But natural gas has come in for some sharp scrutiny of late. First, there was a study by Cornell ecologist Robert Howarth suggesting that natural gas was actually worse for the climate than coal when you take into account the methane leaks from shale-gas development. While that study has been rebutted by other analyses — see, for instance, this new paper in Environment Research Letters — it did rightly call attention to the methane problem. Meanwhile, questions have been raised about how much recoverable gas the Marcellus actually contains, while concerns about the process to extract the gas, hydraulic fracturing, abound.

And now comes along a new study by Tom Wigley of the National Center on Atmospheric Research, raising further questions about natural gas’s climate impact. Wigley’s study, which will be published in Climactic Change Letters next month, finds that a worldwide partial shift from coal to natural gas could actually speed up global warming between now and 2050 (more on why in a sec). After that, global warming would slow slightly, although it would depend on how well the industry could limit methane leaks.

Even in the most optimistic case, natural gas isn’t a panacea — Wigley’s computer simulations suggest that a partial shift to natural gas would, at best, lower the increase in global temperatures about 0.1 degree Celsius in 2100. Here’s a chart showing the difference a natural-gas switch would make — notice that in all scenarios the temperature first increases (versus the coal scenario) and only later declines:

Since natural gas is cleaner and emits fewer sulfates, you’d actually get more warming in the short term. Now, since natural gas emits less carbon dioxide, you would get relatively less warming over a longer timeframe, although even then the net climate impact is fairly small. Ultimately, that’s not an argument against natural gas — after all, a slight improvement is still an improvement. Plus, reducing that sulfate pollution would lead to large public-health benefits. But the climate upside, at least, may not be as sweeping as advertised.