There are two schools of thought which have been around since the dawn of humanity. They have always been at odds with each other, but historians will say that in the early 21st century is when they reached their maximum.
The first is the issue of resources. Some believe that there are too many people for the amount of resources available, and that this will cause great suffering as the resources run out. It doesn't matter if said resources are bone, flint, copper, iron, coal, oil, or uranium: this line of thinking has been around forever.
The other is that essentially everyone who has ever lived has wanted a more comfortable and easy life, which requires the exploitation of resources in ever greater quantities.
Neither of these positions is wrong.
The environment must be protected, if for nothing else than it's the only one we have. On the other hand, it is in our nature to grow and prosper as a species. Denying either of these is wrong. How, then, to reconcile the two?
The solution as I see it, is somewhat contradictory: maximum economic growth.
Let me explain.
Rich societies have two huge advantages over poor ones when it comes to environmental protection. The first is that birthrates fall as income increases, a phenomenon which is well documented. The second is that rich societies have the wherewithal to protect and maintain areas of natural beauty, instead of ripping them down out of desperation.
The secret then, is this: to sustain the maximum economic growth possible, even at the expense of environmental protection. The flip side to that is the realization that the Earth will not be big enough for us, and we must prepare for the days when we leave it.
It is likely that the Earth has enough resources, particularly energy and metals, to support a growing civilization for the next 400 years. This assumes we are willing to do whatever is needed to keep growing. This, then, is the window of time that we have.
Now, you may ask, where else do we go? The answer of course is to spread throughout the solar system.
The asteroid belt contains enough metals to last us for millennia, if not longer. The moon is an ideal place for heavy industry. Mercury can use sun power to perform smelting and refining of metals. Titan has enough hydrocarbons to support a chemical industry lasting for thousands of years. Mars is an ideal place to build colonies. The rings of Saturn can be mined for water for agriculture, rocket fuel, and one day hydrogen for fusion reactors. Jupiter contains all the industrial gasses we will ever need. Taken as a whole, our solar system is so rich in resources that it is difficult to see how we're going to run out.
The resources exist. The question is how to get at them. This is a technological problem, not an inherent problem. Now, there are two ways to do it. The first is a massive government program costing trillions of dollars, but that seems inefficient. The second is to do it slowly, over time, for economic purposes. The second method is superior.
The future of humanity, as I see it, can be broken down into three very broad categories.
In the near term, say the next 100 years, little will change. Everyone will still live on Earth. We'll launch a Mars mission or perhaps several, collecting the data we need to build colonies later. Someone will mine an asteroid for rare, useful metals such as platinum, gold, and tungsten. Space tourism will continue to develop, and orbital space stations will proliferate.
In the next 100 years, people will begin moving off the Earth. The Lunar shipyards will build ships the size of cities, enough to house tens or hundreds of thousands of people. Every oppressed minority on Earth will eventually have had enough, and will move into their own city ship in order to be left in peace. The space based nations will trade metals and manufactured goods with Earth in exchange for food.
The next century is when everything changes. Ships, or perhaps hollowed out asteroids, will be large enough to totally support their populations. Economics will drive the mass migration from Earth: as the Earth no longer produces anything of value, there will be no jobs for anyone. People will move off the planet, with thousands of different places to choose, each with its own culture and laws. People will be able to move freely to the nation that best exemplifies their values.
Eventually, the Earth will be almost entirely uninhabited. The 100 billion people of the space borne nations will agree to maintain the planet as a giant Eco park. As far as we know, the Earth is the only planet capable of supporting complex life, and people four centuries hence will recognize that. The earth will have stopped supporting humanity, and be of no economic use. It can therefore be left to its own devices.
Four hundred years of very hard use of the planet, followed by an eternity of tranquility and healing. Humanity will be far more prosperous than it has ever been, and the environment will be better protected than ever before. It sounds like a win win for everyone.