The idea was, perhaps unsurprisingly, that natural gas will solve the supply problem of "peak oil" - when global oil production starts to decline - and dramatically cut US emissions of greenhouse gases, making it a perfect bridging fuel to a low-carbon future.

With gas prices approaching record highs at the time, I was sceptical to say the least. But things have changed. Today the US is awash with cheap gas, thanks in part to the newfound ability to extract large amounts of shale gas. So could it be that Hefner, despite his obvious commercial interest, was right all along?

Fellow tycoon T. Boone Pickens has also been ...