They say there is not long to go for oil and gas. There is for coal, but of course, if that takes over from the other two, it's time estimate will reduce drastically. Also, the estimates I've read are based on current per capita usage and population, and the population will certainly increase, and if usage trends continue, then per capita usage will increase too.
So picture a few hundred years down the line, we have a warmer world, and no fossil fuels.
We have the middle east and an ideology that doesn't like the west-how powerful will that become amidst the chaos? What kind of lives would we live? How much energy could alternative sources, hydro, wind etc give us, and how would it change our usage of technology? Will technological advance take a back seat, will we be forced to use no more advanced devices than now, or will what we have now be unsustainable with alternative energy?