Continued exponential growth of human population is simply not possible. The fact that we live on a sphere presents an absolute macro-level boundary. The fact that humans need energy and food draws the horizon in quite a bit closer. Humanity as a whole is facing what the Club of Rome calls the "world problematique" - a set of interacting problems that are all really just symptoms of underlying overpopulation. These include:
- Climate change from CO2 and methane release
- Oil depletion (and natural gas as well!)
- Air pollution from smog, particles and chemical fumes
- Water pollution and depletion of fresh water supplies
- Depletion of soil fertility, and soil pollution
- Depletion of ocean fish stocks, possibly to extinction
- Massive extinction rates of all types of species
- Biodiversity loss even in the absence of extinctions
- Economic instability from soaring debt rates and outsourcing
- Social stress from overcrowding and resource competition
In the face of this Gordian Knot of trouble, two things seem obvious to me. One is that we are rapidly approaching a multifactorial crisis point with regard to the continuation of human civilization as we know it. The second is that just solving one problem will not get us out of the woods.
On the specific question of liquid fuels, we are at or near the point of Peak Oil. From all indications the impending decline of the world's great petroleum reservoirs is going to create an increasing shortfall in liquid fuel supply that no amount of biofuels will be able to remedy. If mankind faces an average decline in global oil production of 5% per annum starting in five years, this would result in us losing half our oil production in 15 years. That means a loss of between 40 and 50 million barrels per day. Given that ethanol has a lower energy content than oil, we would need to produce 50 to 60 million barrels of ethanol every day to maintain our current standard of living - even if the world population stopped growing entirely.
To put it baldy, ethanol is a pipe dream. Clinging to the notion of ethanol as a solution to the liquid fuel crisis amounts to whistling past the graveyard. Our current population and its continued growth, considered in the context of the state of the world's resource base and environment, leaves little room for any conclusion except that we are facing a species-wide calamity of the first magnitude.
From the reading and thinking I've done on these topics over the last two years, I'm convinced that humanity is out of both alternatives and time. We have gotten ourselves into a condition of ecological overshoot by treating finite resources (especially oil) as though they were infinite. We have no plan B, and there is no reason to think that any such "Deus Ex Machina" is even possible - especially when the global scale of the problem is factored in.
For an sobering perspective on why this situation is normal, natural and unavoidable, I strongly recommend the book "Overshoot" by William Catton. According to him, we have been in trouble since about 1850 but never realized, simply it because we are not used to thinking of humanity and its actions in ecological terms. I agree with him wholeheartedly.