http://www.theoildrum.com/node/8155 Been saying for years that the model of perpetual growth of 3-4%, which is apparently necessary to avoid recessions etc. etc. are impossible when looked at from a humanity type time frame...it's always countered with words like "replacement" and "efficiencies", but the above link is an interesting argument/thought experiment. Our fossil fuels are stored solar energy, so the premise is that "sustainable" is in equilibrium with solar inputs. Scenario 1 is using P.V. at 20% efficiency, the entire global land mass is covered with P.V. in 275 years...really not that very long. Make the assumption that we get way smart, and make them 100% efficient (impossible), and that factor of 5 only buys another 70 years due to exponential growth...cover the oceans as well, buys another 55 years, again due to the exponential growth. The end part deals with the thermodynamics of the Earth's energy balance should we start to use technologies such as fusion, where the earth has to reject excess heat, which it can only do through rising temperatures, just due to plain laws of physics (not the tabboo stuff on the board, just simple radiative heat transfer. In short, if we keep doing what we are doing, and following the economic lead, our model of society has literally only a couple hundred years ahead of it.