As morbid and cruel as it may sound, I believe the only plausible solutions lead to a dystopian future.
We are using up the Earth's resources at an ever increasing rate, and the degradation of our home world is becoming more and more apparent with each passing year. I fear that there will not be a "permanent" solution for the energy crisis for many, many lifetimes. After we find some substitute for oil, there will be great jubilation, but it is still only a substitute. We derive useful work by driving an ordered system to greater disorder, usually in the accompaniment of lethal Dihydrogen Monoxide.
Unless some unimaginable discovery which shakes the very foundation of our understanding of the mechanics in our universe (for example, how to create a magnetic monopole, or convert non-substantial amounts of energy to matter) our Earth will become inhospitable and we must move on, or perish.
As portrayed in many a Space Opera, Space (also known as SPAAAAAAAAAAAAAAAAAAAAAAAAAAAAAACE) will inevitably become the new frontier. And just like the Wild West, it may very well be lawless, at least in a sense. It is human nature to set up boundaries. Factions will emerge, claims will be staked and not held, and wars will break out over the New Oil. Peace may well come with time, but planets can only offer so much...
Seeing as the thread has been mostly concerning current issues...
Making a statement on modern society is tricky. Which is why historians tend to let a goodly number of years pass by, so as to analyze a period objectively. It is easy to revert to SPAAAAAAAAAAAAAAACE as an option, because it is the Great Unknown, the massive void "Where no Man has gone before." It may hold untold riches, or cold oblivion. It is the hope that I, personally, have for the future of Mankind.
Gah. I was supposed to have that paragraph focus on modern issues... Well, I'm going out for a bite to eat, so here are some quick bullets. [EDIT] Not so quick after all... [EDIT]
* Japan may have a massive population density, but because of the state of its culture and society, the birth rate is quite low. Notice that in many Space Opera anime and Manga, they portray the crowded future streets in a way that much alike a populated area like Tokyo?
* I will not be one to condone the act of setting value to a human life.
* With the advances in medical and mechanical technology, would you be willing to sacrifice your "humanity" for a longer life? I speak of course of Cyborgs and Nanotechnology. With these options, mankind could live a much longer life, provide for the greater good longer, and potentially slow the rate of reproduction. I do not know how the mentality of a people would evolve, but I can guess that with longer life, the idea that beauty is only skin deep will become more profound, and a literal "switch" can be implemented for such things as ovulation.
* I am not a nature freak. I believe that humans are a species of animal as well, and as the dominant species we are capable of many things. Just because you CAN does not been you SHOULD.
* I finished watching Planetes a while ago, and one of the most moving lines was by someone floating through space, gazing down at Earth:
"From here, all I see is Earth." Its meaning is profound. We should unite as a species, and share our world, not carve it up and mutilate it.
* Ever seen Idiocracy? On the note of movies: Judge Dread.
When Fox News starts reporting facts, and stops cherry-picking aspects of the world, I will have renewed faith in America. Power is only powerful if you have an enemy.
Tl;DR~~~~~~~~~~~~~~~~~~
SPAAAAAAAAAAAAAAAAAAAAAAAAAAAACE