With today's technology we can gracefully sustain less than 2% of the global population (<100 million.) By gracefully I mean everyone living like Americans and still maintaining the natural environment.
Of course, we don't have to reduce the population by 98% to meet these objectives "IF" we can transport ourselves in the equivalent of 1500 mile per gallon cars. Light and cool our homes with the equivalent of a 60-watt light bulb. Heat our homes with the energy equivalent of few dozen burning candles, and grow corn at 10,000 bushels per acre.
These are huge technological accomplishments but they are small when compared with the low consumption opportunities if humans didn't have to haul around carbon-based bodies.
You may think I am referring to paying your bills electronically or teleconferencing with family, or telecommuting to work, but I have something bigger in mind. What if technology could create our existence, our thinking, our experience, our creativeness, even our emotions in silicon form, then earth could support a trillion souls. Hurray for technology. It may save us from Malthusian starvation and eco destruction.
There is a small hitch in this wonderful scenario. Scientists need time to create these enormous technological breakthroughs. During this time period civilization must hold together. The scientist must be able to drive to work without being shot. And on the way home the shelves of his or her grocery store cannot be empty.
Social conflict, driven by scarcity, usually crumbles a civilization. "So what!" you say, "Civilization-crumbling does not mean the end of technological progress. Rome fell, but technology continued to advance." Yes, but Rome's failure happen in a fraction of the world. Rome was not consuming to exhaustion nonrenewable resources.
When our integrated global civilization fails most cities will have anarchy in the streets and empty shelves in the grocery. There will be no place for our bright technologists to run and restart. The cheap fuel that twentieth century civilizations used to launch technological revolutions could be gone. "High tech" progress could stop -- maybe permanently.
Logic tells us we are running the human experiment off a cliff. We might already be in mid air, just waiting for the car to hit the rocks below. With our radios still playing and our air conditioners still cooling, it hard to tell which side of the cliff edge we are on.
However, if we are still before that edge, we might have time to write a better scenario. Today we could create a constituency that believes rapid population decline is necessary for the survival of the human experiment. And when that constituency becomes dominant, we just might implement rapid population decline. We just might get the population to decline fast enough to preserve the human experiment.