Why is it important to use substitute energy?

What would we do if it were discovered that there was no solution at all to energy overconsumption and environmental damage?

• So, suppose the nightmare scenario were to come true. We learn that, provably: Fossil fuels are running out fast and will be entirely exhausted in the next few decades with no possibility of discovering more. Renewable and alternative energy sources exist, but cannot substitute for fossil fuels. Ok, technology might be invented that would enable them to, but it's so alien to the results we've found that you might as well hope for a time machine or a Star Trek food replicator. If we reduced our global energy consumption to the level that renewables and alternatives could provide, it would basically unwind most of civilization. With transport not being viable, industrialization and specialization would have to be undone to the point that every drop of energy would have to be used for production of food and heat and the basics - and even then, 40% of the Earth's population would have to starve. No internet, no gadgets, no personal or entertainment based electronics could be used, anywhere and ever, because every watt that went anywhere other than food and heat production would mean someone starving or freezing. (That also means no new technology would come along to solve the problem, because doing science costs energy and...) What would happen? How would it be dealt with? Certainly there would be conflict and many other difficulties. Would it ultimately be rational for humanity to perform the energy reduction, losing 40% of its population, and live as it may for an unknown amount of time? Or would it be more reasonable to ignore the issue, keep energy use high, and march in glory into the oblivion that would be coming sooner or later anyway?

• Answer:

  "Would it ultimately be rational for humanity to perform the energy reduction, losing 40% of its population, and live as it may for an unknown amount of time? Or would it be more reasonable to ignore the issue, keep energy use high, and march in glory into the oblivion "  -- I'm not sure what is more rational, but I know what would happen. The poor nations would starve, we rich would blithly carry on,