substantial methane leakage biases a natural-gas plant towards greater short-term warming than a coal plant with the same output. Natural gas only reduced short-term warming if its associated leakages were low – although, notes Caldeira, average gas leakage in the US is 2.4%. “This means that there would be no short-term benefit if the best natural gas plant were pitted against the best coal plant,” he said.

In the long term, natural gas plants do indeed produce less warming than coal, the researchers found. But that result comes with a caveat: without high-tech carbon storage, neither gas nor coal can achieve the type of greenhouse-gas reductions demanded by international bodies such as the IPCC.

Caldeira, who says he is against the building of more natural-gas power plants, warns that it is all too easy for personal values to creep into scientific work, even if it is not deliberate. “We can at least try to be as objective as possible, and also be up front about our personal values,” he said. “Then people can look for themselves at how well we did at keeping our personal values out of our results.”

Related links

• Key factors for assessing climate benefits of natural gas versus coal electricity generation Xiaochun Zhang et al 2014 Environ. Res. Lett. 9 114022
• Ken Caldeira

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