It's possible for us to revert either to natural gas or to coal or both. Among consequences are the increasing global climate change. But another consequence is, let us suppose you tried to substitute coal for oil. Natural gas is a good substitute and it will last for a while but it will have its own peak one or two decades after oil, so it's only a temporary solution. If you turn to coal, we're now using twice as much energy from oil as we are from coal. So if you want to liquefy coal as a substitute for oil in transportation--which is its most important application--you would have to mine coal at a rate that's many, many times at the rate of what we're doing now. But the conversion process is very inefficient. So you'd have to mine much more than that. If you put that together with the growing world population and the fact that the rest of the world wants to increase its standard of living, you realize that the estimates that say we have hundreds of years worth of coal in the ground are wrong by a factor of ten or more. So we will run out of all fossil fuels. Coal will peak just like any natural resource. We will reach the peak for all fossil fuels by the end of the century.