The start of my reflections is the good old physics law learnt in the middle school: the energy conservation law, which postulates that energy can not evaporate nor be generated from no where. Physical actions only transforms energy from one form to another. Though I am not sure if it stills holds true in the light of modern advances in quantum physics, in my view, on the macro level, it is still of a truth considering the human beings in the equation. 

So on the left of the equation, we have only one energy source in this solar system: the sun, it is the energy source of everything, from rocks to trees to animals. On the right we have all the energy entities: rocks, trees and animals and human beings. From the birth of the race of human, we are constantly trying to transform the energy trapped in other beings: trees and animals into energy of our own. First by inventing tools like arches and spears to aid us in our hunting, then by inventing machines running on coal and oil to create electricity, the life blood of modern society. Looking at the human history, we find more and more advanced mecanismes to suck energy out of nature. And human appetite for energy increases exponentially with the pace of the advancement of the civilization. It is really increasing in that our computers, our televisions, our ipods, our pdas are consuming huge amounts of energy collectively, yet they are leaving not much after them. If we compare the energy footprint of a citizen of Western civilization with that of an emerging economy, we might have 10 fold differences. And the worst thing, or the best thing is, emerging country citizens are catching up… So we are potentially expecting 10 billion souls using energy at the rate of Western citizens… a horror! No wonder the world is desperately scrambling for alternative energy sources. Yet the left of the equation has never changed, the source of energy is always the SUN. The solar flux reaching the earth is about 1000W/m2. Taking the current energy efficiency of solar panels of 10% and the world land area of 148.94 million km2.  That would be 14.894 trillion KW. That means if we cover all the land of the earth with our current inefficient solar panel, we could have a generating capacity of 14.894 trillion KWH/3 =4.964 trillion KWh , because not all the land is lighted at the same time.  The 2007 electricity consumption of United States only, according to the CIA World Fact Book, is  3.816 trillion kWh. I like this kind of number work, here is an example.  What is sustaining the world economy now is in fact the millions of years of sunlight the earth has enjoyed before the advent of human kind. The energy is trapped in the crude oil, in uranium, in coal etc. From the above calculation, human development is simply not sustainable.

And what if the earth can not sustain this huge population with unprecedeted quality of life? Huge declines in population by natural disaster? Armageddon? Nature takes its revenge.