Just finished this book. Greenspan devotes a whole chapter to oil. His view is less dooms-day than most here: He believes that market forces will eventually do away with oil far before it ever disappears from the ground, and his optimism for the amount of oil left in the ground is high. He says at current productions, the there is enough oil to sustain world economic activity up to 130 million barrels a day as projected in 2030, as long as OPEC allows it. He also says that environmental reasons are not enough to disrupt economic activity. The sad reality (and I agree with him) is that serious shifts away from oil as an energy source will only happen when it makes a serious dent on economic activity. So far, $100/barrel oil has not really fazed American, let alone world economic progress.
Anyway, as a self-proclaimed Libertarian-Republican, he seems suggest that market capitalistic forces will eventually solve the world energy crisis, as long as it's allowed to more or less operate freely.