Jesus Christ, you guys are a cheery bunch. I love the optimism here.
Let me say why I think we have a future, and a bright one at that.
The Skylon, the first SSTO craft, is slotted to fly in 2016 for the first time. It will bring the $20,000/kilogram price to $450-$950/kilogram. That's still a lot, but it is a whole lot less. It will be the first step in removing the barrier on craft size, as we can bring up many tiny parts and assemble in orbit.
Staying on with the prospect of space, our engines are on the brink of an explosion of diversity and power. Right now, we have chemical rockets. Those are *** poor excuses for technology, abundantly wasteful, and impractical for actual space exploration/colonization. But look at what is on the horizon. Someone mentioned vacuum-energy technology. That is being investigated. But that's still conjecture, what about things we actually have coming around the corner? Magnetoplasma rockets are, which will do wonders for travelling within the solar system. Opposed to a nine month transit and two year waiting period before another transit for Mars, we could see a two month transit and turn around a month later, for a total of five months instead of about four years.
Or ion engines. They're becoming more powerful, and are already plenty efficient. With being able to assemble space craft in orbit, ion engines could become truly viable to move us at great speeds and low costs.
Then there is the entire world of unexplored physics. We have no true understanding of the universe yet. From all we can tell, we know almost nothing. There will always be something to discover. What if we discover new areas of physics that allow us to break the lightspeed barrier? I don't doubt that we will, it may just take time. Even if we can't, we can colonize the stars with relativistic ships, powered by various methods. One is the use of fusion bombs, but that's a bit crazy. If there were some way to accelerate to .5c, a ship could travel to a distant star with the same crew as upon launch. Hundreds of years would have passed around them, but we'd still be able to perpetuate mankind.
And looking to Earth herself, this cradle of life, you all speak of it becoming Detroit, just on a bigger scale (sorry Detroit, but you do suck.) I cannot see that happening, and that's not based upon opinion, but instead ecology. The carrying capacity of the Earth for human life will be reached at some point. When we get there, people will have children for a while and pass the capacity, until they start seeing many of their kids die. Within one or two generations, natural instincts will kick in to slow down reproduction. We see it in many ecosystems worldwide, in many species across many genera. When they hit the carrying capacity, the population swells above it, dips below, and then stabilizes. It has been selected for and remains in practically every sexual species.
Even so, what of genetic engineering? We are making crops which need not have pesticides, which can use less water, which have more yield and nutrients per calorie, which do less damage to the soil, which grow more quickly. This will expand the carrying capacity further, and allow us to not suffer for it.
As population swells, urban settings will expand. We do see, however, more and more people living in cities, where density is far higher than the country side. We can more efficiently store billions in cities with less land-based impact on the environment. It wouldn't be much more crowded than the metropolises of today. As I recall, we can fit half of the entire human population of Earth today in Rhode Island, if the density were the same as New York City. When more and more people are born, these mega cities will spread; and there will be enough room.
There are two killers here. Fossil fuels and nuclear weapons. If only it weren't so.
Fossil fuels will keep killing our planet for as long as we use them. That is undoubtedly true. We cannot use them forever, though. The supplies are running out. Our consumption of this dwindling resource may be speeding up, but we are already developing means to kick our addiction. When we run out, we will have done irreparable harm to the biosphere. Pollution, in air, land, and sea, will be devastating. Greenhouse effects will change many climates and kill many ecosystems. They are doing this. But these fuels will run out, and we will find an alternative. It may cause undue harm, but it won't kill us, it won't end us. Wars may be fought over the last supplies... and they may be catastrophic in the death toll. So long as nuclear weapons are not used extensively, we will recover. We recovered from WWI in time to have WWII. So we shall again.
But nuclear weapons are the most likely downfall to man. M.A.D. tells us we all will perish if one is launched, and it is indeed true. It is the dice of fate that will tell what happens here. I can only hope we do not murder ourselves foolishly.
I doubt a nuclear war will come. As we go on, as humanity moves forward, we will continue to discover new technologies and new sciences. We will find new ways of manipulating our environment, and I think we will reach the stars. To say we know everything about the lightpseed barrier is ignorant. We are always discovering two questions for every answer, and we may one day find an answer which provides us a cheat. An answer like the Alcubierre drive -- I use this example not because it will come to fruition, but because of the idea behind it. With enough creativity, we can find ways to cheat c, or discover rules which allow presently unimaginable forms of travel.
I see humans as fundamentally good. It is clear most of you disagree. I recognize it takes one bad apple to ruin the bunch, or even that many good people together are bad, but individually, almost all of us are good. Our society will have its inevitable stumbles. War. Famine. Disease. Hatred. But we will overcome. I see growing cooperation between many nations, even as there is evermore polarization. As knowledge spreads, so too does acceptance and tolerance. If we can teach the world, we can fix many problems, and pave a good future.
All this said, I go back to what I opened with. We are like an adolescent on the brink of college. All the possibilities are ahead. It is now that we choose where to go. We are at that pivotal point, and we just need to grasp the reins. I think we can. I disagree with the overwhelming pessimism here.