Space Short
"It is important for the human race to spread out into space for the survival of the species." -Stephen Hawking I can't tell you how much I believe this to be true. Earth cannot solely support our entire race for much longer. If fact it can't support all of us now. With each new child born the resources get spread a little thinner. We need to begin moving into space in order to survive as a species. We are completely capable of doing this. The only thing that is holding us back is our inability to come together and become organized. Unfortunately in order for our civilization to begin spreading into space it is going to take a combined effort of many (to say the least).
It is projected that earth can theoretically sustain a human population of 7.7 billion. The current population is 6,676,349,373 (taken on Jun 3, 2008 at 12:52 GMT) and is growing by at least 90 million a year. From those numbers you can see that we have a rather significant problem looming over us. Think about how many times you have heard about people that are starving because they have no access to food, just in the month or so. We don't even have enough fresh water in the US. Every year there are restrictions put in place in states like Arizona and California. It is estimated that we are a little over half way through our planet's oil resources and not even half the world is even industrialized yet. There are just too many people depending on this planet for their only sources of, well everything. This does not make any sense.
Now so far I have really only mentioned the stuff that we can plan on but in reality the of our dwindling resources is not the only reason to move into space. "Life on Earth is at the ever-increasing risk of being wiped out by a disaster, such as sudden global warming, nuclear war, a genetically engineered virus or other dangers we have not yet thought of." -Stephen Hawking There are countless reasons to not depend on one single place to support all of our growing needs.
To start we should build a very large space station that support the long term life of hundreds of people. This would be like an experiment, just to see all the challenges life in space may present. We could grow large amounts of eatable plants like fruits, vegetables, and nuts and things. Of course there is the problem of having no gravity and technology to create artificial gravity yet. But we can over come that by simply spinning the entire station at a certain rate that mimics the effects of gravity. Chances are, people wouldn't be able to tell the difference. We can power the station by solar energy, or perhaps fusion energy using Helium 3. - see super fuel article I guess the biggest foreseeable problem would be getting all of the material to build and maintain the 'super station' up into space. There are however several theoretical answers to that, like the space elevator.
So look I'm not a specialist in any of the fields of science dealing with type of project but from I have read this all seems completely possible, if there was a source of money and support. The only solution to this I can think of for there to be some kind of very large private organization. Maybe a coalition of many scientific / space driven organizations. If you could get a bunch of places like Life Boat and Space.com to come together to work on putting together a single massive project, by donations and things. Is that not realistic? Can this happen?
From a very early age we are taught to not put all of our eggs in one basket, don't put all you investments in one place and that is exactly what we are all doing. Yet you tell people that we need to begin moving our civilization into space and they scoff at the idea. As if this is not a realistic idea. So once again I ask you: Is this not realistic? Can this happen? Or are we all doomed? The major thinkers agree, we have got to move into space in order for the human race to survive. The numbers add up and we are running out of time and resources. So what are we going to do? Sit and wait for the eventual death of our species or push the envelope and adapt to survive.