If oil and gas production peak within the next decade or two, the obvious fallback will be to coal. Coal reserves worldwide are vast, but there are two reasons to believe they won't last very long if coal again becomes the biggest fossil fuel energy source. First, coal will be used not only to produce electricity, but also as a base for making liquid fuels. That means we will use it up faster, a LOT FASTER, than we are today. Second, like any resource, the best quality coal has been mined first, that is, the coal with the highest energy content. As we mine more and more of it, we can expect the energy yield per unit to steadily decline and then plummet. This piece on From the Wilderness explains the problem in more detail.
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